Segmentation fault thread (monitor) - c

Question

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <errno.h>
#define N_ASS 4
pthread_t tid[N_ASS];
//mutex
pthread_mutex_t mutex;
//variabili condition
pthread_cond_t c_ass[N_ASS], c_pass[N_PASS];
void * checkDoc ()
{
printf("do stuff\n");
}
int main()
{
int err;
int i = 0;
for(i;i<N_ASS;i++){
err = pthread_create (&tid[i], NULL, checkDoc(), NULL);
pthread_cond_wait(&c_ass[i],&mutex);
}
for(i=0;i<N_ASS;i++)
pthread_cond_signal(&c_ass[i]);
for(i=0;i<N_ASS;i++) {
err = pthread_join(tid[i],NULL);
printf("%d joined\n",i);
}
printf("End\n");
}
I just want to say that I'm a little noob who is just learning and this is my first question, if I have to do something else just tell me and I'll do it!
So this is the code, I got one first problem, it makes segmentation-fault and I don't know why.
The output is:
do stuff //1,2,3 or 4 time
Segmentation fault
Nothing else, i runned at least 20 time.

Answer 1

You should be getting compiler warnings here, as the thread function (or rather what is supposed to be the thread function) doesn't return anything.
This will lead to undefined behavior because you don't pass a pointer to the checkDoc function to pthread_create, you actually call the checkDoc function, and use the returned pointer as the pointer to the thread function. As the checkDoc function doesn't actually return anything you will have undefined behavior, and most likely a crash.
Simple solution? Don't call the checkDoc function:
pthread_create (&tid[i], NULL, checkDoc, NULL)
// ^^^^^^^^
// No longer calling the function
And if you state that a function returns a value, you should actually do it. If the returned value is not used then just return NULL.
Also, in C when you declare a function without any formal arguments, it's the same thing as saying that the function accepts any number of arguments of any type. POSIX thread functions must specifically accept a void * argument. If you do not use the argument in the thread function, then simply don't name the argument:
void * checkDoc (void *) { ... }
This last bit also has implications for the main function, as the C specification explicitly says that the main function should either take two arguments (an int and an char *[]), or be void. If you have a function which doesn't take any arguments you must explicitly say that by using void as argument.
There are also other issues with your code. Like you not initializing the structures you use properly. Or like you waiting for the condition variable right after creating the each thread, and as you don't signal the condition variable until after you start waiting for it then you will wait forever and the code will not continue to the part where you actually call the signalling function.