Callbacks with context in C

The C convention for a callback: the function pointer plus a void pointer of your own.

typedef void (*Visitor)(int value, void *user);

void each(const int *values, size_t n, Visitor visit, void *user) {
    for (size_t i = 0; i < n; i++) {
        visit(values[i], user);
    }
}

static void accumulate(int value, void *user) {
    long *total = user;
    *total += value;
}

long sum_via_callback(const int *values, size_t n) {
    long total = 0;
    each(values, n, accumulate, &total);
    return total;
}

How it works

1. The void *user argument carries whatever the caller needs.
2. The library never inspects it, only hands it back.
3. This is how qsort_r, threads and event loops all look.

Keywords and builtins used here

• accumulate
• const
• each
• for
• int
• long
• return
• size_t
• static
• sum_via_callback
• typedef
• void

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Step 3 of 4 in Generic & function pointers, step 8 of 25 in Pointers & memory.

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