How to pass an integer between childprocesses and parent - c

Question

I have a program that is supposed to count the chars in a txt file and use a child process to count the chars and then print out the amount. The parent just feeds the lines in the file to the child in a while loop. The program works in that sense that it can open the file, read it line by line and then if the child process prints the amount it is correct.
But now I want to modify it so that the child process passes back the amount and the parent writes out the nr of chars instead. But when I try to do it the program gives me 1153416175 instead of the real number of chars in the file, and also it just get stuck and I have to kill it. Why is this happening?
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <stdlib.h>
#include <string.h>
#define MAXLINE 100
int main(int argc, char *argv[]) {
int fds[2]; /* file descriptors */
int child; /* child process */
int n; /* size of line in file */
int count, alphacount, total=0; /* used in loop for counting chars in line */
int read_bytes; /* amount of bytes read in read function */
char line[MAXLINE];
FILE *fp;
char *str;
if (argc < 2) {
printf("Format: './rakna <filename>'.\nThe file should have the .txt extension.\n");
exit(1);
} else {
if (pipe(fds) < 0) {
perror("Can't open pipe\n");
exit(1);
}
child = fork();
if (child==-1) {
perror("fork error");
exit(1);
}
/* child that reads chars and returns amount */
if(child == 0)
{
/* close(fds[1]); child process close input of pipe Used to be before ..*/
/* count chars through read */
while (read_bytes = read(fds[0], line, sizeof(line)) > 0) {
/* check if line is received */
if (read_bytes < 0) {
perror("Can't read line");
exit(1);
}
/* count chars in the line */
else {
count=0;
alphacount=0;
while (line[count] != '\0')
{
/* counting chars from 'a' to 'z' */
if (line[count] >= 'a' && line[count] <= 'z')
alphacount++;
/* counting chars from 'A' to 'Z' */
else if (line[count] >= 'A' && line[count] <= 'Z')
alphacount++;
count++;
}
/* adding the nr of chars in line to total */
total += alphacount;
}
}
write(fds[1], &total, sizeof(total)); /* passing total nr of chars to parent */
close(fds[0]); /* closing output part of pipe */
exit(0); /* ending child process */
}
/* parent that sends chars to child-reader and writes out amount */
else
{
/* close(fds[0]); Parent process close output of pipe */
fp = fopen(argv[1], "r");
if (fp == 0) {
perror("Could not read the file.\n");
exit(1);
}
while (fgets(line, MAXLINE, fp) != NULL) {
n = strlen(line);
if (n >= 0) {
line[n]='\0';
if (write(fds[1], line, n) != n) {
perror("write error in pipe");
exit(1);
}
}
else {
perror("problem with fgets");
exit(1);
}
}
int nrofchars;
read_bytes = read(fds[0], &nrofchars, sizeof(nrofchars));
printf("The file has %d nr of chars between a-z\n", nrofchars); //<-- Here it f**ks up, it gives me 1153416175
close(fds[1]); /* closing input part of pipe */
wait(NULL); /* waits for child to read end of file */
}
return 0;
}
}

Answer 1

A pipe is NOT two-way communications, it is unidirectional, fd[0] is the read-end and fd[1] is the write-end.
Your code is using one pipe as bidirectional (you commented out the close of FDs assuming it would become bidirectional) and that's why you are getting the undefined behaviour.
If you want bi-directional you need two pipes :-) OR you can use socketpair(2) to create bidirectional IPC fds.
If you need more help just drop a comment.