Getting file path from FILE pointer in C

barisdemiray · 10-21-2004, 02:53 AM

Hello,
i've quickly searched the archives but found nothing. Do you know a way of getting file name from FILE pointer? I'll use tmpfile() but it does return file pointer only (i know i can use tmpnam() and open the file at the returned path but i'm wondering whether there is a way of doing this). Thanks.

cppkid · 10-21-2004, 05:45 AM

Hi barisdemiray!
Well from my concepts of what filing is, i don't thing that there is a simple way to get the file path from a file pointer, coz the file pointer or stack has no information about the physical path of the file, so you better use the way you use the way you suggested yourself.
Tayyab

Hko · 10-21-2004, 08:54 AM

Quote:

Originally posted by barisdemiray
Do you know a way of getting file name from FILE pointer? I'll use tmpfile() but it does return file pointer only

This is possible, but there two things to keep in mind:

When using tmpfile(), you get an open FILE* to the file, but it also gets unlinked (deleted) by tmpfile() immediately after opening it. So while your process has it still open and it can read and write the file as long as it does not fclose() it, you will not see it with "ls -la /tmp". When your process closes the file, it's gone automatically (see "man tmpfile"). Maybe this is why you are asking this in the first place, because you don't see any file in /tmp?
The only possible way (i know of) to get the full path is sort of a workaround. So it's probably better to use tempnam() or tmpnam().
Geting the file's path name is probably not of any use, since it is already gone.... (except for the program's process itself, which has no need for the name, as it already has the file open)

Anyways, here's how it can be done:
(note the "(deleted)" in the output of this program)

Code:

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

#define MAXLEN 200

int main(int argc, char **argv)
{
    FILE *file;
    int fd;
    char procpath[MAXLEN + 1];
    char filepath[MAXLEN + 1];

    /*
     * Open a temp-file 
     */
    file = tmpfile();
    if (file == NULL) {
        perror(*argv);
        return 1;
    }

    /*
     * Get the low-level file descriptor of the open file 
     */
    fd = fileno(file);
    if (fd < 0) {
        perror(*argv);
        fclose(file);
        return 1;
    }

    /*
     * Construct a string with the /proc path of the file
     * descriptor (which is a symbolic link to the real
     * file).
     */
    snprintf(procpath, MAXLEN, "/proc/self/fd/%d", fd);

    /*
     * Get the path the symlink is pointing to.
     */
    if (readlink(procpath, filepath, (size_t) MAXLEN) < 0) {
        perror(*argv);
        fclose(file);
        return 1;
    }

    /* Output the full path of the temp-file opened by tmpfile() */
    printf("The temp file is: %s\n", filepath);

    return 0;
}

barisdemiray · 10-22-2004, 12:58 AM

Thanks Hko and cppkid! I've written my own tmpfile() function that returns a file name and it uses the outputs of time(NULL), getpid(), random() (but still it worths to read Hko's code and learn something more ;-)) as this:

Code:

/**
 * Returns a random file name
 * @return A temporary file name
 */
string Config::getTempFileName()
{
        FILE *tmpFile;
        stringstream tmpFilePath;

create_again:
        srand(time(NULL));
        tmpFilePath.str("");
        tmpFilePath  << "/tmp/lbr-lb-" << random() << "-" << random()
                     << "-" << getpid() << "-" << time(NULL)%1000;

        /*
         * check the file's existency
         */
        if ( (tmpFile = fopen(tmpFilePath.str().c_str(), "r")) )        {
                fclose(tmpFile);
                goto create_again;
        }

        return tmpFilePath.str();
}

But, there would be a race condition between the time of check in this file (file exists or not) and the time of opening the file at the caller. May be returning a file pointer and placing the file name in a formal parameter may help. Anyway, thanks guys!