C Stop stdout from flushing

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.

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.

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; } } 

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.