13-01-17-->2

POSIX是可移植的操作系统接口(Portable Operating System Interface).该标准的目的是提高应该程序在各种UNIX系统之间的可移植性。

2-3：为路径名动态分配空间

#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);
}

：如若在<limits.h>中定义了常量PATH_MAX,就没有任何问题；如果没有定义，则需要调用pathconf。如果pathconf指明PATH_MAX是不确定的，那么只能猜测这个值。