C Stop stdout from flushing

Question

I am writing a C program where I am printing to stderr and also using putchar() within the code. I want the output on the console to show all of the stderr and then finally flush the stdout before the program ends. Does anyone know of a method that will stop stdout from flushing when a putchar('\n') occurs?

I suppose i could just do an if statement to make sure it doesn't putchar any newlines but I would prefer some line or lines of code to put at the top of the program to stop all flushing until i say fflush(stdout) at the bottom of the program

Why not write to a buffer then write that to stdout at the point you want it to be displayed? — simonc, Oct 25 '13 at 16:12

score 3 · Answer 1 · answered Oct 25 '13 at 16:26

3

What you're trying to do is horribly fragile. C provides no obligation for an implementation of stdio not to flush output, under any circumstances. Even if you get it to work for you, this behavior will be dependent on not exceeding the buffer size. If you really need this behavior, you should probably buffer the output yourself (possibly writing it to a tmpfile() rather than stdout) then copying it all to stdout as the final step before your program exits.

answered Oct 25 '13 at 16:26

R.. GitHub STOP HELPING ICE

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This doesn't work - writing to a temporary still seems to flush periodically as you copy the temporary to stdout. – c z Nov 26 '20 at 10:49

score 1 · Answer 2 · answered Oct 25 '13 at 16:15

1

Run your command from the console using pipeling:

my_command >output.txt

All output witten to stderr will appear immediately. The stuff written to stdout will go to output.txt.

answered Oct 25 '13 at 16:15

alk

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score 0 · Answer 3 · answered Nov 26 '20 at 10:40

Windows only. I'm still looking for the Unix solution myself if anyone has it!

Here is a minimal working example for Windows that sends a buffer to stdout without flushing. You can adjust the maximum buffer size before a flush occurs by changing max_buffer, though I imagine there's some upper limit!

#include <windows.h>
#include <string.h>

int main()
{
    const char* my_buffer = "hello, world!";
    HANDLE hStdout = GetStdHandle(STD_OUTPUT_HANDLE);
    int max_buffer = 1000000;
    int num_remaining = strlen(my_buffer);

    while (num_remaining)
    {
        DWORD num_written = 0;
        int buffer_size = num_remaining < max_buffer ? num_remaining : max_buffer;
        int retval = WriteConsoleA(hStdout, my_buffer, buffer_size, &num_written, 0);

        if (retval == 0 || num_written == 0)
        {
            // Handle error
        }

        num_remaining -= num_written;

        if (num_remaining == 0)
        {
            break;
        }

        my_buffer += num_written;
    }
}

chqrlie · Answer 4 · 2020-11-26T19:35:53.393

You can use setvbuf() to fully buffer output to stdout and provide a large enough buffer size for your purpose:

#include <stdio.h>

int main() {
    // issue this call before any output
    setvbuf(stdout, NULL, _IOFBF, 16384);
    ...
    return 0;
}

Output to stderr is unbuffered by default, so it should go to the console immediately. Output to stdout is line buffered by default when attached to the terminal. Setting it to _IOFBF (fully buffered) should prevent putchar('\n') from flushing the pending output.

C Stop stdout from flushing

4 Answers4

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