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stdoutThis article will demonstrate various methods on how to flush an output stream in C language .

Use function in C language fflushto refresh stdoutoutput stream

I/OThe C standard library provides a stdiobuffered version of the I/O operations that are performed in user space, which improves performance for common use cases. Generally speaking, accessing files and operating on files is provided by operating system services, so users eventually need to make system calls, such as opening a file. Frequent system calls can slow down the program because it requires accessing the operating system's kernel data structures and transferring control back and forth. Therefore, stdiothe C library maintains some buffers to handle input/output operations when using the function calls.

If the user needs to force a write to the kernel buffer, then fflushthe flush is required for the given stream provided by the _flush_flush_function. _flush_flush_flush_function fflushtakes a FILEsingle argument of _flush_flush_flush_function, a pointer to the given stream. Note that fflush_flush_flush_function forces a write to the output stream, while discarding any buffered data for the input stream (there is a seekable file). If the argument is _flush_flush_flush_function NULL, it will flush all open output streams.

Note that fflushthere is no guarantee that the written data is physically stored, as this requires the kernel buffer to be flushed (which can be fsyncdone using the call, see here ).

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

int main(int argc, char *argv[]) {
    char *username;
    size_t len;
    int lnmax = 256;

    username = malloc(lnmax);
    if (username == NULL)
        perror("malloc");

    printf("Username: ");
    fflush(stdout);
    if (fgets(username, lnmax, stdin) == NULL)
        exit(EXIT_FAILURE);

    printf("Your username is set to - %s", username);

    exit(EXIT_SUCCESS);
}

Output:

Username: tmp
Your username is set to - tmp

printfUse functions in C to demonstrate fflushbehavior

Note that some streams (such as stderr) are unbuffered. In contrast, the implicit write function that writes to stdoutthe stream printfis buffered, and if we execute an infinite loop below that prints one character per iteration, nothing will be written to the stream until the internal buffer is full. Therefore, the result of the code example below is a burst of printed bulletcharacters. Note that we call the function on each iteration usleepto slow down the execution speed for the human eye so that it can be clearly observed.

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

int main(int argc, char *argv[]) {

    while (1) {
        printf(".");
        usleep(1e3);
    }

    exit(EXIT_SUCCESS);
}

Alternatively, if we fprintfreplace printfthe call to print to stderrthe stream with , we will get iterative printing of one character per second. Again, the 1 second delay between iterations is just for good demonstration purposes.

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

int main(int argc, char *argv[]) {

    while (1) {
        fprintf(stderr, ".");
        sleep(1);
    }

    exit(EXIT_SUCCESS);
}

Finally, if we need stdoutto emulate the same behavior seen in the previous example code on a stream, we can printfadd fflusha call after the function. This will force the C library buffer to be written to the kernel buffer on each iteration, resulting in similar behavior.

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

int main(int argc, char *argv[]) {

    while (1) {
        printf(".");
        fflush(stdout);
        sleep(1);
    }

    exit(EXIT_SUCCESS);
}