I'm not convinced I should be allowed to do this, C.

onceperthread.m

#import <Foundation/Foundation.h>
#include <pthread.h>

void logExactlyOnce(void)
{
    static __thread dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"Only done once?");
    });
}

void *threadStart(__unused void *unused)
{
    pthread_detach(pthread_self());
    logExactlyOnce();
    return NULL;
}

void makeATonOfThreads()
{
    const int nThreads = 10;
    pthread_t threads[nThreads] = {0};
    for (int i = 0; i < nThreads; i++) {
        pthread_t thisThread = threads[i];
        __unused int win = pthread_create(&thisThread, NULL, threadStart, NULL);
    }
}

int main(int argc, const char * argv[])
{
    @autoreleasepool
    {
        makeATonOfThreads();
        sleep(1);
    }
}

Can't make this run. Getting error "Untitled.m:22:23: error: variable-sized object may not be initialized
pthread_t threads[nThreads] = {0};
^~~~~~~~
1 error generated." Hjälp?

You're building with -std=c99, don't do that and it works :). The array definition requires std=gnu99.

You never said anything about GNU stuff :-p Anyway, I changed nThreads into a #define, that works, too.

Blocks are hardly strict C99…