Semaphore and alternating printing - c

Question

I want the following: first the parent process prints "hello", then the child process prints "world" and this is done 3 times. When I execute my code I only get hello world once but I want to get it printed 3 times.
#include <semaphore.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
int main(int argc, char*argv[]){
char* parent="hello ";
char* child="world";
sem_t *sem = sem_open("/mysem", O_CREAT, 0644, 1);
sem_t *sem1=sem_open("/mysem1", O_CREAT, 0644, 0);
int pid=fork();
if(pid<0){
printf("error in forking");
return -1;
}
int cnt=0, cnt1=0;
if(pid==0){
sem_wait(sem1);
write(1, child, strlen(child));
cnt++;
if(cnt1<3){
sem_post(sem);
}
}else{
sem_wait(sem);
write(1, parent, strlen(parent));
cnt++;
if(cnt<3){
sem_post(sem1);
}
}
return 0;
}

Answer 1

maybe try this :
#include <semaphore.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#define BUFFER_SIZE 7
int main(int argc, char **argv)
{
char parent[BUFFER_SIZE] = "hello ";
char child[BUFFER_SIZE] = "world\n";
sem_t *sem = sem_open("/mysem", O_CREAT, 0644, 1);
sem_t *sem1 = sem_open("/mysem1", O_CREAT, 0644, 0);
int cnt = 0;
int pid = fork();
if (sem == SEM_FAILED || sem1 == SEM_FAILED) {
printf("Error : sem_open() call failed.\n");
return (-1);
}
if (pid < 0) {
printf("error in forking");
return (-1);
}
if (pid == 0) {
while (cnt < 3) {
sem_wait(sem1);
if (write(1, child, strlen(child)) == -1) {
printf("Error : write call failed.\n");
}
cnt++;
sem_post(sem);
}
}
else {
while (cnt < 3) {
sem_wait(sem);
if (write(1, parent, strlen(parent)) == -1) {
printf("Error : write function failed.\n");
}
cnt++;
sem_post(sem1);
}
}
sem_close(sem);
sem_close(sem1);
return (0);
}
Explanation :
#define BUFFER_SIZE 7 this is a macro, 7 because of part 2.
Memory leaks (use valgrind)
Don't do : char *str = "abc";
but do : char str[string_size + 1] = "abc";
Because char *str = doesn't allocate memory so you will probably get memory leaks, always allocate string_size + 1 with C String. You can use malloc too to allocate your own size.
Put your int cnt variables before the fork(), fork will copy all memory so you can put only one variable before your fork call, for more readable code.
You should always call sem_close() as soon as possible and you should always look for returned value from functions that are not your, use C man, for exmaple "man 2 write" in your linux console (not looking for sem_wait semp_post and sem_close returned value shouldn't be problematic here, but you can still print a message if you want).
Your main problem here is that you should switch your if (cnt < 3) into a loop as i did with while, your parent and child process only course once write() if you don't set a loop.
The most important thing, please look for coding style rules to make your code more redeable.
This is pretty long, but you should take at look in all i said.
Compilation :
gcc file.c -pthread
./a.out
I'm on linux, fedora32.
Output :
hello world
hello world
hello world