printf打印相同的语句两次

Question

the next code supposed to write to "file.txt" the PID number and 1 for Parent process or 0 for child process.

Im not sure if the code working as it should but im having a strange problem with Printf() that making trouble . i dont understand why, but printf prints the same statement twice.

Code:

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>


void print_pids(int fd,int n){
int i,p;
char msg[99];

for(i=n;i>0;i--){
    p=fork();
    if(p>0){
        sprintf(msg,"My generation is 1.My pid is %d\n",getpid());
        write(fd,msg,33);
        wait();
    }
    if(p==0){
        sprintf(msg,"My generation is 0.My pid is %d\n",getpid());
        write(fd,msg,33);
    }
    if(p<0){
        printf("cannot fork");
        exit(0);
    }
}


}

void main(){
    int fd;
    char buf[99];
    fd=open("file.txt",O_WRONLY,700);
    print_pids(fd,1);
    close(fd);
    fd=open("file.txt",O_RDONLY,700);
    read(fd,buf,35);
    printf(" %s\n",buf);
    close(fd);
    return;


}

instead of printing

 My generation is 1.My pid is 8022

it prints

 My generation is 1.My pid is 8
 My generation is 1.My pid is 8

why is that?

thanks!

Answer 1

The child doesn't exit in print_pids() , so it goes back to main() and opens the file, reads it, prints it, and then exits. The parent does the same, but only after the child has died. If you printed the PID of the process doing the print operations, you'd be better informed.

The use of write() with fixed size buffers is worrying, too. And there's no error checking.

Here's a fixed version of your code — more relevant headers, calling wait() correctly (you're unlucky your code didn't crash), printing the extra diagnostic information, writing the full length of the messages, reading and printing the full length of the messages (even though there isn't a null terminator), using an octal number ( 0600 ) rather than a decimal number ( 700 ) for the permissions, etc.

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

static void print_pids(int fd, int n)
{
    int i, p;
    char msg[99];

    for (i = n; i > 0; i--)
    {
        p = fork();
        if (p > 0)
        {
            sprintf(msg, "My generation is 1. My pid is %d\n", getpid());
            write(fd, msg, strlen(msg));
            int status;
            int corpse = wait(&status);
            printf("Child %d exited with status 0x%.4X\n", corpse, status);
        }
        if (p == 0)
        {
            sprintf(msg, "My generation is 0. My pid is %d\n", getpid());
            write(fd, msg, strlen(msg));
        }
        if (p < 0)
        {
            printf("cannot fork");
            exit(0);
        }
    }
}

int main(void)
{
    int fd;
    char buf[99];
    fd = open("file.txt", O_WRONLY|O_CREAT|O_TRUNC, 0600);
    print_pids(fd, 1);
    close(fd);
    fd = open("file.txt", O_RDONLY);
    int nbytes = read(fd, buf, sizeof(buf));
    printf("%.5d: %.*s\n", (int)getpid(), nbytes, buf);
    close(fd);
    return 0;
}

Sample output:

33115: My generation is 1. My pid is 33112
My generation is 0. My pid is 33115

Child 33115 exited with status 0x0000
33112: My generation is 1. My pid is 33112
My generation is 0. My pid is 33115

Notice how getting the full length of message helps you see what's going on. Your messages were truncating the output, so you didn't see the full PIDs. And both processes write into the file (about 72 characters in total). (There could be some timing issues to alter what is seen — I got at least one anomalous result with only one of the 'My generation' messages in it, but I couldn't reproduce that reliably.)