c - 空线程阻塞的所有线程均被读取

Question

我试图自学有关C(Linux)中的多线程和多进程编程的知识。我写了一个简短的程序，它产生了一个新线程，该线程转到一个例程，该例程尝试从空FIFO进行阻塞读取，而主线程继续并打印到STDOUT。 (注意:在执行程序之前，我确实在终端中使用mkfifo newfifo创建了一个FIFO)

我期望程序打印到屏幕“主线程”，然后等待我将数据放入FIFO时阻塞。而是，整个过程被阻塞，消息“主线程”仅在我将数据放入FIFO后才打印。

我在这里想念什么吗？即使生成的线程被阻塞，主线程也不应该继续运行吗？我尝试了使用fork创建一个子进程的测试，并得到了相同的结果(两个进程都被从空FIFO读取而阻止了)。

代码如下:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <pthread.h>

#define NEWPIPE "./newfifo"

typedef struct __attribute__ ((__packed__)) {
  int reserved      :30;
  int threadStarted :1;
  int msgRcved      :1;
} Status;

void* thread_routine(int fd, char* buffer, Status* myStatus)
{
  int great_success = 0; 

  myStatus->threadStarted = 1;
  printf("Side thread\n");

  great_success = read(fd, buffer, sizeof(buffer));

  if (great_success < 0) {
    printf("pipe failed\n");
  } else {
    myStatus->msgRcved = 1;
  }
}

void main()
{
  int fd;
  int cnt = 0;
  char buffer[20];
  Status* myStatus;
  pthread_t thread_id;

  myStatus = (Status*) malloc(sizeof(Status));
  myStatus->reserved      = 0;
  myStatus->threadStarted  = 0;
  myStatus->msgRcved      = 0;

  fd = open(NEWPIPE, O_RDONLY);

  pthread_create(&thread_id, 
                  NULL, 
                  (void *) thread_routine(fd, buffer, myStatus), 
                  NULL);

  printf("Main thread\n");

  while (!myStatus->threadStarted) {
    printf("Main thread: side thread started!\n");
  }

  while (!myStatus->msgRcved) {
    sleep(1);
    cnt++;
  } 

  printf("buffer (cnt = %d): %s\n", cnt, buffer);

}

编辑:最新代码

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <pthread.h>

#define NEWPIPE "./newfifo"

struct Status {
  unsigned int reserved      :30;
  unsigned int threadStarted :1;
  unsigned int msgRcved      :1;
};

void* thread_routine(void *arg)
{
  int great_success = 0; 
  int fd;
  char buffer[20];
  struct Status* myStatus;

  fd = open(NEWPIPE, O_RDONLY);

  myStatus = arg;

  myStatus->threadStarted = 1;
  printf("Side thread\n");

  while (1) {
    great_success = read(fd, buffer, 20);

    if (great_success < 0) {
      printf("pipe failed\n");
    } else {
      printf("buffer : %s\n", buffer);
      printf("great_success = %d\n", great_success);
      great_success = 0;
    }
  }
}

void main()
{
  int cnt = 0;
  struct Status* myStatus;
  pthread_t thread_id;

  myStatus = (struct Status*) malloc(sizeof(struct Status));
  myStatus->reserved      = 0;
  myStatus->threadStarted  = 0;
  myStatus->msgRcved      = 0;

  pthread_create(&thread_id, 
                  NULL, 
                  &thread_routine,
                  (void *) myStatus);    // arguments to pass to the function!


  printf("Main thread\n");

  while (!myStatus->msgRcved) {
    printf("Main thread: side thread started!\n");

    if (myStatus->threadStarted) {
      printf("spawned thread started!\n");
    }

    sleep(1);
  }

  pthread_exit(NULL);

}

Answer 1

Instead, the entire process is blocked, and the message "Main thread" only prints after I've put data into the FIFO.

Am I missing something here?

您的主线程在此行被阻止:

fd = open(NEWPIPE, O_RDONLY);

因为FIFO的非阻塞，只读打开将阻塞，直到写入器可用为止。您的主线程最终将被解除阻塞，而不是在将数据写入FIFO时，而只是在打开FIFO进行写入时才解除阻塞。

代码中还有其他问题，如@JohnBollinger discusses in his answer。但是，FIFO语义是为什么您看不到预期的初始输出的原因。