[SOLVED] verify open file descriptor with lsof

verify open file descriptor with lsof

I think that my program leaves one open file descriptor.

int main()
{
    int my_fd = dup(STDOUT_FILENO);
    char *cmd[2];

    cmd[0] = "/bin/ls";
    cmd[1] = NULL;

    dprintf(2, "pid = %d\n", getpid());
    if (fork() != 0)
    {
        // close(my_fd);
        waitpid(-1, NULL, WUNTRACED);
    }
    else
    {
        dup2 (my_fd, STDERR_FILENO);
        // close(my_fd);
        execve(cmd[0], cmd, NULL);
    }
}

The output of the program is:

pid = 8718
myprog  myprog.c

I'd like to see this file descriptor with lsof or another command, I don't see it

I've tried to find similar questions on stackoverflow.

I've tried lsof -c myprog, it shows nothing.

I've tried lsof | grep myprog, it shows :

bash      7020                              an  cwd       DIR                9,0       4096    7082347 /mnt/md0/42/myprog
cpptools  7393                              an  cwd       DIR                9,0       4096    7082347 /mnt/md0/42/myprog
cpptools  7393 7395 cpptools                an  cwd       DIR                9,0       4096    7082347 /mnt/md0/42/myprog
cpptools  7393 7396 cpptools                an  cwd       DIR                9,0       4096    7082347 /mnt/md0/42/myprog
cpptools  7393 7397 cpptools                an  cwd       DIR                9,0       4096    7082347 /mnt/md0/42/myprog
cpptools  7393 7398 cpptools                an  cwd       DIR                9,0       4096    7082347 /mnt/md0/42/myprog
...
etc

I've tried ls -l /proc/$$/fd, it shows

lrwx------ 1 an an 64 oct.  29 23:00 0 -> /dev/pts/0
lrwx------ 1 an an 64 oct.  29 23:00 1 -> /dev/pts/0
lr-x------ 1 an an 64 oct.  29 23:00 103 -> /snap/code/143/usr/share/code/v8_context_snapshot.bin
lrwx------ 1 an an 64 oct.  29 23:00 2 -> /dev/pts/0
lrwx------ 1 an an 64 oct.  29 23:00 255 -> /dev/pts/0
l-wx------ 1 an an 64 oct.  29 23:00 36 -> /home/an/.config/Code/logs/20231029T225917/ptyhost.log
lrwx------ 1 an an 64 oct.  29 23:00 37 -> 'socket:[69635]'
lr-x------ 1 an an 64 oct.  29 23:00 38 -> /snap/code/143/usr/share/code/resources/app/node_modules.asar

I've tried lsof -a -p $$, it shows:

COMMAND  PID USER   FD   TYPE             DEVICE SIZE/OFF    NODE NAME
bash    7020   an  cwd    DIR                9,0     4096 7082347 /mnt/md0/42/myprog
bash    7020   an  rtd    DIR               8,20     4096       2 /
bash    7020   an  txt    REG               8,20  1183448 5245518 /usr/bin/bash
bash    7020   an  mem    REG               8,20   183318 6030673 /usr/share/locale-langpack/fr/LC_MESSAGES/bash.mo
bash    7020   an  mem    REG               8,20    51856 5249542 /usr/lib/x86_64-linux-gnu/libnss_files-2.31.so
bash    7020   an  mem    REG                7,4   340640     187 /snap/code/143/usr/lib/locale/aa_DJ.utf8/LC_CTYPE
bash    7020   an  mem    REG                7,4  2586930     186 /snap/code/143/usr/lib/locale/aa_DJ.utf8/LC_COLLATE
...
etc

I've tried ls -l /proc/8718/fd, it answers:

ls: impossible d'accéder à '/proc/8718/fd': Aucun fichier ou dossier de ce type

Solution

Both your parent and child processes are exiting very quickly (i.e., before you have time to run any of the commands you tried). When a process exits¹, all of its remaining open file descriptors are automatically closed by the kernel. If you want to have time to see them open, pick a longer-running command, e.g.:

    char *cmd[3];

    cmd[0] = "/bin/sleep";
    cmd[1] = "60";
    cmd[2] = NULL;

^{¹: Shenanigans with CLONE_FILES can change this. A more technically correct statement would be "when all threads with a given file descriptor table exit, all of the remaining file descriptors in that table are closed."}