c - 读取文件并强制进程之间的操作顺序

Question

我正在尝试遵循一个教程，该教程要求我编辑示例代码以使程序运行两个进程，这两个进程轮流输出歌曲的歌词(“There's a hole in the bucket”)。

我的问题是文件作为一个整体输出，而不是像我所说的那样应该看到屏幕截图:http://imgur.com/NusvhVA

我的代码如下。谢谢。

    #include <sys/ipc.h>
    #include <sys/sem.h>
    #include <stdio.h>
    #include <stdlib.h>

    #define KEY 87654 //Unique semaphore key

int main()
{
  int id; /* Number by which the semaphore is known within a program */
  FILE *file;
  file = fopen("207song.txt", "r" );
  int c;

  union semun {
    int val;
    struct semid_ds *buf;
    ushort * array;
  } argument;

  argument.val = 1;

  /* Create the semaphore with external key KEY if it doesn't already
     exists. Give permissions to the world. */
  id = semget(KEY, 1, 0666 | IPC_CREAT);

  /* Always check system returns. */
  if(id < 0) {
      fprintf(stderr, "Unable to obtain semaphore.\n");
      exit(0);
  }

  /* What we actually get is an array of semaphores. The second
     argument to semget() was the array dimension - in our case
     1. */

  /* Set the value of the number 0 semaphore in semaphore array
     # id to the value 0. */
  if( semctl(id, 0, SETVAL, argument) < 0) {
      fprintf( stderr, "Cannot set semaphore value.\n");
  } else {
      fprintf(stderr, "Semaphore %d initialized.\n", KEY);
  }

  int pid=fork();


  const int HENRY_DONE = 0;
  const int LIZA_DONE = 1;
  volatile int flag = HENRY_DONE;

  if(pid) {
    struct sembuf operations[1];
    int retval; /* Return value from semop() */

    /* Get the index for the semaphore with external name KEY. */
    id = semget(KEY, 1, 0666);

    if(id < 0){
      /* Semaphore does not exist. */

      fprintf(stderr, "Program sema cannot find semaphore, exiting.\n");
      exit(0);
    }
    operations[0].sem_num = 0;
    /* Which operation? Subtract 1 from semaphore value : */
    operations[0].sem_op = -1;
    /* Set the flag so we will wait : */
    operations[0].sem_flg = 0;

    while(1){
      //Process 1

      //wait
      operations[0].sem_op = -1;
      retval = semop(id, operations, 1);

      //critical section
      printf("Liza's Part: \n");
      fflush(stdout);
      sleep(1);

      while ((c = getc(file)) !=EOF)
                    if (c == "\n") {
                            putchar(c);
                            break;
                            }
                    else
                    putchar(c);
      fflush(stdout);

      operations[0].sem_op = 1;
      //signal
      retval = semop(id, operations, 1);

    }
  }else{
    //Process 2
    struct sembuf operations[1];
    int retval; /* Return value from semop() */
    /* Get the index for the semaphore with external name KEY. */
    id = semget(KEY, 1, 0666);
    if(id < 0){
      /* Semaphore does not exist. */

      fprintf(stderr, "Program sema cannot find semaphore, exiting.\n");
      exit(0);
    }
    operations[0].sem_num = 0;
    /* Which operation? Subtract 1 from semaphore value : */

    operations[0].sem_op = -1;
    /* Set the flag so we will wait : */
    operations[0].sem_flg = 0;

    while(1){

      //wait

      operations[0].sem_op = -1;
      retval = semop(id, operations, 1);

      //critical section

      printf("Henry's Part: \n");
      fflush(stdout);
      sleep(1);


      while ((c = getc(file)) !=EOF)
                    if (c == "\n") {
                            putchar(c);
                            break;
                            }
                    else
                    putchar(c);
      fflush(stdout);

      //signal
      operations[0].sem_op = 1;
      retval = semop(id, operations, 1);

    }

  }

}

Answer 1

如果你的 while 循环有:

while ((c = getc(file)) !=EOF)
                if (c == "\n") {

getc 返回一个整数，“\n”是 char* 类型的 C 字符串。那比较不匹配，导致第一个用户显示整个文件。

你可能想要

c == '\n'

注意单引号 '而不是双引号 "单引号将是一个 char，它将与 int 进行合理比较。