fflush(NULL) deadlock (C/C++)

案例代码

#include <stdio.h>
#include <time.h>
#include <pthread.h>
#include <unistd.h>

void *func(void *arg)
{
  char str[50];
  FILE* fp = fdopen((int*)arg, "r");
  printf("fgets\n");
  fgets(str, 20, fp);
  exit(-1);
}

void main() {
  pthread_t ntid;
  int pipefd[2] = {0};

  if (pipe(pipefd) == -1)
  {
    perror("pipe");
  }
  pthread_create(&ntid, NULL, func, (void*)pipefd[0]);
  sleep(1);
  printf("fflush\n");
  fflush(NULL);
  close(pipefd[0]);
  close(pipefd[1]);
  printf("success\n");
}

现象

fflush(NULL) 卡死.
pstack 显示卡在 __lll_lock_wait_private

调用栈信息:

Thread 2 (Thread 0x7ffff7dcf700 (LWP 8812)):
#0  0x00007ffff7ec1f84 in read () from /lib64/libc.so.6
#1  0x00007ffff7e51c98 in __GI__IO_file_underflow () from /lib64/libc.so.6
#2  0x00007ffff7e52dd6 in _IO_default_uflow () from /lib64/libc.so.6
#3  0x00007ffff7e4622a in _IO_getline_info () from /lib64/libc.so.6
#4  0x00007ffff7e4523f in fgets () from /lib64/libc.so.6
#5  0x000000000040122e in p ()
#6  0x00007ffff7fa14a7 in start_thread () from /lib64/libpthread.so.0
#7  0x00007ffff7ed13e3 in clone () from /lib64/libc.so.6
Thread 1 (Thread 0x7ffff7dd0740 (LWP 8811)):
#0  0x00007ffff7e551cb in __lll_lock_wait_private () from /lib64/libc.so.6
#1  0x00007ffff7e5382c in _IO_flush_all_lockp () from /lib64/libc.so.6
#2  0x00000000004011eb in thr_fn ()
#3  0x00000000004012b6 in main ()

原因分析

先看看 fflush 的官方描述

If the given stream was open for writing (or if it was open for updating and the last i/o operation was an output operation) any unwritten data in its output buffer is written to the file.
If stream is a null pointer, all such streams are flushed.

简单来说就是 fflush 会刷新打开的fd stream.
如果入参是NULL, 则刷新所有的fd sream.

再看看fgets

Reads characters from stream and stores them as a C string into str until (num-1) characters have been read or either a newline or the end-of-file is reached, whichever happens first.
A newline character makes fgets stop reading, but it is considered a valid character by the function and included in the string copied to str.

简单来说，fgets 会从给定的fd stream 读取数据，并存入buffer.
但这里没有说到的一点是：fgets is a blocking read.
它会一直阻塞直到fd stream 有数据。

所以如果当fd steam 没有数据，又没有结束，比如fd是一个还没传数据的pipe，那么read这个动作一直会阻塞在这个fd 上。
然后，当fflush 尝试去刷新这个fd时，会因为拿不到锁（锁一直被read拿着）而一直阻塞。

解决办法

应该尽量避免在代码中以阻塞的方式读fd stream, 除非我们确定当前一定有数据。
可以通过select, poll, epoll等异步I/O 来等到fd stream 有可用数据后，再去读，避免阻塞。

修改后的代码

#include <stdio.h>
#include <time.h>
#include <pthread.h>
#include <unistd.h>
#include <poll.h>

void *func(void *arg)
{
  char str[50];
  FILE* fp = fdopen((int*)arg, "r");
  struct pollfd readFds = {0};
  readFds.fd = (int*)arg;
  readFds.events = POLLIN;

  printf("poll\n");
  if (0 < poll(&readFds, 1, -1))
  {
    printf("fgets\n");
    fgets(str, 20, fp);
  }
  exit(-1);
}

void main() {
  pthread_t ntid;
  int pipefd[2] = {0};

  if (pipe(pipefd) == -1)
  {
    perror("pipe");
  }
  pthread_create(&ntid, NULL, func, (void*)pipefd[0]);
  sleep(1);
  printf("fflush\n");
  fflush(NULL);
  close(pipefd[0]);
  close(pipefd[1]);
  printf("success\n");
}