// Retrieves the current reading position in a file of any process on linux

Rarely I cross-post messages but for me this information was like a revelation so … have patience.

Question

How to get the current reading position in a file of a process on your machine ?

This information can be useful in an avalanche of circumstances

  • you are importing 10G dump.sql into your new database
  • a friend is downloading your last recorded song (copyright free, obviously ;-) ) from your home web-server and you want to know how many bytes you must still send to him before you can turn off the laptop
  • how long you must wait before a grep command finishes its work

Right Answer

Use cat /proc/<process pid>/fdinfo/<file_handle>

Example

For a demonstration keep this micro-program readdy.c and compile it:

// compile me with gcc -o readdy readdy.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    int fh, ret = 1;
    char bf[64];

    if ((fh = open(argv[1], O_RDONLY)) < 0)
        exit(1);

    ret = 1;
    while(ret) {
        fgetc(stdin);
        ret = read(fh, bf, 64);
    }

    close(fh);
    
    exit (0);
    return (0);
}

Run as

$ ./readdy readdy.c

and in another console find its pid with ps.

$ ps ax | grep readdy.c 

on my box I found:

10352 pts/5    S+     0:00 ./readdy readdy.c

I want to investigate about process file handlers with the command

$ ls -l /proc/10352/fd

obtaining:

total 0
lrwx------ 1 nastasi nastasi 64 Jan 28 14:53 0 -> /dev/pts/5
lrwx------ 1 nastasi nastasi 64 Jan 28 14:53 1 -> /dev/pts/5
lrwx------ 1 nastasi nastasi 64 Jan 28 14:50 2 -> /dev/pts/5
lr-x------ 1 nastasi nastasi 64 Jan 28 14:53 3 -> /home/nastasi/src/readdy/readdy.c

at this point I can retrieve the current read position of the file readdy.c with:

$ cat /proc/10352/fdinfo/3

and obtain:

pos:	0
flags:	0100000

return on the console with ./readdy running and press enter key only one time

on the other console type again

$ cat /proc/10352/fdinfo/3

and this time obtain:

pos:	64
flags:	0100000

and so on.

If you have root permissions you can investigate each file opened by each process of your machine.

Simple and powerful.

Wrong Answers

Before this 'discover' I tried 'inotify' file system inquiry software interface but the maximum information level was “someone access the file” without any info about position.

Another tentative was with strace but no info about position are associated with read system call.

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