Destroying pthread mutex/rwlock in signal handler - c

Question

How to correctly destroy pthread mutex or rwlock in signal handler? For example - in the code below i has main thread and 3 another threads. All threads do some tasks in infinity loop on some array, using mutexes and locks. Because main thread is also doing some task, the only way to exit from program - using signal handler. But in this way i can't destroy my mutex/rwlock object, because there's no guarantee that object is unlocked. And if I'll try to unlock it, of course one of thread will lock it again. And when i'm trying to launch my program again, that print corrupted result. So how can i solve this problem? There is example of my code with rwlocks:
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#include <string.h>
#include <ctype.h>
#include <sys/types.h>
#include <signal.h>
#include <stdlib.h>
#include "thread_data.h"
#include "exchange_types.h"
pthread_rwlock_t rwlock;
unsigned int main_time = 500000;
unsigned int shift_time = 1000000;
unsigned int mirror_time = 2000000;
unsigned int count_time = 4000000;
void signal_handler(int signo) {
// Destroying locked lock or mutex is UB
pthread_rwlock_destroy(&rwlock);
exit(1);
}
void* shift_letter_case_async( void* array ) {
while(1) {
if( pthread_rwlock_rdlock(&rwlock) < 0 )
handle_error("rdlock[shift]");
carray_t* arr = (carray_t*) array;
shift_letter_case( arr->array, arr->size );
if( pthread_rwlock_unlock(&rwlock) < 0 )
handle_error("unlock[shift]");
usleep(shift_time);
}
return NULL;
}
void* mirror_array_async( void* array ) {
while(1) {
if( pthread_rwlock_rdlock(&rwlock) < 0 )
handle_error("rdlock[mirror]");
carray_t* arr = (carray_t*) array;
mirror_array( arr->array, arr->size );
if( pthread_rwlock_unlock(&rwlock) < 0 )
handle_error("unlock[mirror]");
usleep(mirror_time);
}
return NULL;
}
void* count_async( void* array ) {
while(1) {
if( pthread_rwlock_wrlock(&rwlock) < 0 )
handle_error("wrlock[count]");
carray_t* arr = (carray_t*) array;
count_upper_letters( arr->array, arr->size );
if( pthread_rwlock_unlock(&rwlock) < 0 )
handle_error("unlock[count]");
usleep(count_time);
}
return NULL;
}
int main( int argc, char** argv ) {
/* Common data */
char letters[ 'z' - 'a' + 1 ];
size_t letter_len;
carray_t transferer;
/* pthread data */
pthread_t threads[3];
/* Initializing array */
letter_len = sizeof(letters);
for( int i = 0; i < letter_len; i++ )
letters[i] = 'a' + i;
transferer.array = letters;
transferer.size = letter_len;
/* Initializing signal handlers */
if ( signal(SIGINT, signal_handler) == SIG_ERR )
handle_error("signal[SIGINT]");
if ( signal(SIGTERM, signal_handler) == SIG_ERR )
handle_error("signal[SIGTERM]");
/* Initializing locks */
if( pthread_rwlock_init(&rwlock, NULL) < 0 )
handle_error("pthread_rwlock_init");
/* Initializing threads */
if( pthread_create( &threads[0], NULL, shift_letter_case_async, &transferer ) != 0 )
handle_error("phtread_create[shift_letter_case]");
if( pthread_create( &threads[1], NULL, mirror_array_async, &transferer ) != 0 )
handle_error("phtread_create[mirror_array]");
if( pthread_create( &threads[2], NULL, count_async, &transferer ) != 0 )
handle_error("phtread_create[count]");
while(1) {
if( pthread_rwlock_wrlock(&rwlock) < 0 )
handle_error("wrlock[main]");
print_array(letters, letter_len);
if( pthread_rwlock_unlock(&rwlock) < 0 )
handle_error("unlock[main]");
usleep(main_time);
}
return 0;
}

Answer 1

Instead of the very risky-sounding approach you propose, consider shutting down by arranging to signal every thread, to which each one responds by performing an orderly shutdown. Have the main thread join the others in that case so that it knows when they have finished; then it can cleanly tear down any persistent synchronization structures, temporary files, etc..
Alternatively, signal the main thread only, and have it -- in a properly synchronized way -- raise a flag that each other thread will recognize as an instruction to shut down, then proceed as above (join the worker threads, then tear down).

Answer 2

I think my first naive attempt at this would be to change the while(1) of the threads and main to while(running) with "running" defined by:
volatile int running = 1;
Change the signal_handler to:
void signal_handler(int signo) {
running = 0;
}
You can join the threads as normal before the return in main. I didn't run this so I may be completely wrong. Also, you may wish to have "if (running)" predicates for the usleep functions.