Question

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Getting Filename from file descriptor in C

Is there a simple and (reasonably) portable way of getting the filename from a FILE*?

I open a file using f = fopen(filename, ...) and then pass down f to various other functions, some of which may report an error. I’d like to report the filename in the error message but avoid having to pass around the extra parameter.

I could create a custom wrapper struct { FILE *f, const char *name }, but is there perhaps a simpler way? (If the FILE* wasn’t opened using fopen I don’t care about the result.)

Answer 1

On some platforms (such as Linux), you may be able to fetch it by reading the link of /proc/self/fd/<number>, as so:

#include <stdio.h>
#include <unistd.h>
#include <string.h>

int main(void)
{
    char path[1024];
    char result[1024];

    /* Open a file, get the file descriptor. */
    FILE *f = fopen("/etc/passwd", "r");
    int fd = fileno(f); 

    /* Read out the link to our file descriptor. */
    sprintf(path, "/proc/self/fd/%d", fd);
    memset(result, 0, sizeof(result));
    readlink(path, result, sizeof(result)-1);

    /* Print the result. */
    printf("%s\n", result);
}

This will, on my system, print out /etc/passwd, as desired.