increasing standard output buffer size

Question

I'm running a program which starts a process and then that process writes to stdout which my program uses. Problem is, the output I need is about 42000 bytes. It seems the stdout buffer size is 8192 and I don't want it to flush until it gets to the 42000. Is there a way to set this?

I tried this:

setvbuf ( stdout , NULL , _IOFBF , 50000 ); // ie set it to 50000 bytes

on the code for the subprocess, but it doesn't seem to work at all. Does anyone have any ideas?

Answer 1

You need to supply a buffer to setvbuf() to work.

static char buf[50000]; /* buf must survive until stdout is closed */
setvbuf ( stdout , buf , _IOFBF , sizeof(buf) );

From the man page:

int setvbuf(FILE *stream, char *buf, int mode , size_t size);
...
Except for unbuffered files, the buf argument should point to a buffer at least size bytes long; this buffer will be used instead of the current buffer. If the argument buf is NULL, only the mode is affected; a new buffer will be allocated on the next read or write operation.

Here is a sample program:

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

int main (int argc, char *argv[]) {
    char msg[42000];
    char buf[50000];

    setvbuf(stdout, buf, _IOFBF, sizeof(buf));
    memset(msg, 'a', sizeof(msg));
    msg[sizeof(msg)-1] = '\0';
    puts(msg);
    exit(0);
}

On my system, the strace output shows a single write of 42000 bytes.

Answer 2

As additional information to jxh answer:

char buf[50000];
setvbuf ( stdout , buf , _IOFBF , sizeof(buf) );
/*close stdout before the programs finishes or even before buf goes out of scope*/
fclose(stdout);

you'll have to close stdout aswell because man setvbuff also mentions:

You must make sure that the space that buf points to still exists by the time stream is closed, which also happens at program termination.