Context manipulation

libcfg+: command line and configuration file parsing library
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# 2.2. Context manipulation

The parsing capabilities of libcfg+ are accessed through a context, which is defined by the CFG_CONTEXT structure. By using contexts, one can parse several configuration files and the command line at the same time.

The functions used for creating a new context are described below. Those used for freeing (destroying) a context are described at the end of this page.

    #include <cfg+.h>

``CFG_CONTEXT cfg_get_context`(struct cfg_option *options);`

``CFG_CONTEXT cfg_get_cmdline_context`(long begin_pos, long size, char **argv, struct cfg_option options);`

``CFG_CONTEXT cfg_get_cfgfile_context`(long begin_pos, long size, char *filename, struct cfg_option options);`

``void cfg_set_cmdline_context`(CFG_CONTEXT con, long begin_pos, long size, char **argv);`

``void cfg_set_cfgfile_context`(CFG_CONTEXT con, long begin_pos, long size, char *filename);`

The function `cfg_get_context()` initializes a context but does not associate it with either the command line or a file. It takes a pointer to the option set `*options` and returns a new context. The association with the command line or a file can be done with `cfg_set_cmdline_context()` and `cfg_set_cfgline_context()` respectively.

The function `cfg_get_cmdline_context()` is used for full context initialization, which includes the initialization itself and assigning the newly created context to the command line. The `begin_pos` parameter stores the starting parsing position in the array of command line arguments. The `size` parameter defines the number of arguments (array elements) to parse. If a negative value is passed, the parsing will stop only when an `NULL` is encountered. The `argv` parameter is a pointer to a `NULL` terminated array of strings (options + arguments) to be parsed. As most of you probably know, `argv` is the second parameter passed to the `main()` function.

The function `cfg_get_cfgfile_function()` initializes a new context and associates it with a configuration file. The parameter `begin_pos` determines the file offset at which the parsing will start. Whether the offset is expressed in bytes or lines depends on the value of the context flag `CFG_FILE_LINE_POS_USAGE`. If the flag is unset (the default value), `begin_pos` should be interpreted as a byte offset. If the flag is set, `begin_pos` is a line offset. See the Context flags section for more information about this. Whatever the case, the offset is zero based. The `size` parameter defines the number of bytes or lines to parse. If the value is negative, the file will be parsed in its entirety. The name of the file to parse is passed in `filename`.

`cfg_set_cmdline_context()` and `cfg_set_cfgfile_context()` work on an already existing context. These functions accept the existing context in their first parameter. The rest of their arguments are the same as those of `cfg_get_cfgfile_function()` and `cfg_get_cmdline_context()`. Note that the two sets of functions differ in their return values. The context setting functions return void while the context creating functions return a new context. Working with existing contexts may be more convenient than creating new contexts from scratch, especially when there is a need to switch from parsing a file to parsing the command line and vice versa.

Note, that while `begin_pos` of the file context defines the starting parsing offset in a file, the same parameter for the command line context specifies the offset in the `argv` array. Note also that all offsets are zero-based. For example, the third line of a file is at offset `2`. However, the error handling routines of libcfg+ increment all offsets by one when reporting an error, thus human readable messages are returned. So for the first position on command line it will print something like `on position 1`, for second line in configuration file it will print `on line 2` and so on.

Starting from version 0.5.2, the `cfg_get_cmdline_context_argc()` and `cfg_set_cmdline_context_argc()` functions became available. It creates a new context by passing the `argc` and `argv` parameters directly from `main()` function. Its parameters are similar to those of `cfg_get_cmdline_context()`, but instead of `begin_pos` and `size`, only `argc` is present. Whether one is more convenient than the other depends on the circumstances and it is for you to decide which one suits you best.

There are also two groups of macros for managing contexts. The first group includes `cfg_get_cmdline_context_pos()` and `cfg_get_cfgfile_context_pos()` and is used for creating a new context of the needed type. You can use the second group, which consist of `cfg_set_cmdline_context_pos()` and `cfg_set_cfgfile_context_pos()`, for manipulating an existing context. The difference between these four macros and their function counterparts is in the `size` parameter, which is replaced by `end_pos` and specifies the offset of the last element to be parsed. For a file context the value is the byte offset (or line number) while for a command line context it is the offset in the `argv` array.

The following functions are used for reseting and freeing a context.

    #include <cfg+.h>

``void cfg_reset_context`(CFG_CONTEXT con);`

``void cfg_free_context`(CFG_CONTEXT con);`

You may come across situations when a context reset is necessary. You can use `cfg_reset_context()` to perform this task. When this function is called on a command line context, the offset in the `argv` is set to the value of `begin_pos`. For a file context, the offset is reset to the value of `begin_pos`. Context flags and context properties remain unchanged after a call to `cfg_reset_context()`.

After you are done with a context, you should use `cfg_free_context()` to free the resources associated with it. The function takes a context in its only parameter `con`. The function does not deallocate the memory for any arguments or argument arrays you might have allocated in your option set. You need to free that memory manually. However, it will free all internal context structures, which includes the context flags, context properties, and the context itself.

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