Can a program call fflush() on the same FILE* concurrently in C?

In C, FILE* is a pointer used to represent a file stream, and the fflush() function is used to flush the buffer of an output or update stream, writing the buffered data to the underlying file.

Theoretically, calling fflush() multiple times on the same FILE* is feasible, but in practice, it may introduce race conditions, especially in multithreaded environments.

Race Condition

When multiple threads or processes attempt to modify the same data concurrently, the final output depends on thread scheduling and execution order, which is known as a race condition. Without synchronization mechanisms, multiple threads may concurrently write to the same file stream, leading to data corruption or program crashes.

Solution

To safely use FILE* in multithreaded contexts, implement appropriate synchronization mechanisms such as mutexes to prevent race conditions. For example, acquire the mutex before calling fflush() and release it afterward.

Example

Assume we have a log file that multiple threads need to write to. Ensure that the file stream is not interrupted by other threads during fflush() calls.

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

FILE *log_file;
pthread_mutex_t lock;

void log_message(const char *message) {
    pthread_mutex_lock(&lock);
    fprintf(log_file, "%s\n", message);
    fflush(log_file);
    pthread_mutex_unlock(&lock);
}

int main() {
    pthread_mutex_init(&lock, NULL);
    log_file = fopen("log.txt", "a");

    if (log_file == NULL) {
        perror("Failed to open file");
        return 1;
    }

    // Example: multithreaded writing
    // Threads can be created to call log_message

    fclose(log_file);
    pthread_mutex_destroy(&lock);
    return 0;
}

In this example, we use a mutex to ensure that when one thread executes fflush(), no other thread can write to the file stream. This enables safe usage of FILE* and fflush() in multithreaded environments.

In conclusion, although calling fflush() multiple times on the same FILE* is possible, it requires caution in multithreaded contexts and appropriate synchronization to maintain data consistency and program stability.