进程创建子进程，将x增加1，然后将x值发送到子进程，子进程可以将x乘以10，然后将x传递回父进程，依此类推。你应该至少这样做5次。
输出应如下所示：
初始值0
父：
操作后x值：1
子：
操作后x值：10
父：
操作后x值：11
子：
操作后x值：110
父：
操作后x值：111
子
操作后x值：1110
我所拥有的是以下.......
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/syscall.h>
#define READ 0
#define WRITE 1
int main()
{
pid_t pid;
int mypipefd[2];
int ret;
int i;
int x=0;
int result,result2;
ret = pipe(mypipefd);
if(ret ==-1) //test for pipe
{
perror("pipe"); //show error and exit
exit(1);
}
printf("initial value %d\n", x);
pid = fork();
for(i=0;i<5;i++)
{
if(pid == 0)
{
/*Child process*/
result2=result*10;
write(mypipefd[WRITE],&result2,sizeof(result2));
printf("Child:\nx value after operation: %d\n", x);
close(mypipefd[WRITE]);
read(mypipefd[READ],&result2,sizeof(result2));
printf("=======================================\n");
close(mypipefd[READ]);
}
else if(pid > 0)
{
/*Parent process*/
result=x++;
write(mypipefd[READ],&result,sizeof(result));
printf("Parent:\nx value after operation: %d\n", x);
close(mypipefd[WRITE]);
read(mypipefd[WRITE],&result,sizeof(result));
printf("=======================================\n");
close(mypipefd[READ]);
exit(0);
}
else
{
perror("fork");
exit(1);
}
}
}
问题是，我的代码出了什么问题？我试着读取和写回管道，但似乎没有工作......
我的代码现在的输出是什么：
初始值0
父：
操作后x值：1
子：
操作后x值：0
子：
操作后x值：0
子：
操作后x值：0
子：
操作后x值：0
子：
操作后x值：0
答案 0 :(得分：4)
我认为要解决的第一个问题是，当您尝试读取/写入数据时，管道的两端都会打开。如果您要将数据写入管道，那么
close(fd[READ_END])
write(...)
close(fd[WRITE_END])
其次，unix中的管道是单工的。您似乎同时尝试从管道读取和写入。如果你想这样做，你将不得不打开两个管道。
我修改了你的程序，向你展示如何从管道读取，然后写入父/子管道。希望这会对你有所帮助。
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <string.h>
#define BUFFER_SIZE 25
#define READ 0
#define WRITE 1
int main(void)
{
pid_t pid;
//open two pipes, one for each direction
int mypipefd[2];
int mypipefd2[2];
/* create the pipe */
if (pipe(mypipefd) == -1 || pipe(mypipefd2) == -1) {
fprintf(stderr,"Pipe failed");
return 1;
}
/* now fork a child process */
pid = fork();
if (pid < 0) {
fprintf(stderr, "Fork failed");
return 1;
}
if (pid > 0) { /* parent process */
int writeValue=10;
int readValue=0;
close(mypipefd[READ]); //close read end, write and then close write end
write(mypipefd[WRITE],&writeValue,sizeof(writeValue)); //write to pipe one
printf("Parent: writes value : %d\n", writeValue);
close(mypipefd[WRITE]);
close(mypipefd2[WRITE]); //close write end, read, and then close read end
read(mypipefd2[READ],&readValue,sizeof(readValue));
printf("Parent: reads value : %d\n", readValue);
close(mypipefd2[READ]);
}
else { /* child process */
int writeValue=20;
int readValue=0;
close(mypipefd[WRITE]); //close write end, read, and then close read end
read(mypipefd[READ],&readValue,sizeof(readValue));
printf("child: read value : %d\n", readValue);
writeValue+=readValue;
close(mypipefd[READ]);
close(mypipefd2[READ]); //close read end, write and then close write end
write(mypipefd2[WRITE],&writeValue,sizeof(writeValue));
printf("child: writeValue value : %d\n", writeValue);
close(mypipefd2[WRITE]);
}
return 0;
}
这打开两个管道。父级将值写入第一个管道，子级读取该值。子项更新此值，并将其写入第二个方向管道。然后父母读取这个值和vuela！双向管道通信。
输出：
Parent: writes value : 10
child: read value : 10
child: write value : 30
Parent: reads value : 30
这可以扩展到您的应用程序
if (pid > 0) { /* parent process */
result1++;
close(mypipefd[READ]); //close read end, write and then close write end
write(mypipefd[WRITE],&result1,sizeof(result1)); //write to pipe one
printf("Parent:\n x value after operation: %d\n", result1);
close(mypipefd[WRITE]);
close(mypipefd2[WRITE]); //close write end, read, and then close read end
read(mypipefd2[READ],&result1,sizeof(result1));
close(mypipefd2[READ]);
}
else { /* child process */
close(mypipefd[WRITE]); //close write end, read, and then close read end
read(mypipefd[READ],&result2,sizeof(result2));
result2*=10;
printf("child:\n x value after operation %d\n", result2);
close(mypipefd[READ]);
close(mypipefd2[READ]); //close read end, write and then close write end
write(mypipefd2[WRITE],&result2,sizeof(result2));
close(mypipefd2[WRITE]);
}
如果你把它放在循环中那么它理论上会起作用。但是，由于上下文切换和其他与操作系统相关的调度，执行顺序将在两个进程之间混乱。这意味着读取和写入不一定按照您的方式顺序执行，并且它不会工作。阅读这个问题 How to wait till data is written on the other end of pipe