$ cat producer1.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <semaphore.h>
#include <sys/sem.h>
#define MEM_SZ 4096
struct shared_use_st
{
        sem_t p;
        sem_t c;
        char some_text[MEM_SZ];
};
int main()
{
int running = 1;
void *shared_memory = (void *)0;
struct shared_use_st *shared_stuff;
char buffer[MEM_SZ];
int shmid;
shmid = shmget((key_t)1234, MEM_SZ, 0666 | IPC_CREAT);
if(shmid == -1)
{
printf("shmget failed\n");
exit(EXIT_FAILURE);
}
shared_memory = shmat(shmid, NULL, 0);
if(shared_memory == (void *)-1)
{
printf("\nshmat failed\n");
exit(EXIT_FAILURE);
}
printf("Memory attached at %x\n", (int)shared_memory);
shared_stuff = (struct shared_use_st *)shared_memory;
sem_init(&(shared_stuff->p), 1, 1);
sem_init(&(shared_stuff->c), 1, 0);
while(running)
{
        printf("\nwaiting for client...\n");
        sem_wait(&(shared_stuff->p));
        printf("Enter some text: ");
        fgets(buffer, MEM_SZ, stdin);
        strcpy(shared_stuff->some_text, buffer);
        if(strncmp(buffer, "end", 3) == 0)
        {
                running = 0;
        }
        sem_post(&(shared_stuff->c));
}
if(shmdt(shared_memory) == -1)
{
printf("shmdt failed\n");
exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}

 
$ gcc producer1.c -o producer1
Undefined                       first referenced
 symbol                             in file
sem_init                            /var/tmp//ccjymEo2.o
sem_post                            /var/tmp//ccjymEo2.o
sem_wait                            /var/tmp//ccjymEo2.o
ld: fatal: Symbol referencing errors. No output written to producer1
collect2: ld returned 1 exit status