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#include <stdlib.h>
#include <stdio.h>
#include <limits.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
char *realpath(const char *restrict filename, char *restrict resolved)
{
int fd;
ssize_t r;
struct stat st1, st2;
char buf[15+3*sizeof(int)];
int alloc = 0;
if (!filename) {
errno = EINVAL;
return 0;
}
fd = open(filename, O_RDONLY|O_NONBLOCK|O_CLOEXEC);
if (fd < 0) return 0;
snprintf(buf, sizeof buf, "/proc/self/fd/%d", fd);
if (!resolved) {
alloc = 1;
resolved = malloc(PATH_MAX);
if (!resolved) return 0;
}
r = readlink(buf, resolved, PATH_MAX-1);
if (r < 0) goto err;
resolved[r] = 0;
fstat(fd, &st1);
r = stat(resolved, &st2);
if (r<0 || st1.st_dev != st2.st_dev || st1.st_ino != st2.st_ino) {
if (!r) errno = ELOOP;
goto err;
}
close(fd);
return resolved;
err:
if (alloc) free(resolved);
close(fd);
return 0;
}
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