How to check if a file is already opened in C

问题

I am working on a multithreaded system where a file can be shared among different threads based on the file access permissions . How can I check if file is already opened by another thread. Thanks in advance

回答1:

To find out if a named file is already opened on linux, you can scan the /proc/self/fd directory to see if the file is associated with a file descriptor. The program below sketches out a solution:

DIR *d = opendir("/proc/self/fd");
if (d) {
    struct dirent *entry;
    struct dirent *result;

    entry = malloc(sizeof(struct dirent) + NAME_MAX + 1);
    result = 0;
    while (readdir_r(d, entry, &result) == 0) {
        if (result == 0) break;
        if (isdigit(result->d_name[0])) {
            char path[NAME_MAX+1];
            char buf[NAME_MAX+1];
            snprintf(path, sizeof(path), "/proc/self/fd/%s",
                     result->d_name);
            ssize_t bytes = readlink(path, buf, sizeof(buf));
            buf[bytes] = '\0';
            if (strcmp(file_of_interest, buf) == 0) break;
        }
    }
    free(entry);
    closedir(d);
    if (result) return FILE_IS_FOUND;
}
return FILE_IS_NOT_FOUND;

From your comment, it seems what you want to do is to retrieve an existing FILE * if one has already been created by a previous call to fopen() on the file. There is no mechanism provided by the standard C library to iterate through all currently opened FILE *. If there was such a mechanism, you could derive its file descriptor with fileno(), and then query /proc/self/fd/# with readlink() as shown above.

This means you will need to use a data structure to manage your open FILE *s. Probably a hash table using the file name as the key would be the most useful for you.

回答2:

You can use int flock(int fd, int operation); to mark a file as locked and also to check if it is locked.

   Apply or remove an advisory lock on the open file specified by fd.
   The argument operation is one of the following:

       LOCK_SH  Place a shared lock.  More than one process may hold a
                shared lock for a given file at a given time.

       LOCK_EX  Place an exclusive lock.  Only one process may hold an
                exclusive lock for a given file at a given time.

       LOCK_UN  Remove an existing lock held by this process.

flock should work in a threaded app if you open the file separately in each thread: multiple threads able to get flock at the same time

There's more information about flock and it's potential weaknesses here:

回答3:

I don't know much in the way of multithreading on Windows, but you have a lot of options if you're on Linux. Here is a FANTASTIC resource. You might also take advantage of any file-locking features offered inherently or explicitly by the OS (ex: fcntl). More on Linux locks here. Creating and manually managing your own mutexes offers you more flexibility than you would otherwise have. user814064's comment about flock() looks like the perfect solution, but it never hurts to have options!

Added a code example:

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

FILE *fp;
int counter;
pthread_mutex_t fmutex = PTHREAD_MUTEX_INITIALIZER;

void *foo() {
        // pthread_mutex_trylock() checks if the mutex is
        // locked without blocking
        //int busy = pthread_mutex_trylock(&fmutex);

        // this blocks until the lock is released
        pthread_mutex_lock(&fmutex);
        fprintf(fp, "counter = %d\n", counter);
        printf("counter = %d\n", counter);
        counter++;
        pthread_mutex_unlock(&fmutex);
}

int main() {

        counter = 0;
        fp = fopen("threads.txt", "w");

        pthread_t thread1, thread2;

        if (pthread_create(&thread1, NULL, &foo, NULL))
                printf("Error creating thread 1");
        if (pthread_create(&thread2, NULL, &foo, NULL))
                printf("Error creating thread 2");

        pthread_join(thread1, NULL);
        pthread_join(thread2, NULL);

        fclose(fp);
        return 0;
}

回答4:

If you tend to do it in shell, you can simply use lsof $filename.

来源：https://stackoverflow.com/questions/18648291/how-to-check-if-a-file-is-already-opened-in-c

标签

Linux