symlink.c: an example of using O_PATH to read symlink
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	kib
	Sun, Feb 8, 1:42 AM

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symlink.c: an example of using O_PATH to read symlink
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	/* $Id: symlink.c,v 1.7 2026/02/08 01:42:12 kostik Exp kostik $ */

	#include <sys/stat.h>
	#include <err.h>
	#include <fcntl.h>
	#include <stdio.h>
	#include <unistd.h>

	static void
	onelink(const char *name)
	{
	struct stat st;
	long plen;
	ssize_t llen;
	int fd;

	fd = open(name, O_PATH \| O_NOFOLLOW);
	if (fd == -1) {
	warn("open \"%s\"", name);
	return;
	}
	if (fstatat(fd, "", &st, AT_EMPTY_PATH \| AT_SYMLINK_NOFOLLOW) == -1) {
	warn("fstatat \"%s\"", name);
	goto ret;
	}
	if (!S_ISLNK(st.st_mode)) {
	warnx("%s: not a symlink", name);
	goto ret;
	}
	#if defined __FreeBSD__
	plen = fpathconf(fd, _PC_SYMLINK_MAX);
	if (plen == -1) {
	warn("fpathconf \"%s\"", name);
	goto ret;
	}
	#elif defined __linux__
	/* On Linux, fpathconf() does not work on O_PATH-fd. */
	plen = 2048;
	#endif

	{
	char path[plen + 1];

	llen = readlinkat(fd, "", path, plen);
	if (llen == -1) {
	warn("readlinkat \"%s\"", name);
	goto ret;
	}
	path[llen] = '\0';
	printf("%s => %s\n", name, path);
	}

	ret:
	close(fd);
	}

	int
	main(int argc, char *argv[])
	{
	for (int i = 1; i < argc; i++)
	onelink(argv[i]);
	}

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