### 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)

> Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

> 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;
         }