The GNU C Library - String Parameters

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.

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;
	}