Interprocess communication sending structure via pipe - c

Question

I have to show the elapsed time to run a command using pipes, so I use the gettimeofday function and pass the pointer through the pipe, but when I'm reading it in the father process I get a different result than expected. I'm supossed to get 1604626608037896 but instead I have this 94762421350317.
This is my code:
if (pipe(p) < 0)
exit(1);
pid=fork();
if (pid == -1){
printf("can't fork, error occured\n");
exit(EXIT_FAILURE);
}else if (pid == 0){
//This is the child I get the time
gettimeofday(&current,NULL);
//I write it in the pipe, if I print the time here i get the result I want
write(p[1], &current, sizeof(current));
close(p[1]);
//then I execute the command
}else{
//this is the parent
if (waitpid(pid, &status, 0) > 0){
close(p[1]);
struct timeval inicio;
//Here is the problem I think, once I tried to read it I get a random number
read(p[0], &inicio, sizeof(inicio));
printf(": %ld,%ld\n", inicio.tv_sec,inicio.tv_usec);
}else{
printf("waitpid() failed\n");
}
exit(0);
}

Answer 1

I believe you are expecting that the if block runs in the child process and else block is running in parent process and the fork is called before the code segment. In that case the p in two process are different for two process and they are not connected.
Your read call if throwing a error. Can you check the return value at of the read system call?
The modified code snipped should looks like
if (pipe(p) < 0)
exit(1);
if (fork() < 0)
exit(1);
if (pid == 0){
//I get the time
gettimeofday(&current,NULL);
//I write it in the pipe, if I print the time here i get the result I want
if (write(p[1], &current, sizeof(current)) < 0) {
perror("write");
exit(1);
}
close(p[1]);
//then I execute the command
}else{
if (waitpid(pid, &status, 0) > 0){
close(p[1]);
struct timeval inicio;
//Here is the problem I think, once I tried to read it I get a random number
if (read(p[0], &inicio, sizeof(inicio)) < 0) {
perror("read");
exit(1);
}
printf(": %ld,%ld\n", inicio.tv_sec,inicio.tv_usec);
}else{
printf("waitpid() failed\n");
}
exit(0);
}