Pipe buffer size is 4k or 64k?

Question

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??

Answer 1

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

Answer 2

It's programmable now

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As of Linux 2.6.35 you can use fcntl(2) with the F_SETPIPE_SZ operation to set the pipe buffer up to /proc/sys/fs/pipe-max-size. This is by default 1 MB; see proc(5).