Here is what my code is intended to do :
The child should:
1. Sends a character to the parent
2. Receive integer from parent and print it
The parent should:
1. Read the character sent from the child and print it
2. Cast it to an integer and send the result to the child
Here is the code I wrote:
#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<string.h>
#include<sys/wait.h>
int main()
{
int fd1[2];
int fd2[2];
pid_t p;
if (pipe(fd1)==-1)
{
fprintf(stderr, "Pipe Failed" );
return 1;
}
if (pipe(fd2)==-1)
{
fprintf(stderr, "Pipe Failed" );
return 1;
}
p = fork();
if (p<0)
{
fprintf(stderr, "fork Failed" );
return 1;
}
if (p==0){
char c='a';
int received;
close(fd1[0]);
write(fd1[1], c, sizeof(char));
close(fd1[1]);
close(fd2[1]);
read(fd2[0], received, sizeof(int));
printf("Printing from child ");
printf(" ");
printf("%d", received);
close(fd2[0]);
}
if (p >0)
{
char received;
close(fd1[1]);
read(fd1[0], received, sizeof(char));
printf("Printing from parent ");
printf(" ");
printf("%c", received);
close(fd1[0]);
close(fd2[0]);
int test=(int)received;
write(fd2[1], test, sizeof(test));
close(fd2[1]);
}
}
My current output is the following: Printing from parent Printing from child 0
I am assuming the parent is reading from the pipe before the child writes to it, how to fix that?
I am assuming the parent is reading from the pipe before the child writes to it, how to fix that?
This assumption is false. The error is one that a good compiler should have warned about - you missed that not the values of the variables c, received and test have to be passed to write and read, but their addresses:
write(fd1[1], &c, sizeof(char));
…
read(fd2[0], &received, sizeof(int));
…
read(fd1[0], &received, sizeof(char));
…
write(fd2[1], &test, sizeof(test));
May I ask how the computer ensures the scenario I assumed doesn't happen?
The read from the pipe, just as with a terminal device, simply blocks until there's something to read (provided that the file descriptor hasn't explicitly been set to non-blocking).