fork子进程文件复制正常，父进程生成空文件的作业问题求助

Hey there! Let's figure out why your parent process's mainlog.txt is ending up empty. Based on your code snippet and the test file content, I can spot a couple of key issues causing this:

1. Shared File Resources Conflicting After fork() #

When you open files before calling fork(), both the parent and child processes get copies of the user-space FILE structs, but they share the underlying kernel file descriptors and file offset pointers. This leads to messy behavior:

• If the child reads first, it moves the shared kernel file offset, so the parent might start reading from the end of the file (hence nothing to write).
• C's buffered I/O adds another layer of confusion—each process has its own buffer, so data read by one might not be visible to the other.

Fix Option 1: Open Files After Fork (Cleanest Approach) #

Have each process open its own file handles after forking. This completely avoids shared resource conflicts:

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

int main(){ 
    int c1; 
    char str[100]; 

    c1 = fork(); 
    if(c1 == 0){
        // Child process: handle lines starting with '1'
        FILE *fp_in = fopen("test.txt","r");
        FILE *fp_out = fopen("child1log.txt","w");
        
        // Always check if file opens succeeded!
        if(!fp_in || !fp_out){
            perror("Failed to open file");
            exit(1);
        }

        while(fgets(str, sizeof(str), fp_in) != NULL){
            if(str[0] == '1'){
                fputs(str, fp_out);
            }
        }

        fclose(fp_in);
        fclose(fp_out);
        exit(0); // Exit child to avoid running parent code
    } else if(c1 > 0){
        // Parent process: handle lines starting with '0'
        FILE *fp_in = fopen("test.txt","r");
        FILE *fp_out = fopen("mainlog.txt","w");
        
        if(!fp_in || !fp_out){
            perror("Failed to open file");
            exit(1);
        }

        while(fgets(str, sizeof(str), fp_in) != NULL){
            if(str[0] == '0'){
                fputs(str, fp_out);
            }
        }

        fclose(fp_in);
        fclose(fp_out);
        wait(NULL); // Wait for child to finish to avoid zombies
    } else {
        perror("Fork failed");
        exit(1);
    }
    return 0;
}

2. Missing Critical Guardrails in Your Original Code #

If you must open files before forking, you need to fix the buffer and offset issues:

• Call fflush(fp1) in both parent and child right after forking to sync user-space buffers with the kernel.
• Use fseek(fp1, 0, SEEK_SET) to reset the file pointer to the start for each process.
• Always check fopen() return values—you might be silently failing to open a file.
• Have the child call exit(0) after its work, so it doesn't execute the parent's code path.
• Parent must call wait() to ensure the child finishes its I/O before exiting.

Quick Test Tip #

After running your fixed code, verify:

• child1log.txt contains 1 text to be copied to child1.
• mainlog.txt contains 0 text to be copied to parent

内容的提问来源于stack exchange，提问作者Elvyn Liew