c - How to prevent fork from returning twice,

问题描述

so I am working on this C code, that print a list of child PID and parent PID

the problem that my output looks like this

digraph {
           "12896" [ label="pid 12896, level 0" ];
    "12897" [ label="pid 12897, level 1" ];
    "12896" -> "12897";
    "12898" [ label="pid 12898, level 2" ];
    "12897" -> "12898";
    "12899" [ label="pid 12899, level 3" ];
    "12898" -> "12899";
    "12900" [ label="pid 12900, level 4" ];
    "12899" -> "12900";
}digraph {
           "12896" [ label="pid 12896, level 0" ];
    "12897" [ label="pid 12897, level 1" ];
    "12896" -> "12897";
    "12898" [ label="pid 12898, level 2" ];
    "12897" -> "12898";
    "12899" [ label="pid 12899, level 3" ];
    "12898" -> "12899";
digraph {
           "12896" [ label="pid 12896, level 0" ];
    "12897" [ label="pid 12897, level 1" ];
    "12896" -> "12897";
    "12898" [ label="pid 12898, level 2" ];
    "12897" -> "12898";
digraph {
           "12896" [ label="pid 12896, level 0" ];
    "12897" [ label="pid 12897, level 1" ];
    "12896" -> "12897";
digraph {

there is 4 different digraph object, I only need the first one. also I am not sure why the first line inside digraph is tabbed, if anyone knows

here is the C code of my fork


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

int main(void){

    int i, status;
    printf( "Enter a value for n :");
    int n;
    scanf("%d", &n);

    FILE *file;
    file = fopen("./diagraph.txt", "w+"); //my output file
    fprintf(file, "digraph {\n");
    for (i=0; i<=n; i++){

        //fflush(stdout);
        int pid = fork();

        if (pid == 0 ) {

                if(i == 0){ //first level
                        fprintf(file, "    \"%d\" [ label=\"pid %d, level %d\" ];\n", getppid(), getppid(), i, file);
                        fprintf(file, "    \"%d\" [ label=\"pid %d, level %d\" ];\n", getpid(), getpid(), i +1, file);
                        fprintf(file, "    \"%d\" -> \"%d\";\n", getppid(), getpid(), file);
                }else{
                        fprintf(file, "    \"%d\" [ label=\"pid %d, level %d\" ];\n", getpid(), getpid(), i +1, file);
                        fprintf(file, "    \"%d\" -> \"%d\";\n", getppid(), getpid(), file);

                }
        }
        else{ //parent process, we will wait for the children.
        pid = waitpid(-1, &status, 0);
        return 0;
        }
    }

fprintf(file, "}");
fclose(file);

}

I am not sure of the reason of this behavior, I would like to understand it and know how to fix it thank you in advance

标签： c

默认情况下，当您使用它写入文件时，fprintf它实际上并没有写入文件（还）——它只是写入缓冲区。当缓冲区填满时，或者您稍后调用fflushorfclose时，缓冲区中的数据将写入文件。缓冲区是存储在进程内存中的 FILE 对象的一部分，因此当您调用时fork，缓冲区及其内容会被复制。这导致子和父可能将相同的数据写入文件的情况，从而导致重复。

避免这种情况的最简单方法是确保在调用 fork 之前调用fflush所有FILE对象，以便缓冲区全部为空并且没有数据重复。

c - How to prevent fork from returning twice,

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