管道缓冲区有多大？

作为注释，我对为什么makefile中的“ | true”与“ || true”用户cjm 产生相同的效果感到困惑：

避免的另一个原因 true是如果命令产生的输出足以填充管道缓冲区，它将阻塞等待true的读取。

我们是否可以找出管道缓冲区的大小？

管道缓冲区的容量在不同系统之间有所不同（甚至在同一系统上可能有所不同）。我不确定是否有一种快速，轻松和跨平台的方法来查找管道的容量。

例如，Mac OS X默认使用16384字节的容量，但是如果对管道进行大量写入，则可以切换到65336字节的容量，如果已经有太多的内核内存，则可以切换到单个系统页面的容量。由管道缓冲区使用（请参阅xnu/bsd/sys/pipe.h和xnu/bsd/kern/sys_pipe.c；由于它们来自FreeBSD，因此在此处也可能发生相同的行为）。

一个Linux pipe（7）手册页指出，自Linux 2.6.11起，管道容量为65536字节，而在此之前的单个系统页（例如，（32位）x86系统上为4096字节）。代码（include/linux/pipe_fs_i.h和fs/pipe.c）似乎使用16个系统页面（如果系统页面为4 KiB，则为64 KiB），但是每个管道的缓冲区都可以通过管道上的fcntl进行调整（最大容量默认为1048576）个字节，但可以通过/proc/sys/fs/pipe-max-size））进行更改。

这是一些bash / perl组合，用于测试系统上的管道容量：

#!/bin/bash
test $# -ge 1 || { echo "usage: $0 write-size [wait-time]"; exit 1; }
test $# -ge 2 || set -- "$@" 1
bytes_written=$(
{
    exec 3>&1
    {
        perl -e '
            $size = $ARGV[0];
            $block = q(a) x $size;
            $num_written = 0;
            sub report { print STDERR $num_written * $size, qq(\n); }
            report; while (defined syswrite STDOUT, $block) {
                $num_written++; report;
            }
        ' "$1" 2>&3
    } | (sleep "$2"; exec 0<&-);
} | tail -1
)
printf "write size: %10d; bytes successfully before error: %d\n" \
    "$1" "$bytes_written"

这是我在Mac OS X 10.6.7系统上以各种写入大小运行它的结果（请注意，写入大于16KiB的更改）：

% /bin/bash -c 'for p in {0..18}; do /tmp/ts.sh $((2 ** $p)) 0.5; done'
write size:          1; bytes successfully before error: 16384
write size:          2; bytes successfully before error: 16384
write size:          4; bytes successfully before error: 16384
write size:          8; bytes successfully before error: 16384
write size:         16; bytes successfully before error: 16384
write size:         32; bytes successfully before error: 16384
write size:         64; bytes successfully before error: 16384
write size:        128; bytes successfully before error: 16384
write size:        256; bytes successfully before error: 16384
write size:        512; bytes successfully before error: 16384
write size:       1024; bytes successfully before error: 16384
write size:       2048; bytes successfully before error: 16384
write size:       4096; bytes successfully before error: 16384
write size:       8192; bytes successfully before error: 16384
write size:      16384; bytes successfully before error: 16384
write size:      32768; bytes successfully before error: 65536
write size:      65536; bytes successfully before error: 65536
write size:     131072; bytes successfully before error: 0
write size:     262144; bytes successfully before error: 0

Linux 3.19上的相同脚本：

/bin/bash -c 'for p in {0..18}; do /tmp/ts.sh $((2 ** $p)) 0.5; done'
write size:          1; bytes successfully before error: 65536
write size:          2; bytes successfully before error: 65536
write size:          4; bytes successfully before error: 65536
write size:          8; bytes successfully before error: 65536
write size:         16; bytes successfully before error: 65536
write size:         32; bytes successfully before error: 65536
write size:         64; bytes successfully before error: 65536
write size:        128; bytes successfully before error: 65536
write size:        256; bytes successfully before error: 65536
write size:        512; bytes successfully before error: 65536
write size:       1024; bytes successfully before error: 65536
write size:       2048; bytes successfully before error: 65536
write size:       4096; bytes successfully before error: 65536
write size:       8192; bytes successfully before error: 65536
write size:      16384; bytes successfully before error: 65536
write size:      32768; bytes successfully before error: 65536
write size:      65536; bytes successfully before error: 65536
write size:     131072; bytes successfully before error: 0
write size:     262144; bytes successfully before error: 0

注意：PIPE_BUFC头文件中定义的值（以及的pathconf值_PC_PIPE_BUF）没有指定管道的容量，而是指定了可以自动写入的最大字节数（请参阅POSIX write（2））。

引用自include/linux/pipe_fs_i.h：

/* Differs from PIPE_BUF in that PIPE_SIZE is the length of the actual
   memory allocation, whereas PIPE_BUF makes atomicity guarantees.  */

此shell行也可以显示管道缓冲区的大小：

M=0; while true; do dd if=/dev/zero bs=1k count=1 2>/dev/null; \
       M=$(($M+1)); echo -en "\r$M KB" 1>&2; done | sleep 999

（将1k块发送到阻塞的管道，直到缓冲区已满）...一些测试输出：

64K (intel-debian), 32K (aix-ppc), 64K (jslinux bellard.org)      ...Ctrl+C.

使用printf的最短bash-one-liner：

M=0; while printf A; do >&2 printf "\r$((++M)) B"; done | sleep 999

以下是一些仅使用shell命令来探索实际管道缓冲区容量的替代方法：

# get pipe buffer size using Bash
yes produce_this_string_as_output | tee >(sleep 1) | wc -c

# portable version
( (sleep 1; exec yes produce_this_string_as_output) & echo $! ) | 
     (pid=$(head -1); sleep 2; kill "$pid"; wc -c </dev/stdin)

# get buffer size of named pipe
sh -c '
  rm -f fifo
  mkfifo fifo
  yes produce_this_string_as_output | tee fifo | wc -c &
  exec 3<&- 3<fifo
  sleep 1
  exec 3<&-
  rm -f fifo
'

# Mac OS X
#getconf PIPE_BUF /
#open -e /usr/include/limits.h /usr/include/sys/pipe.h
# PIPE_SIZE
# BIG_PIPE_SIZE
# SMALL_PIPE_SIZE
# PIPE_MINDIRECT

这是Ubuntu 12.04，YMMV上的快速而肮脏的黑客

cat >pipesize.c

#include <unistd.h>
#include <errno.h>
#include </usr/include/linux/fcntl.h>
#include <stdio.h>

void main( int argc, char *argv[] ){
  int fd ;
  long pipesize ;

  if( argc>1 ){
  // if command line arg, associate a file descriptor with it
    fprintf( stderr, "sizing %s ... ", argv[1] );
    fd = open( argv[1], O_RDONLY|O_NONBLOCK );
  }else{
  // else use STDIN as the file descriptor
    fprintf( stderr, "sizing STDIN ... " );
    fd = 0 ;
  }

  fprintf( stderr, "%ld bytes\n", (long)fcntl( fd, F_GETPIPE_SZ ));
  if( errno )fprintf( stderr, "Uh oh, errno is %d\n", errno );
  if( fd )close( fd );
}

gcc -o pipesize pipesize.c

mkfifo /tmp/foo

./pipesize /tmp/foo

>sizing /tmp/foo ... 65536 bytes

date | ./pipesize

>sizing STDIN ... 65536 bytes

$ ulimit -a | grep pipe
pipe size            (512 bytes, -p) 8

因此，在我的Linux机器上，默认情况下，我有8 * 512 = 4096字节的管道。

Solaris和许多其他系统具有类似的ulimit功能。

如果您需要Python> = 3.3中的值，这是一个简单的方法（假设您可以运行调用dd）：

from subprocess import Popen, PIPE, TimeoutExpired
p = Popen(["dd", "if=/dev/zero", "bs=1"], stdin=PIPE, stdout=PIPE)
try: 
    p.wait(timeout=1)
except TimeoutExpired: 
    p.kill()
    print(len(p.stdout.read()))