Why is it that my pipe does not read in the printf despite replacing stdin?

问题

The objective is to use dup, dup2 and pipe to communicate between parent and child processes. Just to get a feel of how to use dup and pipes. The function multby, called in by the child process, takes a number as an argument (3) and multiply it with a user input (in this case 5 as printed in parent) to get a product. (which should be 15)

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

void main(int argv, char *argc) { 
    int testpipe[2]; 
    pipe(testpipe); 
    int PID = fork(); 
    if (PID == 0) { 
        dup2(testpipe[0], 0); 
        close(testpipe[1]); 
        execl("./multby", "multby", "3", NULL); 
        close(testpipe[0]); 
        close(0); 
    } else { 
        dup2(testpipe[1], 1); 
        close(testpipe[0]); 
        printf("5"); 
        close(1); 
        close(testpipe[1]); 
        wait(NULL); 
    }  
}

Here is the code for multby. I also added some fprintf statement to troubleshoot. In particular, it prints out the 2 numbers to be multiplied (which should be 3 and 5 as mentioned above).

#include <stdio.h>

int main(int argv, char *argc[]) {
  fprintf(stderr,"exec successful\n");
  if (argv < 2) {
    fprintf(stderr, "Usage: %s <factor>\n", argc[0]);
    return 1;
  }      

  size_t a, b;
  sscanf(argc[1], "%zu", &a);
  fprintf(stderr, "a: %zu\n", a);

  if (scanf("%zu", &b) != 1) {
    fprintf(stderr, "No input %zu\n", a);
    return 1; 
  } 
  fprintf(stderr, "b: %zu\n", b);
  printf("%zu", a*b);
  fprintf(stderr, "multby successful\n");
  return 0;
}

However, this is my result: exec successful a: 3 No input 3

5 was not registered from the read end of the pipe.

Could someone please advise me on what I am doing wrong?

回答1:

You use printf(“5”); to relay the data to the child, but standard output is at least line-buffered so nothing is written until a newline is sent (or fflush(stdout)). Then you close(1); to close standard output, which does not flush the standard I/O buffers. So nothing is sent to the child. Use fclose(stdout); to get the 5 sent to the child.

来源：https://stackoverflow.com/questions/60250374/why-is-it-that-my-pipe-does-not-read-in-the-printf-despite-replacing-stdin

标签

pipe

dup