What you need to do is to call pthread_mutex_lock to secure a mutex, like this:
pthread_mutex_lock(&mutex);
Once you do this, any other calls to pthread_mutex_lock(mutex) will not return until you call pthread_mutex_unlock in this thread. So if you try to call pthread_create, you will be able to create a new thread, and that thread will be able to (incorrectly) use the shared resource. You should call pthread_mutex_lock from within your fooAPI function, and that will cause the function to wait until the shared resource is available.
So you would have something like this:
#include <pthread.h>
#include <stdio.h>
int sharedResource = 0;
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
void* fooAPI(void* param)
{
pthread_mutex_lock(&mutex);
printf("Changing the shared resource now.\n");
sharedResource = 42;
pthread_mutex_unlock(&mutex);
return 0;
}
int main()
{
pthread_t thread;
// Really not locking for any reason other than to make the point.
pthread_mutex_lock(&mutex);
pthread_create(&thread, NULL, fooAPI, NULL);
sleep(1);
pthread_mutex_unlock(&mutex);
// Now we need to lock to use the shared resource.
pthread_mutex_lock(&mutex);
printf("%d\n", sharedResource);
pthread_mutex_unlock(&mutex);
}
Edit: Using resources across processes follows this same basic approach, but you need to map the memory into your other process. Here's an example using shmem:
#include <stdio.h>
#include <unistd.h>
#include <sys/file.h>
#include <sys/mman.h>
#include <sys/wait.h>
struct shared {
pthread_mutex_t mutex;
int sharedResource;
};
int main()
{
int fd = shm_open("/foo", O_CREAT | O_TRUNC | O_RDWR, 0600);
ftruncate(fd, sizeof(struct shared));
struct shared *p = (struct shared*)mmap(0, sizeof(struct shared),
PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
p->sharedResource = 0;
// Make sure it can be shared across processes
pthread_mutexattr_t shared;
pthread_mutexattr_init(&shared);
pthread_mutexattr_setpshared(&shared, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&(p->mutex), &shared);
int i;
for (i = 0; i < 100; i++) {
pthread_mutex_lock(&(p->mutex));
printf("%d\n", p->sharedResource);
pthread_mutex_unlock(&(p->mutex));
sleep(1);
}
munmap(p, sizeof(struct shared*));
shm_unlink("/foo");
}
Writing the program to make changes to p->sharedResource is left as an exercise for the reader. :-)
Forgot to note, by the way, that the mutex has to have the PTHREAD_PROCESS_SHARED attribute set, so that pthreads will work across processes.