Unix file descriptor

Question

Today I found very interesting behavior of file descriptors in Linux. Look at that code:

#include <dirent.h>     /* Defines DT_* constants */
#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <errno.h>


#define handle_error(msg) \
do { trace(msg); exit(0); } while (0)
#define trace printf

int createFile(const char* name) {
    int r;
    r = ::open( name, 0 );
    if (r < 0)
    {
        trace("create file : %s\n", name);
        r = ::open( name, O_CREAT, 0666 );
        if (r < 0)
            trace("error r < 0 %d\n",errno);
    }
    return r;
}

int createDir(const char* name) {
    int r = ::mkdir( name, 0777 );
    if (r != 0) {
        trace("error r!=0\n");
    }
    r = open(name, 0);
    if (r < 0) {
        trace("error create dir r <0\n");
    }
    return r;
}

struct linux_dirent {
    long           d_ino;
    off_t          d_off;
    unsigned short d_reclen;
    char           d_name[];
};

#include <sys/types.h>
#include <dirent.h>
void test123(int fd) {
    int nread;
    char buf[1024];
    unsigned char buffer[1024];
    struct linux_dirent *d;
    int bpos,r;
    char d_type;


    if (fd == -1)
        handle_error("open");

    for ( ; ; ) {
        nread = syscall(SYS_getdents, fd, buf, 1024);
        if (nread == -1)
            handle_error("getdents");

        if (nread == 0)
            break;

        trace("--------------- nread=%d ---------------\n", nread);
        trace("i-node#  file type  d_reclen  d_off   d_name\n");
        for (bpos = 0; bpos < nread;) {
            d = (struct linux_dirent *) (buf + bpos);
            trace("%8ld  ", d->d_ino);
            d_type = *(buf + bpos + d->d_reclen - 1);
            trace("%4d %10lld  %s\n", d->d_reclen,
                  (long long) d->d_off, d->d_name);
            bpos += d->d_reclen;
        }
    }
}


int main(int argc, const char * argv[]) {



    int dir = createDir("test");

    int file = createFile("test/file.gg");

    test123(dir);
    close(dir);
    close(file);


    return 0;
}

in that code I create folder, save its file descriptor, create file in that folder and after I want to print all files in that directory via file descriptors. However I get this output:

create file : test/file.gg
--------------- nread=32 ---------------
i-node#  file type  d_reclen  d_off   d_name
   48879    16          1  .
   48880    16          2  ..

There is no file.gg file in that folder. So, my question is - how it can be and how to work correctly with file descriptors? As I understand file descriptor is just an index in local for process table with all opened files and directories. But it is looks like that folder descriptor caches somehow files in that folder. How to work correctly with descriptors in my case?

Answer 1

Try to do an fsync on your directory. You should open directory with O_RDONLY flags. O_WRONLY will fail. Create a file and sync may not sync metadata for this file. More informations in this article