GNU C Library (libc) Programming Guide

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31.12 String-Valued Parameters

POSIX.2 defines a way to get string-valued parameters from the operating system with the function confstr:

— Function: size_t confstr (int parameter, char *buf, size_t len)

This function reads the value of a string-valued system parameter, storing the string into len bytes of memory space starting at buf. The parameter argument should be one of the `_CS_' symbols listed below.
The normal return value from confstr is the length of the string value that you asked for. If you supply a null pointer for buf, then confstr does not try to store the string; it just returns its length. A value of 0 indicates an error.
If the string you asked for is too long for the buffer (that is, longer than len - 1), then confstr stores just that much (leaving room for the terminating null character). You can tell that this has happened because confstr returns a value greater than or equal to len.
The following errno error conditions are defined for this function:

EINVAL
The value of the parameter is invalid.

Currently there is just one parameter you can read with confstr:

_CS_PATH: This parameter's value is the recommended default path for searching for executable files. This is the path that a user has by default just after logging in.
_CS_LFS_CFLAGS: The returned string specifies which additional flags must be given to the C compiler if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS_LDFLAGS: The returned string specifies which additional flags must be given to the linker if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS_LIBS: The returned string specifies which additional libraries must be linked to the application if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS_LINTFLAGS: The returned string specifies which additional flags must be given to the lint tool if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS64_CFLAGS: The returned string specifies which additional flags must be given to the C compiler if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS64_LDFLAGS: The returned string specifies which additional flags must be given to the linker if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS64_LIBS: The returned string specifies which additional libraries must be linked to the application if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS64_LINTFLAGS: The returned string specifies which additional flags must be given to the lint tool if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.

The way to use confstr without any arbitrary limit on string size is to call it twice: first call it to get the length, allocate the buffer accordingly, and then call confstr again to fill the buffer, like this:

     char *
     get_default_path (void)
     {
       size_t len = confstr (_CS_PATH, NULL, 0);
       char *buffer = (char *) xmalloc (len);
     
       if (confstr (_CS_PATH, buf, len + 1) == 0)
         {
           free (buffer);
           return NULL;
         }
     
       return buffer;
     }