linux编程|fork函数讲解

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[root@slave bdkyr]# cat fork_test.c

* create by bdkyr

* date 2015-1-22

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
#include <stdarg.h>
#include <string.h>

#define MAXLINE 4096   /* max line length */

int glob = 6;
char buf[]= "a write to stdout\n";

void err_sys(const char *fmt, ...);
static void err_doit(int, int, const char *, va_list);

int main(void){
        int var;
        pid_t pid;
        var = 88;
        if (write (STDOUT_FILENO, buf, sizeof(buf) -1) != sizeof(buf)-1)
                err_sys("write error");
        printf("before fork\n");
        if((pid = fork()) < 0){
                err_sys("fork error");
        }else if(pid == 0){
                glob++;
                var++;
        }else{
                sleep(2);
        }
        printf("pid=%d, glob = %d, var = %d\n", getpid(),glob, var);
        exit(0);
}

void err_sys(const char *fmt, ...){
        va_list ap;
        va_start(ap, fmt);
        err_doit(1, errno, fmt, ap);
        va_end(ap);
        exit(1);
}

static void err_doit(int errnoflag, int error, const char *fmt, va_list ap){
        vsnprintf(buf, MAXLINE, fmt, ap);
        if(errnoflag)
                snprintf(buf+strlen(buf), MAXLINE-strlen(buf), ": %s", strerror(error));
        strcat(buf, "\n");
        fflush(stdout);
        fputs(buf, stderr);
        fflush(NULL);
}
[root@slave bdkyr]# gcc fork_test.c -o fork_test
[root@slave bdkyr]# ./fork_test
a write to stdout
before fork
pid=5209, glob = 7, var = 89
pid=5208, glob = 6, var = 88

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