I read in multiple places that the default buffer size for a pipe is 4kB (for instance, here), and my ulimit -a
tends to confirm that statement:
$ ulimit -a
core file size (blocks, -c) 0
data seg size (kbytes, -d) unlimited
scheduling priority (-e) 0
file size (blocks, -f) unlimited
pending signals (-i) 15923
max locked memory (kbytes, -l) 64
max memory size (kbytes, -m) unlimited
open files (-n) 1024
pipe size (512 bytes, -p) 8 // 8 * 512B = 4kB
POSIX message queues (bytes, -q) 819200
real-time priority (-r) 0
stack size (kbytes, -s) 10240
cpu time (seconds, -t) unlimited
max user processes (-u) 1024
virtual memory (kbytes, -v) unlimited
file locks (-x) unlimited
But when I use a little program to test the buffer size (by writing into the pipe until the write() blocks), I see a limit of 64kB!
See this program:
#include <stdio.h>
#include <unistd.h>
#include <limits.h>
int main(void)
{
int tube[2];
char c = 'c';
int i;
fprintf(stdout, "Tube Creation\n");
fprintf(stdout, "Theoretical max size: %d\n", PIPE_BUF);
if( pipe(tube) != 0)
{
perror("pipe");
_exit(1);
}
fprintf(stdout, "Writing in pipe\n");
for(i=0;; i++)
{
fprintf(stdout, "%d bytes written\n", i+1);
if( write(tube[1], &c, 1) != 1)
{
perror("Write");
_exit(1);
}
}
return 0;
}
And its output:
$ ./test_buf_pipe
Tube Creation
Theoretical max size: 4096
Writing in pipe
1 bytes written
2 bytes written
3 bytes written
4 bytes written
[...]
65535 bytes written
[blocks here]
It strongly suggests that the pipe buffer size is actually 64k! What is happening here??
The other answers tell you that the pipe size is 64 KB. The reason why PIPE_BUF is 4KB is that PIPE_BUF is the largest size for which writes are guaranteed to be atomic. See http://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html