Growing a buffer in C

Doubling on demand, and never assigning realloc's result over your only pointer.

#include <stdlib.h>

int push(int **values, size_t *len, size_t *cap, int value) {
    if (*len == *cap) {
        size_t bigger = (*cap == 0) ? 4 : *cap * 2;
        int *grown = realloc(*values, bigger * sizeof(int));
        if (grown == NULL) {
            return 0;
        }
        *values = grown;
        *cap = bigger;
    }
    (*values)[*len] = value;
    (*len)++;
    return 1;
}

How it works

1. Growth by doubling keeps the amortised cost constant.
2. If realloc fails it returns NULL and the old block survives.
3. So assign to a temporary and only then replace the pointer.

Keywords and builtins used here

• NULL
• if
• int
• push
• return
• size_t
• sizeof

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Step 4 of 5 in Dynamic memory, step 13 of 25 in Pointers & memory.

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