Linux System Calls problems using Fork() passing ints to child and parent processes

问题

I am working on a program that will take an integer and create two processes, a parent and a child. The parent will subtract 5 from the integer, pass it to the child who will divide it by 5, and then they will repeat this process 5 times, each time printing the current value of the integer.

The integer can be passed through a text file and be both written and read off of, or a pipeline can be used which ever is simpler.

I have been looking up the systems calls needed, and have a semi working program. I have been stuck for hours however, and I think my issue is that I can't get them to wait for each other to finish because my output is incorrect.

Here is what I got so far.

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

int main(void)
{    
    int x=19530;
    int w=1;
    int fd[2];
    pipe(fd);
    int pid = fork();

    int k;
    for (k=0; k<5; k++) {
        if (pid>0) {
            //int x = 19530;

            if ((close(fd[0]))==-1) {
                perror("Close:");
            }

            read(fd[0], &x, sizeof(int));
            x=x-5;
            write (fd[1], &x, sizeof(int));
            printf("X in parent %d\n", x);

            close(fd[1]);
            close(fd[0]);
        } else if (pid==0) {

            if ((close(fd[1]))==-1) {
                perror("Close:");
            }

            read(fd[0], &x, sizeof(int));
            x=x/5;
            printf("X in child %d\n", x);
            write (fd[1], &x, sizeof(int));

            close(fd[1]);
            close(fd[0]);
        }
    }
    return 0;
}

However my output is an issue, I am getting:

X in parent 19525
X in child 3905
Close:: Bad file descriptor
X in parent 19520
Close:: Bad file descriptor
X in parent 19515
Close:: Bad file descriptor
X in parent 19510
Close:: Bad file descriptor
X in parent 19505
Close:: Bad file descriptor
X in child 781
Close:: Bad file descriptor
X in child 156
Close:: Bad file descriptor
X in child 31
Close:: Bad file descriptor
X in child 6

It seems to start off well, but then the child doesn't pass back properly then the parent runs too many times in a row before the child catches up. I also been trying to fix that bad file descriptor but to no avail.

Any help would be greatly appreciated.

Thanks.

回答1:

#include <stdio.h>
#include <unistd.h>
#include <err.h>

#define ok(x) ({ int i_ = (x); if (i_ == -1) err(1, #x); i_; })

enum { p_, c_ }; // parent, child
enum { r_, w_ }; // read, write

int main(void)
{
        int x = 19530;
        int fd[2][2];
        int pid;

        ok(pipe(fd[0]));
        ok(pipe(fd[1]));

        ok(pid = fork());

        close(fd[p_][pid ? r_ : w_]);
        close(fd[c_][pid ? w_ : r_]);

        for (int i = 0; i < 5; i++) {
                if (pid) {
                        x -= 5;
                        printf("X in parent %d\n", x);
                        ok(write(fd[p_][w_], &x, sizeof(x)));
                        ok(read(fd[c_][r_], &x, sizeof(x)));
                }
                else {
                        ok(read(fd[p_][r_], &x, sizeof(x)));
                        x /= 5;
                        printf("X in child %d\n", x);
                        ok(write(fd[c_][w_], &x, sizeof(x)));
                }
        }
        return 0;
}

Pipes are unidirectional, so you need two. I used some enums to try and make things easier to read.

来源：https://stackoverflow.com/questions/35302981/linux-system-calls-problems-using-fork-passing-ints-to-child-and-parent-proces

标签

Linux

fork