New AT_ flags for restricting pathname lookup

#define _GNU_SOURCE

#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/stat.h>

static char *cwd[] = {
    ".",
    NULL
};

int main(int argc, char **argv)
{
    DIR *dir;
    struct dirent *d_ent;
    struct stat st;
    int dirfd, rc;
    
    ++argv;
    if (!*argv) argv = cwd;
    do {
        dirfd = open(*argv, O_RDONLY, 0);
        if (dirfd == -1) {
            perror("open");
            continue;
        }
        
        dir = fdopendir(dirfd);
        if (!dir) {
            perror("fdopendir");
            continue;
        }

        printf("-----\nfiles in %s\n-----\n", *argv);
        while ((d_ent = readdir(dir))) {
            rc = fstatat(dirfd, d_ent->d_name, &, 0);
            if (rc == -1) {
                if (errno != ENOENT) perror("fstatat");
                continue;
            }

            if (S_ISREG(st.st_mode))
                printf("%s\t\t%zu bytes\n", d_ent->d_name, (size_t)st.st_size);
        }

        closedir(dir);
    } while (*++argv);

    return 0;
}

Also, chdir is basically unusable in multi-threaded processes as it changes the working directory of the process, ie, it affects all threads, not just the one executing it and, as seen by another thread, the cwd change is an unpredictable, asynchronously occuring event. Eg, a thread desiring to create two files in the same directory might end up creating them in different directories.

Lastly, the directory a process was started in might have been picked intentionally, eg, as location where core dumps should go to, and the process shouldn't change it except if there's a very good reason for that (and this should be documented).