@node Library version handling
@section Library version handling

The module @samp{check-version} can be useful when your gnulib
application is a system library.  You will typically wrap the call to
the @code{check_version} function through a library API, your library
header file may contain:

@example
#define STRINGPREP_VERSION "0.5.18"
...
  extern const char *stringprep_check_version (const char *req_version);
@end example

To avoid ELF symbol collisions with other libraries that use the
@samp{check-version} module, add to @file{config.h} through a
AC_DEFINE something like:

@example
AC_DEFINE(check_version, stringprep_check_version,
          [Rename check_version.])
@end example

The @code{stringprep_check_version} function will thus be implemented
by the @code{check_version} module.

There are two uses of the interface.  The first is a way to provide
for applications to find out the version number of the library it
uses.  The application may contain diagnostic code such as:

@example
  printf ("Stringprep version: header %s library %s",
          STRINGPREP_VERSION,
          stringprep_check_version (NULL));
@end example

Separating the library and header file version can be useful when
searching for version mismatch related problems.

The second uses is as a rudimentary test of proper library version, by
making sure the application get a library version that is the same, or
newer, than the header file used when building the application.  This
doesn't catch all problems, libraries may change backwards incompatibly
in later versions, but enable applications to require a certain
minimum version before it may proceed.

Typical uses look like:

@example
       /* Check version of libgcrypt. */
       if (!gcry_check_version (GCRYPT_VERSION))
         die ("version mismatch\n");
@end example