bad output from pipe with multiple processes - c

Question

In the following code, I have two pipes, one, fd[] handles ferrying a range variable to the child processes. The other pipe rw[] is responsible for printing the results of the method. fd works correctly, but rw prints garbage. Both range and narc_num are long and I have sucessfully printed a string and a char through the rw pipe. Any ideas? Thanks.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <math.h>
#include <string.h>
/**
* process propigating version of narcisstic base-10 number generator
**/
/**
* A Narcissistic number is a number where all of its digits, when raised to the power n where n is the number of digits in its number, equla the number itseld.
* examples:
* 2^1 = 2
**/
void isNarcissisticNumber(int rw[], long min, long max){
// printf("min %ld max %ld\n", min, max);
long n;
for(n = min; n <= max; n++){
long num = n;
int digits = floor(log10(num)) + 1;
int digit_arr[digits];
long narc_num = 0;
int index = 0;
do{
digit_arr[index++] = num%10;
num /= 10;
}while(num > 0);
index = 0;
for(index; index < digits; index++){
narc_num += pow(digit_arr[index], digits);
}
if(narc_num == n){
printf("%ld\n", n);
// parent: writing only, so close read-descriptor.
close(rw[0]);
write(rw[1], &n, sizeof(long));
// close the write descriptor
close(rw[1]);
}
}
}
int main(int argc, // Number of strings in array argv
char *argv[]){ // Array of command-line argument strings)
//check that there is only one passed in parameter in addition to the program name at argv[0]
if(argc != 2){
printf("Args found: %d. 1 arg required\n", argc);
return;
}
//check that argv passed in is an int
if(!atoi(argv[1])){
printf("argument shoud be the # of processes to proc.\n");
return;
}
int num_processes = atoi(argv[1]);
//counter for narcissistic numbers
long offset = 10000;
long range= -offset;
//file pipe
int fd[2];
//printing pipe
int rw[2];
int n = 0;
long output;
while(n < num_processes){ // -1 offset to line up array index with num_processes arg
pipe(rw);
pipe(fd);
pid_t process = fork();
pid_t child_process;
int status;
if(process == 0){ //chid process --> execute program
child_process = process;
/* Duplicate the input side of pipe to stdin */
// chid: reading only, so close the write-descriptor
close(fd[1]);
read(fd[0], &range, sizeof(range));
close(fd[0]);
isNarcissisticNumber(rw, range, range+offset-1);
}else if(process != 0){
// parent: writing only, so close read-descriptor.
close(fd[0]);
range += offset;
write(fd[1], &range, sizeof(range));
// close the write descriptor
close(fd[1]);
// for the current child process to complete its routine befire checking for output
wait(&child_process);
//read from the printing pipe
close(rw[1]);
while(read(rw[0], &output, sizeof(long))){
printf("printer %ld\n", output);
}
close(rw[0]);
}else{ //failed to fork
printf("process failed to fork!\n");
return -1;
}
n++;
}
}
EDIT #1: while that made the parent only check after a child completed, it doesn't fix the output of the pipe, which is now just 0 even as the printf shows otherwise.

Answer 1

I'm not sure this will fix all the problems but you definitely have a logic issue. In your computation routine you can write zero or more longs to the output. In your print (parent) routine, you expect exactly one value to come through. So if nothing is sent, your read will fail. If multiple values are sent, you only read the first one. First fix would be to check your return value from read and also loop on it.
Note also - probably also a good idea to wait() on your child process.