C program in Linux to read a file descriptor passed as a terminal argument

Question

I've been looking into different discussions how to get the open file descriptors for a current process on Linux from ac program, but could not find the following example:

./sample 4</some_file 5<some_other_file

Is there any way to get those file descriptors in a sample.c program in this case. I found out that those redirections are not treated as command line arguments. Hope someone can help.

Ofcourse fds 4 and 5 are given just as example, I would like the program to find out which fds were open on execution.

Answer 1

Given sample is started by

./sample 4</some_file 5<some_other_file

this will provide file descriptors that can be used to access those files:

int fd_for_some_file = 4;
int fd_for_some_other_file = 5;

If you don't want to assume file descriptors are fixed values, don't assign the files to hardcoded descriptor values when you start your process.

Edit:

I would like the program to find out which fds were open on execution.

In general, I don't think you can.

If, however, your code to identify pre-opened file descriptors runs before any invocation of open , you may be able to just run through values greater than 2 to see what they are using OS-specific means. On Linux:

for ( int fd = 3; fd < fd_max; fd++ )
{
    sprintf( linkname, "/proc/self/fd/%d", fd );
    int rc = readlink( linkname, linkvalue, sizeof( linkvalue ) ); 
    if ( rc == 0 )
    {
         // found a file opened by calling process
    }
}

Yes, that's inherently racy for multithreaded programs. If you're using GCC, you can put the code in a function with __attribute__(( constructor )) and it will run before main() is called. But even that could identify files opened by other such functions as being passed by the parent process.