FIFO pipe content contains wrong chars after read

Question

I created a program which looking for a maximum in an integer array parallel on 10 thread and has a parent and a child process. Child calculate the maximum of interval's (array has 1000 elements) maximum and write to the named pipe, parent just reads the max number from the FIFO and print it. When I wanna read content from FIFO, the read content contains some wrong characters but the input string is ok. How can I avoid this problem?

My code:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <sys/wait.h>

#define max 1000

#define Th_max 10

int numbers[max];

int max_num[Th_max] = { 0 };
int thread_no = 0;

void *maximum(void* arg)
  ....
}

int main()
{
    int maxs = 0;
    int i;
    char buf[32];
    int fd, ret;
    int pipefd[2];  
     pid_t cpid;
    pthread_t threads[Th_max];

    srand(time(0));
    for( i = 0; i < 1000; i++ ) {
        numbers[i] = rand();
   }
    cpid = fork();
    if (cpid == -1) {
       perror("fork");
       exit(-1);
    }
    if (cpid == 0) {
        printf("PID: %d : Child process \n",getpid());

        for (i = 0; i < Th_max; i++)
            pthread_create(&threads[i], NULL,
                        maximum, (void*)NULL);



        for (i = 0; i < Th_max; i++)
            pthread_join(threads[i], NULL);


        for (i = 0; i < Th_max; i++) {
            if (max_num[i] > maxs)
                maxs = max_num[i];
        }   
        fd=open("Pipe",O_RDWR);
        
        sprintf(buf, "%d", maxs);
        printf("PID: %d :Write this to FIFO: %s:%ld\n",getpid(),buf,strlen(buf));
        write(fd,buf,strlen(buf)); 

        exit(0);
        
        
     } else {
           printf("PID: %d : Parent process\n",getpid());

           ret=mkfifo("Pipe",00666);    
           if (ret == -1) {
               perror("mkfifo()");
               exit(-1);
            }

           fd=open("Pipe",O_RDWR);
           if (fd == -1) {
               perror("open() error!");
               exit(-1);
            }


            wait(NULL);   
            ret=read(fd,buf,32); 
            printf("PID: %d :Read %d byte: %s\n",getpid(),ret,buf);

            close(fd);
            
            unlink("LA3WXZ");   
    }


    return 0;
    
}

`content contains wrong characters` Could you post an example?> — KamilCuk, May 03 '21 at 21:49
You read 10 characters and then printed more than 10 characters. How were you expecting the `printf` line to know how many characters to print? — David Schwartz, May 03 '21 at 22:06

chux - Reinstate Monica · Accepted Answer · 2021-05-04T01:57:30.423

OP's code is attempting to print an array of characters as a string. It is not the desired string as it lacks a null character after the 10th character..

Instead use a precision to limit printing a character array.

//printf("PID: %d :Read %d byte: %s\n",getpid(),ret,buf);
printf("PID: %d :Read %d byte: %.*s\n",getpid(),ret, ret, buf);
//                              ^^                   ^^^ precision

Or only read up to 1 less character and append a 0.

// ret=read(fd,buf,32); 
ret = read(fd, buf, sizeof buf - 1); 
if (ret >= 0) {
  buf[ret] = '\0';
  ...

Or send the null character.

// write(fd,buf,strlen(buf));
write(fd,buf,strlen(buf) + 1);

Or ...

Note: getpid() does not certainly return an int. Cast to a wide type and print.

printf("%lld\n", (long long) getpid());

FIFO pipe content contains wrong chars after read

1 Answers1