通过FIFO传递结构

Question

我编写了一个简单的代码，其中一个进程将一个结构传递给另一个进程，而父进程正在编写数据，但是子进程无法访问它。我使用fork()系统调用来创建两个进程。但是我在这里遇到了问题，我调用了两次的父进程和子进程在运行程序时也被调用了两次。有人能让我知道我犯了什么错吗。

#include <stdlib.h>
#include <signal.h>
#include <stdio.h>
#include <errno.h>
#include <sys/errno.h>
#include <fcntl.h>
#include <string.h>

#define MAX_LINE_LEN    100
#define FIFO_NAME "my_fifo"

typedef struct student_info  {
char *name;
int age;
char *sex;
}student;

int w_byte_parent = 0, r_byte_parent = 0;
int w_byte_child = 0, r_byte_child = 0;

void do_child() {
  int fifo;
  char buffer[MAX_LINE_LEN];
  int i = 0;
  student child;

  printf("Child opening FIFO \n");
  if( (fifo = open(FIFO_NAME, O_RDWR)) < 0 )   {
    perror("open in child failed");
    exit(EXIT_FAILURE);
  }
  printf("Child  reading  from FIFO \n");
  r_byte_child = read(fifo, child, sizeof(student));
  if(r_byte_child < 0)
    printf("Read failed by child process\n");
  printf("%d Bytes read by child process\n", r_byte_child);
  }

 int main(int argc, char **argv)  {
 int fifo;
 char buffer[MAX_LINE_LEN];
 int ch = 0, i = 0;

 /*
 ** Create a FIFO
 */
 /* Parent creating FIFO */
if( (mkfifo(FIFO_NAME, 0666)) < 0)  {
    if( errno != EEXIST ) {
        perror( "mkfifo" );
        exit( EXIT_FAILURE );
    }
}
/*
 ** Create other process 
 */

switch(fork())  {
    case -1:
        perror("fork()");
        exit(EXIT_FAILURE);
    case 0: /* Child Process */
        do_child();
        break;
    default:/* Parent Process */
        break;
}
/* Pass a structure to FIFO */
student *info;
info = (student *)malloc( sizeof (student)) ;
info->name = (char *)calloc(sizeof(char), 10);
strcpy(info->name, "jack");
info->age = 27;
info->sex = (char *)calloc(sizeof(char), 10);
strcpy(info->sex , "Male");

/* Parent Opening FIFO */
printf("parent opening FIFO \n");
if( (fifo = open(FIFO_NAME,  O_RDWR)) < 0 )   {
    perror("open in parent failed");
    exit(EXIT_FAILURE);
}

/*
 ** Write some thing into FIFO so child can read it
 */
printf("parent writing to FIFO \n");
w_byte_parent = write( fifo, info, sizeof(struct student_info));
if(w_byte_parent < 0)
    printf("Nothing was written to FIFO by parent\n");
printf("Wrote %d bytes to FIFO\n",w_byte_parent);
}

Answer 1

让我们从基础开始。用-Wall选项编译GCC代码：

$ gcc fifo.c -o fifo -Wall
fifo.c: In function ‘do_child’:
fifo.c:33: warning: implicit declaration of function ‘read’
fifo.c:24: warning: unused variable ‘i’
fifo.c:23: warning: unused variable ‘buffer’
fifo.c: In function ‘main’:
fifo.c:48: warning: implicit declaration of function ‘mkfifo’
fifo.c:58: warning: implicit declaration of function ‘fork’
fifo.c:88: warning: implicit declaration of function ‘write’
fifo.c:42: warning: unused variable ‘i’
fifo.c:42: warning: unused variable ‘ch’
fifo.c:41: warning: unused variable ‘buffer’
fifo.c:92: warning: control reaches end of non-void function

忽略所有未使用的变量问题，您需要包括<unistd.h>和<sys/stat.h>，以获得read、mkfifo、fork和write的正确声明。再次汇编：

$ gcc fifo.c -o fifo -Wall -Wno-unused
fifo.c: In function ‘do_child’:
fifo.c:35: error: incompatible type for argument 2 of ‘read’
fifo.c: In function ‘main’:
fifo.c:94: warning: control reaches end of non-void function

您将student实例作为参数2传递给read，但它需要一个void*指针。您需要将student实例的地址作为&child传递。此外，显式返回值表单main()也很好，尽管这并不是绝对必要的，因为未能返回值表单main()隐式返回0(但只返回main()函数)。

但是，即使纠正了这些错误，您仍然可以看到父进程的输出两次。为什么？因为父进程和子进程都在开关语句下面执行代码块。子进程调用do_child()函数，永不退出，因此它返回并继续执行。

对此的简单修正是确保退出子进程，或者在do_child()结束时，或者在调用返回后立即退出。