## 4. Memcheck: a heavyweight memory checker

To use this tool, you may specify `--tool=memcheck` on the Valgrind command line. You don't have to, though, since Memcheck is the default tool.

## 4.1. Kinds of bugs that Memcheck can find

Memcheck is Valgrind's heavyweight memory checking tool. All reads and writes of memory are checked, and calls to malloc/new/free/delete are intercepted. As a result, Memcheck can detect the following problems:

- Use of uninitialised memory

- Reading/writing memory after it has been free'd

- Reading/writing off the end of malloc'd blocks

- Reading/writing inappropriate areas on the stack

- Memory leaks - where pointers to malloc'd blocks are lost forever

- Mismatched use of malloc/new/new [] vs free/delete/delete []

- Overlapping `src` and `dst` pointers in `memcpy()` and related functions

## 4.2. Command-line flags specific to Memcheck

`--leak-check=<no|summary|yes|full> [default: summary]`

  When enabled, search for memory leaks when the client program finishes. A memory leak means a malloc'd block, which has not yet been free'd, but to which no pointer can be found. Such a block can never be free'd by the program, since no pointer to it exists. If set to `summary`, it says how many leaks occurred. If set to `full` or `yes`, it gives details of each individual leak.

`--show-reachable=<yes|no> [default: no]`

  When disabled, the memory leak detector only shows blocks for which it cannot find a pointer to at all, or it can only find a pointer to the middle of. These blocks are prime candidates for memory leaks. When enabled, the leak detector also reports on blocks which it could find a pointer to. Your program could, at least in principle, have freed such blocks before exit. Contrast this to blocks for which no pointer, or only an interior pointer could be found: they are more likely to indicate memory leaks, because you do not actually have a pointer to the start of the block which you can hand to `free`, even if you wanted to.

`--leak-resolution=<low|med|high> [default: low]`

  When doing leak checking, determines how willing `memcheck` is to consider different backtraces to be the same. When set to `low`, only the first two entries need match. When `med`, four entries have to match. When `high`, all entries need to match.

  For hardcore leak debugging, you probably want to use `--leak-resolution=high` together with `--num-callers=40` or some such large number. Note however that this can give an overwhelming amount of information, which is why the defaults are 4 callers and low-resolution matching.

  Note that the `--leak-resolution=` setting does not affect `memcheck's` ability to find leaks. It only changes how the results are presented.

`--freelist-vol=<number> [default: 10000000]`

  When the client program releases memory using `free` (in `C`) or delete (`C++`), that memory is not immediately made available for re-allocation. Instead, it is marked inaccessible and placed in a queue of freed blocks. The purpose is to defer as long as possible the point at which freed-up memory comes back into circulation. This increases the chance that `memcheck` will be able to detect invalid accesses to blocks for some significant period of time after they have been freed.

  This flag specifies the maximum total size, in bytes, of the blocks in the queue. The default value is ten million bytes. Increasing this increases the total amount of memory used by `memcheck` but may detect invalid uses of freed blocks which would otherwise go undetected.

`--workaround-gcc296-bugs=<yes|no> [default: no]`

  When enabled, assume that reads and writes some small distance below the stack pointer are due to bugs in gcc 2.96, and does not report them. The "small distance" is 256 bytes by default. Note that gcc 2.96 is the default compiler on some ancient Linux distributions (RedHat 7.X) and so you may need to use this flag. Do not use it if you do not have to, as it can cause real errors to be overlooked. A better alternative is to use a more recent gcc/g++ in which this bug is fixed.

  You may also need to use this flag when working with gcc/g++ 3.X or 4.X on 32-bit PowerPC Linux. This is because gcc/g++ generates code which occasionally accesses below the stack pointer, particularly for floating-point to/from integer conversions. This is in violation of the 32-bit PowerPC ELF specification, which makes no provision for locations below the stack pointer to be accessible.

`--partial-loads-ok=<yes|no> [default: no]`

  Controls how `memcheck` handles word-sized, word-aligned loads from addresses for which some bytes are addressable and others are not. When `yes`, such loads do not produce an address error. Instead, loaded bytes originating from illegal addresses are marked as uninitialised, and those corresponding to legal addresses are handled in the normal way.

  When `no`, loads from partially invalid addresses are treated the same as loads from completely invalid addresses: an illegal-address error is issued, and the resulting bytes are marked as initialised.

  Note that code that behaves in this way is in violation of the the ISO C/C++ standards, and should be considered broken. If at all possible, such code should be fixed. This flag should be used only as a last resort.

`--undef-value-errors=<yes|no> [default: yes]`

  Controls whether `memcheck` detects dangerous uses of undefined value errors. Set this to `no` if you don't like seeing undefined value errors; it also has the side effect of speeding `memcheck` up somewhat.

`--malloc-fill=<hexnumber>`

  Fills blocks allocated by `malloc`, `new`, etc, but not by `calloc`, with the specified byte. This can be useful when trying to shake out obscure memory corruption problems. The allocated area is still regarded by Memcheck as undefined -- this flag only affects its contents.

`--free-fill=<hexnumber>`

  Fills blocks freed by `free`, `delete`, etc, with the specified byte. This can be useful when trying to shake out obscure memory corruption problems. The freed area is still regarded by Memcheck as not valid for access -- this flag only affects its contents.

## 4.3. Explanation of error messages from Memcheck

Despite considerable sophistication under the hood, Memcheck can only really detect two kinds of errors: use of illegal addresses, and use of undefined values. Nevertheless, this is enough to help you discover all sorts of memory-management problems in your code. This section presents a quick summary of what error messages mean. The precise behaviour of the error-checking machinery is described in Details of Memcheck's checking machinery.

### 4.3.1. Illegal read / Illegal write errors

For example:

    Invalid read of size 4
       at 0x40F6BBCC: (within /usr/lib/libpng.so.2.1.0.9)
       by 0x40F6B804: (within /usr/lib/libpng.so.2.1.0.9)
       by 0x40B07FF4: read_png_image(QImageIO *) (kernel/qpngio.cpp:326)
       by 0x40AC751B: QImageIO::read() (kernel/qimage.cpp:3621)
     Address 0xBFFFF0E0 is not stack'd, malloc'd or free'd

This happens when your program reads or writes memory at a place which Memcheck reckons it shouldn't. In this example, the program did a 4-byte read at address 0xBFFFF0E0, somewhere within the system-supplied library libpng.so.2.1.0.9, which was called from somewhere else in the same library, called from line 326 of `qpngio.cpp`, and so on.

Memcheck tries to establish what the illegal address might relate to, since that's often useful. So, if it points into a block of memory which has already been freed, you'll be informed of this, and also where the block was free'd at. Likewise, if it should turn out to be just off the end of a malloc'd block, a common result of off-by-one-errors in array subscripting, you'll be informed of this fact, and also where the block was malloc'd.

In this example, Memcheck can't identify the address. Actually the address is on the stack, but, for some reason, this is not a valid stack address -- it is below the stack pointer and that isn't allowed. In this particular case it's probably caused by gcc generating invalid code, a known bug in some ancient versions of gcc.

Note that Memcheck only tells you that your program is about to access memory at an illegal address. It can't stop the access from happening. So, if your program makes an access which normally would result in a segmentation fault, you program will still suffer the same fate -- but you will get a message from Memcheck immediately prior to this. In this particular example, reading junk on the stack is non-fatal, and the program stays alive.

### 4.3.2. Use of uninitialised values

For example:

    Conditional jump or move depends on uninitialised value(s)
       at 0x402DFA94: _IO_vfprintf (_itoa.h:49)
       by 0x402E8476: _IO_printf (printf.c:36)
       by 0x8048472: main (tests/manuel1.c:8)

An uninitialised-value use error is reported when your program uses a value which hasn't been initialised -- in other words, is undefined. Here, the undefined value is used somewhere inside the printf() machinery of the C library. This error was reported when running the following small program:

    int main()
    {
      int x;
      printf ("x = %d\n", x);
    }

It is important to understand that your program can copy around junk (uninitialised) data as much as it likes. Memcheck observes this and keeps track of the data, but does not complain. A complaint is issued only when your program attempts to make use of uninitialised data. In this example, x is uninitialised. Memcheck observes the value being passed to `_IO_printf` and thence to `_IO_vfprintf`, but makes no comment. However, `_IO_vfprintf` has to examine the value of x so it can turn it into the corresponding ASCII string, and it is at this point that Memcheck complains.

Sources of uninitialised data tend to be:

- Local variables in procedures which have not been initialised, as in the example above.

- The contents of malloc'd blocks, before you write something there. In C++, the new operator is a wrapper round malloc, so if you create an object with new, its fields will be uninitialised until you (or the constructor) fill them in.

### 4.3.3. Illegal frees

For example:

    Invalid free()
       at 0x4004FFDF: free (vg_clientmalloc.c:577)
       by 0x80484C7: main (tests/doublefree.c:10)
     Address 0x3807F7B4 is 0 bytes inside a block of size 177 free'd
       at 0x4004FFDF: free (vg_clientmalloc.c:577)
       by 0x80484C7: main (tests/doublefree.c:10)

Memcheck keeps track of the blocks allocated by your program with malloc/new, so it can know exactly whether or not the argument to free/delete is legitimate or not. Here, this test program has freed the same block twice. As with the illegal read/write errors, Memcheck attempts to make sense of the address free'd. If, as here, the address is one which has previously been freed, you wil be told that -- making duplicate frees of the same block easy to spot.

### 4.3.4. When a block is freed with an inappropriate deallocation function

In the following example, a block allocated with `new[]` has wrongly been deallocated with `free`:

    Mismatched free() / delete / delete []
       at 0x40043249: free (vg_clientfuncs.c:171)
       by 0x4102BB4E: QGArray::~QGArray(void) (tools/qgarray.cpp:149)
       by 0x4C261C41: PptDoc::~PptDoc(void) (include/qmemarray.h:60)
       by 0x4C261F0E: PptXml::~PptXml(void) (pptxml.cc:44)
     Address 0x4BB292A8 is 0 bytes inside a block of size 64 alloc'd
       at 0x4004318C: operator new[](unsigned int) (vg_clientfuncs.c:152)
       by 0x4C21BC15: KLaola::readSBStream(int) const (klaola.cc:314)
       by 0x4C21C155: KLaola::stream(KLaola::OLENode const *) (klaola.cc:416)
       by 0x4C21788F: OLEFilter::convert(QCString const &) (olefilter.cc:272)

In `C++` it's important to deallocate memory in a way compatible with how it was allocated. The deal is:

- If allocated with `malloc`, `calloc`, `realloc`, `valloc` or `memalign`, you must deallocate with `free`.

- If allocated with `new[]`, you must deallocate with `delete[]`.

- If allocated with `new`, you must deallocate with `delete`.

The worst thing is that on Linux apparently it doesn't matter if you do mix these up, but the same program may then crash on a different platform, Solaris for example. So it's best to fix it properly. According to the KDE folks "it's amazing how many C++ programmers don't know this".

The reason behind the requirement is as follows. In some C++ implementations, `delete[]` must be used for objects allocated by `new[]` because the compiler stores the size of the array and the pointer-to-member to the destructor of the array's content just before the pointer actually returned. This implies a variable-sized overhead in what's returned by `new` or `new[]`.

### 4.3.5. Passing system call parameters with inadequate read/write permissions

Memcheck checks all parameters to system calls:

- It checks all the direct parameters themselves.

- Also, if a system call needs to read from a buffer provided by your program, Memcheck checks that the entire buffer is addressable and has valid data, ie, it is readable.

- Also, if the system call needs to write to a user-supplied buffer, Memcheck checks that the buffer is addressable.

After the system call, Memcheck updates its tracked information to precisely reflect any changes in memory permissions caused by the system call.

Here's an example of two system calls with invalid parameters:

      #include <stdlib.h>
      #include <unistd.h>
      int main( void )
      {
        char* arr  = malloc(10);
        int*  arr2 = malloc(sizeof(int));
        write( 1 /* stdout */, arr, 10 );
        exit(arr2[0]);
      }

You get these complaints ...

      Syscall param write(buf) points to uninitialised byte(s)
         at 0x25A48723: __write_nocancel (in /lib/tls/libc-2.3.3.so)
         by 0x259AFAD3: __libc_start_main (in /lib/tls/libc-2.3.3.so)
         by 0x8048348: (within /auto/homes/njn25/grind/head4/a.out)
       Address 0x25AB8028 is 0 bytes inside a block of size 10 alloc'd
         at 0x259852B0: malloc (vg_replace_malloc.c:130)
         by 0x80483F1: main (a.c:5)

      Syscall param exit(error_code) contains uninitialised byte(s)
         at 0x25A21B44: __GI__exit (in /lib/tls/libc-2.3.3.so)
         by 0x8048426: main (a.c:8)

... because the program has (a) tried to write uninitialised junk from the malloc'd block to the standard output, and (b) passed an uninitialised value to `exit`. Note that the first error refers to the memory pointed to by `buf` (not `buf` itself), but the second error refers directly to `exit`'s argument `arr2[0]`.

### 4.3.6. Overlapping source and destination blocks

The following C library functions copy some data from one memory block to another (or something similar): `memcpy()`, `strcpy()`, `strncpy()`, `strcat()`, `strncat()`. The blocks pointed to by their `src` and `dst` pointers aren't allowed to overlap. Memcheck checks for this.

For example:

    ==27492== Source and destination overlap in memcpy(0xbffff294, 0xbffff280, 21)
    ==27492==    at 0x40026CDC: memcpy (mc_replace_strmem.c:71)
    ==27492==    by 0x804865A: main (overlap.c:40)

You don't want the two blocks to overlap because one of them could get partially overwritten by the copying.

You might think that Memcheck is being overly pedantic reporting this in the case where `dst` is less than `src`. For example, the obvious way to implement `memcpy()` is by copying from the first byte to the last. However, the optimisation guides of some architectures recommend copying from the last byte down to the first. Also, some implementations of `memcpy()` zero `dst` before copying, because zeroing the destination's cache line(s) can improve performance.

In addition, for many of these functions, the POSIX standards have wording along the lines "If copying takes place between objects that overlap, the behavior is undefined." Hence overlapping copies violate the standard.

The moral of the story is: if you want to write truly portable code, don't make any assumptions about the language implementation.

### 4.3.7. Memory leak detection

Memcheck keeps track of all memory blocks issued in response to calls to malloc/calloc/realloc/new. So when the program exits, it knows which blocks have not been freed.

If `--leak-check` is set appropriately, for each remaining block, Memcheck scans the entire address space of the process, looking for pointers to the block. Each block fits into one of the three following categories.

- Still reachable: A pointer to the start of the block is found. This usually indicates programming sloppiness. Since the block is still pointed at, the programmer could, at least in principle, free it before program exit. Because these are very common and arguably not a problem, Memcheck won't report such blocks unless `--show-reachable=yes` is specified.

- Possibly lost, or "dubious": A pointer to the interior of the block is found. The pointer might originally have pointed to the start and have been moved along, or it might be entirely unrelated. Memcheck deems such a block as "dubious", because it's unclear whether or not a pointer to it still exists.

- Definitely lost, or "leaked": The worst outcome is that no pointer to the block can be found. The block is classified as "leaked", because the programmer could not possibly have freed it at program exit, since no pointer to it exists. This is likely a symptom of having lost the pointer at some earlier point in the program.

For each block mentioned, Memcheck will also tell you where the block was allocated. It cannot tell you how or why the pointer to a leaked block has been lost; you have to work that out for yourself. In general, you should attempt to ensure your programs do not have any leaked or dubious blocks at exit.

For example:

    8 bytes in 1 blocks are definitely lost in loss record 1 of 14
       at 0x........: malloc (vg_replace_malloc.c:...)
       by 0x........: mk (leak-tree.c:11)
       by 0x........: main (leak-tree.c:39)

    88 (8 direct, 80 indirect) bytes in 1 blocks are definitely lost
                               in loss record 13 of 14
       at 0x........: malloc (vg_replace_malloc.c:...)
       by 0x........: mk (leak-tree.c:11)
       by 0x........: main (leak-tree.c:25)

The first message describes a simple case of a single 8 byte block that has been definitely lost. The second case mentions both "direct" and "indirect" leaks. The distinction is that a direct leak is a block which has no pointers to it. An indirect leak is a block which is only pointed to by other leaked blocks. Both kinds of leak are bad.

The precise area of memory in which Memcheck searches for pointers is: all naturally-aligned machine-word-sized words found in memory that Memcheck's records indicate is both accessible and initialised.

## 4.4. Writing suppression files

The basic suppression format is described in Suppressing errors.

The suppression-type (second) line should have the form:

    Memcheck:suppression_type

The Memcheck suppression types are as follows:

- `Value1`, `Value2`, `Value4`, `Value8`, `Value16`, meaning an uninitialised-value error when using a value of 1, 2, 4, 8 or 16 bytes.

- `Cond` (or its old name, `Value0`), meaning use of an uninitialised CPU condition code.

- `Addr1`, `Addr2`, `Addr4`, `Addr8`, `Addr16`, meaning an invalid address during a memory access of 1, 2, 4, 8 or 16 bytes respectively.

- `Jump`, meaning an jump to an unaddressable location error.

- `Param`, meaning an invalid system call parameter error.

- `Free`, meaning an invalid or mismatching free.

- `Overlap`, meaning a `src` / `dst` overlap in `memcpy()` or a similar function.

- `Leak`, meaning a memory leak.

`Param` errors have an extra information line at this point, which is the name of the offending system call parameter. No other error kinds have this extra line.

The first line of the calling context: for Value and Addr errors, it is either the name of the function in which the error occurred, or, failing that, the full path of the .so file or executable containing the error location. For Free errors, is the name of the function doing the freeing (eg, `free`, `__builtin_vec_delete`, etc). For Overlap errors, is the name of the function with the overlapping arguments (eg. `memcpy()`, `strcpy()`, etc).

Lastly, there's the rest of the calling context.

## 4.5. Details of Memcheck's checking machinery

Read this section if you want to know, in detail, exactly what and how Memcheck is checking.

### 4.5.1. Valid-value (V) bits

It is simplest to think of Memcheck implementing a synthetic CPU which is identical to a real CPU, except for one crucial detail. Every bit (literally) of data processed, stored and handled by the real CPU has, in the synthetic CPU, an associated "valid-value" bit, which says whether or not the accompanying bit has a legitimate value. In the discussions which follow, this bit is referred to as the V (valid-value) bit.

Each byte in the system therefore has a 8 V bits which follow it wherever it goes. For example, when the CPU loads a word-size item (4 bytes) from memory, it also loads the corresponding 32 V bits from a bitmap which stores the V bits for the process' entire address space. If the CPU should later write the whole or some part of that value to memory at a different address, the relevant V bits will be stored back in the V-bit bitmap.

In short, each bit in the system has an associated V bit, which follows it around everywhere, even inside the CPU. Yes, all the CPU's registers (integer, floating point, vector and condition registers) have their own V bit vectors.

Copying values around does not cause Memcheck to check for, or report on, errors. However, when a value is used in a way which might conceivably affect the outcome of your program's computation, the associated V bits are immediately checked. If any of these indicate that the value is undefined, an error is reported.

Here's an (admittedly nonsensical) example:

    int i, j;
    int a[10], b[10];
    for ( i = 0; i < 10; i++ ) {
      j = a[i];
      b[i] = j;
    }

Memcheck emits no complaints about this, since it merely copies uninitialised values from `a[]` into `b[]`, and doesn't use them in a way which could affect the behaviour of the program. However, if the loop is changed to:

    for ( i = 0; i < 10; i++ ) {
      j += a[i];
    }
    if ( j == 77 )
      printf("hello there\n");

then Memcheck will complain, at the `if`, that the condition depends on uninitialised values. Note that it doesn't complain at the `j += a[i];`, since at that point the undefinedness is not "observable". It's only when a decision has to be made as to whether or not to do the `printf` -- an observable action of your program -- that Memcheck complains.

Most low level operations, such as adds, cause Memcheck to use the V bits for the operands to calculate the V bits for the result. Even if the result is partially or wholly undefined, it does not complain.

Checks on definedness only occur in three places: when a value is used to generate a memory address, when control flow decision needs to be made, and when a system call is detected, Memcheck checks definedness of parameters as required.

If a check should detect undefinedness, an error message is issued. The resulting value is subsequently regarded as well-defined. To do otherwise would give long chains of error messages. In other words, once Memcheck reports an undefined value error, it tries to avoid reporting further errors derived from that same undefined value.

This sounds overcomplicated. Why not just check all reads from memory, and complain if an undefined value is loaded into a CPU register? Well, that doesn't work well, because perfectly legitimate C programs routinely copy uninitialised values around in memory, and we don't want endless complaints about that. Here's the canonical example. Consider a struct like this:

    struct S { int x; char c; };
    struct S s1, s2;
    s1.x = 42;
    s1.c = 'z';
    s2 = s1;

The question to ask is: how large is `struct S`, in bytes? An `int` is 4 bytes and a `char` one byte, so perhaps a `struct S` occupies 5 bytes? Wrong. All non-toy compilers we know of will round the size of `struct S` up to a whole number of words, in this case 8 bytes. Not doing this forces compilers to generate truly appalling code for accessing arrays of `struct S`'s on some architectures.

So `s1` occupies 8 bytes, yet only 5 of them will be initialised. For the assignment `s2 = s1`, gcc generates code to copy all 8 bytes wholesale into `s2` without regard for their meaning. If Memcheck simply checked values as they came out of memory, it would yelp every time a structure assignment like this happened. So the more complicated behaviour described above is necessary. This allows `gcc` to copy `s1` into `s2` any way it likes, and a warning will only be emitted if the uninitialised values are later used.

### 4.5.2. Valid-address (A) bits

Notice that the previous subsection describes how the validity of values is established and maintained without having to say whether the program does or does not have the right to access any particular memory location. We now consider the latter question.

As described above, every bit in memory or in the CPU has an associated valid-value (V) bit. In addition, all bytes in memory, but not in the CPU, have an associated valid-address (A) bit. This indicates whether or not the program can legitimately read or write that location. It does not give any indication of the validity or the data at that location -- that's the job of the V bits -- only whether or not the location may be accessed.

Every time your program reads or writes memory, Memcheck checks the A bits associated with the address. If any of them indicate an invalid address, an error is emitted. Note that the reads and writes themselves do not change the A bits, only consult them.

So how do the A bits get set/cleared? Like this:

- When the program starts, all the global data areas are marked as accessible.

- When the program does malloc/new, the A bits for exactly the area allocated, and not a byte more, are marked as accessible. Upon freeing the area the A bits are changed to indicate inaccessibility.

- When the stack pointer register (`SP`) moves up or down, A bits are set. The rule is that the area from `SP` up to the base of the stack is marked as accessible, and below `SP` is inaccessible. (If that sounds illogical, bear in mind that the stack grows down, not up, on almost all Unix systems, including GNU/Linux.) Tracking `SP` like this has the useful side-effect that the section of stack used by a function for local variables etc is automatically marked accessible on function entry and inaccessible on exit.

- When doing system calls, A bits are changed appropriately. For example, `mmap` magically makes files appear in the process' address space, so the A bits must be updated if `mmap` succeeds.

- Optionally, your program can tell Memcheck about such changes explicitly, using the client request mechanism described above.

### 4.5.3. Putting it all together

Memcheck's checking machinery can be summarised as follows:

- Each byte in memory has 8 associated V (valid-value) bits, saying whether or not the byte has a defined value, and a single A (valid-address) bit, saying whether or not the program currently has the right to read/write that address.

- When memory is read or written, the relevant A bits are consulted. If they indicate an invalid address, Memcheck emits an Invalid read or Invalid write error.

- When memory is read into the CPU's registers, the relevant V bits are fetched from memory and stored in the simulated CPU. They are not consulted.

- When a register is written out to memory, the V bits for that register are written back to memory too.

- When values in CPU registers are used to generate a memory address, or to determine the outcome of a conditional branch, the V bits for those values are checked, and an error emitted if any of them are undefined.

- When values in CPU registers are used for any other purpose, Memcheck computes the V bits for the result, but does not check them.

- Once the V bits for a value in the CPU have been checked, they are then set to indicate validity. This avoids long chains of errors.

- When values are loaded from memory, Memcheck checks the A bits for that location and issues an illegal-address warning if needed. In that case, the V bits loaded are forced to indicate Valid, despite the location being invalid.

  This apparently strange choice reduces the amount of confusing information presented to the user. It avoids the unpleasant phenomenon in which memory is read from a place which is both unaddressable and contains invalid values, and, as a result, you get not only an invalid-address (read/write) error, but also a potentially large set of uninitialised-value errors, one for every time the value is used.

  There is a hazy boundary case to do with multi-byte loads from addresses which are partially valid and partially invalid. See details of the flag `--partial-loads-ok` for details.

Memcheck intercepts calls to malloc, calloc, realloc, valloc, memalign, free, new, new[], delete and delete[]. The behaviour you get is:

- malloc/new/new[]: the returned memory is marked as addressable but not having valid values. This means you have to write to it before you can read it.

- calloc: returned memory is marked both addressable and valid, since calloc clears the area to zero.

- realloc: if the new size is larger than the old, the new section is addressable but invalid, as with malloc.

- If the new size is smaller, the dropped-off section is marked as unaddressable. You may only pass to realloc a pointer previously issued to you by malloc/calloc/realloc.

- free/delete/delete[]: you may only pass to these functions a pointer previously issued to you by the corresponding allocation function. Otherwise, Memcheck complains. If the pointer is indeed valid, Memcheck marks the entire area it points at as unaddressable, and places the block in the freed-blocks-queue. The aim is to defer as long as possible reallocation of this block. Until that happens, all attempts to access it will elicit an invalid-address error, as you would hope.

## 4.6. Client Requests

The following client requests are defined in `memcheck.h`. See `memcheck.h` for exact details of their arguments.

- `VALGRIND_MAKE_MEM_NOACCESS`, `VALGRIND_MAKE_MEM_UNDEFINED` and `VALGRIND_MAKE_MEM_DEFINED`. These mark address ranges as completely inaccessible, accessible but containing undefined data, and accessible and containing defined data, respectively. Subsequent errors may have their faulting addresses described in terms of these blocks. Returns a "block handle". Returns zero when not run on Valgrind.

- `VALGRIND_MAKE_MEM_DEFINED_IF_ADDRESSABLE`. This is just like `VALGRIND_MAKE_MEM_DEFINED` but only affects those bytes that are already addressable.

- `VALGRIND_DISCARD`: At some point you may want Valgrind to stop reporting errors in terms of the blocks defined by the previous three macros. To do this, the above macros return a small-integer "block handle". You can pass this block handle to `VALGRIND_DISCARD`. After doing so, Valgrind will no longer be able to relate addressing errors to the user-defined block associated with the handle. The permissions settings associated with the handle remain in place; this just affects how errors are reported, not whether they are reported. Returns 1 for an invalid handle and 0 for a valid handle (although passing invalid handles is harmless). Always returns 0 when not run on Valgrind.

- `VALGRIND_CHECK_MEM_IS_ADDRESSABLE` and `VALGRIND_CHECK_MEM_IS_DEFINED`: check immediately whether or not the given address range has the relevant property, and if not, print an error message. Also, for the convenience of the client, returns zero if the relevant property holds; otherwise, the returned value is the address of the first byte for which the property is not true. Always returns 0 when not run on Valgrind.

- `VALGRIND_CHECK_VALUE_IS_DEFINED`: a quick and easy way to find out whether Valgrind thinks a particular value (lvalue, to be precise) is addressable and defined. Prints an error message if not. Returns no value.

- `VALGRIND_DO_LEAK_CHECK`: runs the memory leak detector right now. Is useful for incrementally checking for leaks between arbitrary places in the program's execution. Returns no value.

- `VALGRIND_COUNT_LEAKS`: fills in the four arguments with the number of bytes of memory found by the previous leak check to be leaked, dubious, reachable and suppressed. Again, useful in test harness code, after calling `VALGRIND_DO_LEAK_CHECK`.

- `VALGRIND_GET_VBITS` and `VALGRIND_SET_VBITS`: allow you to get and set the V (validity) bits for an address range. You should probably only set V bits that you have got with `VALGRIND_GET_VBITS`. Only for those who really know what they are doing.

## 4.7. Memory Pools: describing and working with custom allocators

Some programs use custom memory allocators, often for performance reasons. Left to itself, Memcheck is unable to "understand" the behaviour of custom allocation schemes and so may miss errors and leaks in your program. What this section describes is a way to give Memcheck enough of a description of your custom allocator that it can make at least some sense of what is happening.

There are many different sorts of custom allocator, so Memcheck attempts to reason about them using a loose, abstract model. We use the following terminology when describing custom allocation systems:

- Custom allocation involves a set of independent "memory pools".

- Memcheck's notion of a a memory pool consists of a single "anchor address" and a set of non-overlapping "chunks" associated with the anchor address.

- Typically a pool's anchor address is the address of a book-keeping "header" structure.

- Typically the pool's chunks are drawn from a contiguous "superblock" acquired through the system malloc() or mmap().

Keep in mind that the last two points above say "typically": the Valgrind mempool client request API is intentionally vague about the exact structure of a mempool. There is no specific mention made of headers or superblocks. Nevertheless, the following picture may help elucidate the intention of the terms in the API:

       "pool"
       (anchor address)
       |
       v
       +--------+---+
       | header | o |
       +--------+-|-+
                  |
                  v                  superblock
                  +------+---+--------------+---+------------------+
                  |      |rzB|  allocation  |rzB|                  |
                  +------+---+--------------+---+------------------+
                             ^              ^
                             |              |
                           "addr"     "addr"+"size"

Note that the header and the superblock may be contiguous or discontiguous, and there may be multiple superblocks associated with a single header; such variations are opaque to Memcheck. The API only requires that your allocation scheme can present sensible values of "pool", "addr" and "size".

Typically, before making client requests related to mempools, a client program will have allocated such a header and superblock for their mempool, and marked the superblock NOACCESS using the `VALGRIND_MAKE_MEM_NOACCESS` client request.

When dealing with mempools, the goal is to maintain a particular invariant condition: that Memcheck believes the unallocated portions of the pool's superblock (including redzones) are NOACCESS. To maintain this invariant, the client program must ensure that the superblock starts out in that state; Memcheck cannot make it so, since Memcheck never explicitly learns about the superblock of a pool, only the allocated chunks within the pool.

Once the header and superblock for a pool are established and properly marked, there are a number of client requests programs can use to inform Memcheck about changes to the state of a mempool:

- `VALGRIND_CREATE_MEMPOOL(pool, rzB, is_zeroed)`: This request registers the address "pool" as the anchor address for a memory pool. It also provides a size "rzB", specifying how large the redzones placed around chunks allocated from the pool should be. Finally, it provides an "is_zeroed" flag that specifies whether the pool's chunks are zeroed (more precisely: defined) when allocated.

  Upon completion of this request, no chunks are associated with the pool. The request simply tells Memcheck that the pool exists, so that subsequent calls can refer to it as a pool.

- `VALGRIND_DESTROY_MEMPOOL(pool)`: This request tells Memcheck that a pool is being torn down. Memcheck then removes all records of chunks associated with the pool, as well as its record of the pool's existence. While destroying its records of a mempool, Memcheck resets the redzones of any live chunks in the pool to NOACCESS.

- `VALGRIND_MEMPOOL_ALLOC(pool, addr, size)`: This request informs Memcheck that a "size"-byte chunk has been allocated at "addr", and associates the chunk with the specified "pool". If the pool was created with nonzero "rzB" redzones, Memcheck will mark the "rzB" bytes before and after the chunk as NOACCESS. If the pool was created with the "is_zeroed" flag set, Memcheck will mark the chunk as DEFINED, otherwise Memcheck will mark the chunk as UNDEFINED.

- `VALGRIND_MEMPOOL_FREE(pool, addr)`: This request informs Memcheck that the chunk at "addr" should no longer be considered allocated. Memcheck will mark the chunk associated with "addr" as NOACCESS, and delete its record of the chunk's existence.

- `VALGRIND_MEMPOOL_TRIM(pool, addr, size)`: This request "trims" the chunks associated with "pool". The request only operates on chunks associated with "pool". Trimming is formally defined as:

  - All chunks entirely inside the range [addr,addr+size) are preserved.

  - All chunks entirely outside the range [addr,addr+size) are discarded, as though `VALGRIND_MEMPOOL_FREE` was called on them.

  - All other chunks must intersect with the range [addr,addr+size); areas outside the intersection are marked as NOACCESS, as though they had been independently freed with `VALGRIND_MEMPOOL_FREE`.

  This is a somewhat rare request, but can be useful in implementing the type of mass-free operations common in custom LIFO allocators.

- `VALGRIND_MOVE_MEMPOOL(poolA, poolB)`: This request informs Memcheck that the pool previously anchored at address "poolA" has moved to anchor address "poolB". This is a rare request, typically only needed if you realloc() the header of a mempool.

  No memory-status bits are altered by this request.

- `VALGRIND_MEMPOOL_CHANGE(pool, addrA, addrB, size)`: This request informs Memcheck that the chunk previously allocated at address "addrA" within "pool" has been moved and/or resized, and should be changed to cover the region [addrB,addrB+size). This is a rare request, typically only needed if you realloc() a superblock or wish to extend a chunk without changing its memory-status bits.

  No memory-status bits are altered by this request.

- `VALGRIND_MEMPOOL_EXISTS(pool)`: This request informs the caller whether or not Memcheck is currently tracking a mempool at anchor address "pool". It evaluates to 1 when there is a mempool associated with that address, 0 otherwise. This is a rare request, only useful in circumstances when client code might have lost track of the set of active mempools.

## 4.8. Debugging MPI Parallel Programs with Valgrind

Valgrind supports debugging of distributed-memory applications which use the MPI message passing standard. This support consists of a library of wrapper functions for the `PMPI_*` interface. When incorporated into the application's address space, either by direct linking or by `LD_PRELOAD`, the wrappers intercept calls to `PMPI_Send`, `PMPI_Recv`, etc. They then use client requests to inform Valgrind of memory state changes caused by the function being wrapped. This reduces the number of false positives that Memcheck otherwise typically reports for MPI applications.

The wrappers also take the opportunity to carefully check size and definedness of buffers passed as arguments to MPI functions, hence detecting errors such as passing undefined data to `PMPI_Send`, or receiving data into a buffer which is too small.

Unlike most of the rest of Valgrind, the wrapper library is subject to a BSD-style license, so you can link it into any code base you like. See the top of `auxprogs/libmpiwrap.c` for license details.

### 4.8.1. Building and installing the wrappers

The wrapper library will be built automatically if possible. Valgrind's configure script will look for a suitable `mpicc` to build it with. This must be the same `mpicc` you use to build the MPI application you want to debug. By default, Valgrind tries `mpicc`, but you can specify a different one by using the configure-time flag `--with-mpicc=`. Currently the wrappers are only buildable with `mpicc`s which are based on GNU `gcc` or Intel's `icc`.

Check that the configure script prints a line like this:

    checking for usable MPI2-compliant mpicc and mpi.h... yes, mpicc

If it says `... no`, your `mpicc` has failed to compile and link a test MPI2 program.

If the configure test succeeds, continue in the usual way with `make` and `make install`. The final install tree should then contain `libmpiwrap.so`.

Compile up a test MPI program (eg, MPI hello-world) and try this:

    LD_PRELOAD=$prefix/lib/valgrind/<platform>/libmpiwrap.so   \
               mpirun [args] $prefix/bin/valgrind ./hello

You should see something similar to the following

    valgrind MPI wrappers 31901: Active for pid 31901
    valgrind MPI wrappers 31901: Try MPIWRAP_DEBUG=help for possible options

repeated for every process in the group. If you do not see these, there is an build/installation problem of some kind.

The MPI functions to be wrapped are assumed to be in an ELF shared object with soname matching `libmpi.so*`. This is known to be correct at least for Open MPI and Quadrics MPI, and can easily be changed if required.

### 4.8.2. Getting started

Compile your MPI application as usual, taking care to link it using the same `mpicc` that your Valgrind build was configured with.

Use the following basic scheme to run your application on Valgrind with the wrappers engaged:

    MPIWRAP_DEBUG=[wrapper-args]                                  \
       LD_PRELOAD=$prefix/lib/valgrind/<platform>/libmpiwrap.so   \
       mpirun [mpirun-args]                                       \
       $prefix/bin/valgrind [valgrind-args]                       \
       [application] [app-args]

As an alternative to `LD_PRELOAD`ing `libmpiwrap.so`, you can simply link it to your application if desired. This should not disturb native behaviour of your application in any way.

### 4.8.3. Controlling the wrapper library

Environment variable `MPIWRAP_DEBUG` is consulted at startup. The default behaviour is to print a starting banner

    valgrind MPI wrappers 16386: Active for pid 16386
    valgrind MPI wrappers 16386: Try MPIWRAP_DEBUG=help for possible options

and then be relatively quiet.

You can give a list of comma-separated options in `MPIWRAP_DEBUG`. These are

- `verbose`: show entries/exits of all wrappers. Also show extra debugging info, such as the status of outstanding `MPI_Request`s resulting from uncompleted `MPI_Irecv`s.

- `quiet`: opposite of `verbose`, only print anything when the wrappers want to report a detected programming error, or in case of catastrophic failure of the wrappers.

- `warn`: by default, functions which lack proper wrappers are not commented on, just silently ignored. This causes a warning to be printed for each unwrapped function used, up to a maximum of three warnings per function.

- `strict`: print an error message and abort the program if a function lacking a wrapper is used.

If you want to use Valgrind's XML output facility (`--xml=yes`), you should pass `quiet` in `MPIWRAP_DEBUG` so as to get rid of any extraneous printing from the wrappers.

### 4.8.4. Abilities and limitations

#### 4.8.4.1. Functions

All MPI2 functions except `MPI_Wtick`, `MPI_Wtime` and `MPI_Pcontrol` have wrappers. The first two are not wrapped because they return a `double`, and Valgrind's function-wrap mechanism cannot handle that (it could easily enough be extended to). `MPI_Pcontrol` cannot be wrapped as it has variable arity: `int MPI_Pcontrol(const int level, ...)`

Most functions are wrapped with a default wrapper which does nothing except complain or abort if it is called, depending on settings in `MPIWRAP_DEBUG` listed above. The following functions have "real", do-something-useful wrappers:

    PMPI_Send PMPI_Bsend PMPI_Ssend PMPI_Rsend

    PMPI_Recv PMPI_Get_count

    PMPI_Isend PMPI_Ibsend PMPI_Issend PMPI_Irsend

    PMPI_Irecv
    PMPI_Wait PMPI_Waitall
    PMPI_Test PMPI_Testall

    PMPI_Iprobe PMPI_Probe

    PMPI_Cancel

    PMPI_Sendrecv

    PMPI_Type_commit PMPI_Type_free

    PMPI_Pack PMPI_Unpack

    PMPI_Bcast PMPI_Gather PMPI_Scatter PMPI_Alltoall
    PMPI_Reduce PMPI_Allreduce PMPI_Op_create

    PMPI_Comm_create PMPI_Comm_dup PMPI_Comm_free PMPI_Comm_rank PMPI_Comm_size

    PMPI_Error_string
    PMPI_Init PMPI_Initialized PMPI_Finalize

A few functions such as `PMPI_Address` are listed as `HAS_NO_WRAPPER`. They have no wrapper at all as there is nothing worth checking, and giving a no-op wrapper would reduce performance for no reason.

Note that the wrapper library itself can itself generate large numbers of calls to the MPI implementation, especially when walking complex types. The most common functions called are `PMPI_Extent`, `PMPI_Type_get_envelope`, `PMPI_Type_get_contents`, and `PMPI_Type_free`.

#### 4.8.4.2. Types

MPI-1.1 structured types are supported, and walked exactly. The currently supported combiners are `MPI_COMBINER_NAMED`, `MPI_COMBINER_CONTIGUOUS`, `MPI_COMBINER_VECTOR`, `MPI_COMBINER_HVECTOR` `MPI_COMBINER_INDEXED`, `MPI_COMBINER_HINDEXED` and `MPI_COMBINER_STRUCT`. This should cover all MPI-1.1 types. The mechanism (function `walk_type`) should extend easily to cover MPI2 combiners.

MPI defines some named structured types (`MPI_FLOAT_INT`, `MPI_DOUBLE_INT`, `MPI_LONG_INT`, `MPI_2INT`, `MPI_SHORT_INT`, `MPI_LONG_DOUBLE_INT`) which are pairs of some basic type and a C `int`. Unfortunately the MPI specification makes it impossible to look inside these types and see where the fields are. Therefore these wrappers assume the types are laid out as `struct { float val; int loc; }` (for `MPI_FLOAT_INT`), etc, and act accordingly. This appears to be correct at least for Open MPI 1.0.2 and for Quadrics MPI.

If `strict` is an option specified in `MPIWRAP_DEBUG`, the application will abort if an unhandled type is encountered. Otherwise, the application will print a warning message and continue.

Some effort is made to mark/check memory ranges corresponding to arrays of values in a single pass. This is important for performance since asking Valgrind to mark/check any range, no matter how small, carries quite a large constant cost. This optimisation is applied to arrays of primitive types (`double`, `float`, `int`, `long`, `long long`, `short`, `char`, and `long double` on platforms where `sizeof(long double) == 8`). For arrays of all other types, the wrappers handle each element individually and so there can be a very large performance cost.

### 4.8.5. Writing new wrappers

For the most part the wrappers are straightforward. The only significant complexity arises with nonblocking receives.

The issue is that `MPI_Irecv` states the recv buffer and returns immediately, giving a handle (`MPI_Request`) for the transaction. Later the user will have to poll for completion with `MPI_Wait` etc, and when the transaction completes successfully, the wrappers have to paint the recv buffer. But the recv buffer details are not presented to `MPI_Wait` -- only the handle is. The library therefore maintains a shadow table which associates uncompleted `MPI_Request`s with the corresponding buffer address/count/type. When an operation completes, the table is searched for the associated address/count/type info, and memory is marked accordingly.

Access to the table is guarded by a (POSIX pthreads) lock, so as to make the library thread-safe.

The table is allocated with `malloc` and never `free`d, so it will show up in leak checks.

Writing new wrappers should be fairly easy. The source file is `auxprogs/libmpiwrap.c`. If possible, find an existing wrapper for a function of similar behaviour to the one you want to wrap, and use it as a starting point. The wrappers are organised in sections in the same order as the MPI 1.1 spec, to aid navigation. When adding a wrapper, remember to comment out the definition of the default wrapper in the long list of defaults at the bottom of the file (do not remove it, just comment it out).

### 4.8.6. What to expect when using the wrappers

The wrappers should reduce Memcheck's false-error rate on MPI applications. Because the wrapping is done at the MPI interface, there will still potentially be a large number of errors reported in the MPI implementation below the interface. The best you can do is try to suppress them.

You may also find that the input-side (buffer length/definedness) checks find errors in your MPI use, for example passing too short a buffer to `MPI_Recv`.

Functions which are not wrapped may increase the false error rate. A possible approach is to run with `MPI_DEBUG` containing `warn`. This will show you functions which lack proper wrappers but which are nevertheless used. You can then write wrappers for them.

A known source of potential false errors are the `PMPI_Reduce` family of functions, when using a custom (user-defined) reduction function. In a reduction operation, each node notionally sends data to a "central point" which uses the specified reduction function to merge the data items into a single item. Hence, in general, data is passed between nodes and fed to the reduction function, but the wrapper library cannot mark the transferred data as initialised before it is handed to the reduction function, because all that happens "inside" the `PMPI_Reduce` call. As a result you may see false positives reported in your reduction function.