2.2 为路径名动态分配空间

lib/pathalloc.c

#include "apue.h"
#include <errno.h>
#include <limits.h>
#ifdef	PATH_MAX
static int	pathmax = PATH_MAX;
#else
static int	pathmax = 0;
#endif
#define SUSV3	200112L
static long	posix_version = 0;
/* If PATH_MAX is indeterminate, no guarantee this is adequate */
#define	PATH_MAX_GUESS	1024
char *
path_alloc(int *sizep) /* also return allocated size, if nonnull */
{
	char	*ptr;
	int	size;
	if (posix_version == 0)
		posix_version = sysconf(_SC_VERSION);
	if (pathmax == 0) {		/* first time through */
		errno = 0;
		if ((pathmax = pathconf("/", _PC_PATH_MAX)) < 0) {
			if (errno == 0)
				pathmax = PATH_MAX_GUESS;	/* it's indeterminate */
			else
				err_sys("pathconf error for _PC_PATH_MAX");
		} else {
			pathmax++;		/* add one since it's relative to root */
		}
	}
	if (posix_version < SUSV3)
		size = pathmax + 1;
	else
		size = pathmax;
	if ((ptr = malloc(size)) == NULL)
		err_sys("malloc error for pathname");
	if (sizep != NULL)
		*sizep = size;
	return(ptr);
}