Read-write locks in C

Many readers together, or one writer alone.

#define _POSIX_C_SOURCE 200809L

#include <pthread.h>

typedef struct {
    int values[16];
    size_t count;
    pthread_rwlock_t lock;
} Table;

void table_init(Table *t) {
    t->count = 0;
    pthread_rwlock_init(&t->lock, NULL);
}

int table_append(Table *t, int value) {
    pthread_rwlock_wrlock(&t->lock);
    int ok = t->count < 16;
    if (ok) {
        t->values[t->count++] = value;
    }
    pthread_rwlock_unlock(&t->lock);
    return ok;
}

size_t table_size(Table *t) {
    pthread_rwlock_rdlock(&t->lock);
    size_t n = t->count;
    pthread_rwlock_unlock(&t->lock);
    return n;
}

How it works

rdlock may be held by several threads at once.
wrlock waits for every reader to leave.
Both are released by the same unlock call.

Keywords and builtins used here

NULL
if
int
return
size_t
struct
table_append
table_init
table_size
typedef
void

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Step 3 of 4 in Locking, step 6 of 10 in Threads & concurrency.

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