How do I find a filename, given a FILE pointer? [duplicate]

Question

Possible Duplicate:
Getting Filename from file descriptor in C
Obtain filename from file pointer in C

I have a function here:

handle_file(FILE *file)
{
    if(condition)
    {
        print filename and other msg;
    }
}

The only thing we know here is the a pointer to the file; is it possible to get the filename according to the pointer?

Answer 1

Check out this answer to obtain file descriptor and this answer to get file name from file descriptor. Should be OK on Linux (not sure about other operating systems).

Here's a quick working example (tested under Cygwin/Win7):

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

int main()
{
    int MAXSIZE = 0xFFF;
    char proclnk[0xFFF];
    char filename[0xFFF];
    FILE *fp;
    int fno;
    ssize_t r;

    // test.txt created earlier
    fp = fopen("test.txt", "r");
    if (fp != NULL)
    {
        fno = fileno(fp);
        sprintf(proclnk, "/proc/self/fd/%d", fno);
        r = readlink(proclnk, filename, MAXSIZE);
        if (r < 0)
        {
            printf("failed to readlink\n");
            exit(1);
        }
        filename[r] = '\0';
        printf("fp -> fno -> filename: %p -> %d -> %s\n",
                fp, fno, filename);
    }
    return 0;
}

Output:

fp -> fno -> filename: 0x80010294 -> 3 -> /tmp/test.txt

Answer 2

This can be done in 2 stages. First, you will need to get the file descriptor, then you will need to recover the filename. The following is an example, but has some serious buffer overflow vulnerabilities!

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

char * recover_filename(FILE * f) {
  int fd;
  char fd_path[255];
  char * filename = malloc(255);
  ssize_t n;

  fd = fileno(f);
  sprintf(fd_path, "/proc/self/fd/%d", fd);
  n = readlink(fd_path, filename, 255);
  if (n < 0)
      return NULL;
  filename[n] = '\0';
  return filename;
}