Unix管道实现报错：Ubuntu下调试C程序管道示例遇问题

Hey there! Let's break down the problems in your current code and get those two programs communicating over a pipe properly, since you're working through System Programming with C and Unix by Adam Hoover.

Key Problems in Your Current Code #

• Buffering Gotcha: When you run summer | bingo, summer's stdout is connected to a pipe (not a terminal), so printf("#?") gets stuck in stdio's full buffer instead of showing up immediately. Terminal output uses line buffering, but pipes switch to full buffering by default.
• Non-Standard main: void main() isn't compliant with the C standard—stick with int main() and return a status code (like 0 for success).
• Mismatched String Matching: Your summer program outputs sum = %d (note the space between sum and =), but you're checking for "sum=5" in bingo. On top of that, scanf("%s", s) splits input on spaces, so it'll only read "sum" or "=" or "5" separately—never the full line.
• Incomplete Logic: Your bingo code cuts off at if ( strcmp(s,"sum=5") == 0 )...—we need to add the actual "bingo" behavior.

Revised Code #

summer.c #

#include<stdio.h>
#include<stdlib.h>

int main() {
    int x, sum = 0;
    while (1) {
        printf("#?");
        fflush(stdout); // Force output to show immediately, even through a pipe
        
        // Handle cases where input isn't a number or EOF is reached
        if (scanf("%d", &x) != 1) {
            break;
        }
        
        if (x == 0) {
            break;
        }
        
        sum += x;
        printf("sum = %d\n", sum);
        fflush(stdout); // Make sure the sum is sent to the pipe right away
    }
    return 0;
}

bingo.c #

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

int main() {
    char line[80];
    // Read full lines instead of splitting on spaces
    while (fgets(line, sizeof(line), stdin) != NULL) {
        // Remove the trailing newline character that fgets includes
        line[strcspn(line, "\n")] = '\0';
        
        if (strcmp(line, "sum = 5") == 0) {
            printf("BINGO! You hit the target sum of 5!\n");
            // Optional: Exit once we hit bingo, or keep listening
            // exit(EXIT_SUCCESS);
        }
        
        // Uncomment below for debugging to see what's being received
        // printf("Received line: %s\n", line);
    }
    return 0;
}

How to Compile and Run #

1. Compile both programs first:

   gcc summer.c -o summer
   gcc bingo.c -o bingo
   

2. Run them connected with a pipe:

   ./summer | ./bingo
   

   Now when you enter numbers, once the sum hits 5, bingo will shout out its success!

Quick Notes #

• The fflush(stdout) calls are critical here—without them, summer's output gets buffered up and bingo won't see it until the buffer fills up, which breaks the interactive flow.
• Using fgets instead of scanf("%s") ensures we capture the entire line of output from summer, including spaces, so the string comparison works as expected.
• Adding error handling for scanf prevents the program from hanging if you enter something that isn't a number.

内容的提问来源于stack exchange，提问作者BlackBrain