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Flexible Array Member Allocation and Memory Layout Optimization

C99 introduced Flexible Array Members (FAMs), allowing a structure to end with an incomplete array type. This technique enables a single malloc call to allocate both the descriptor metadata and contiguous dynamically sized payload data. Performing single-block allocation maximizes cache locality and eliminates secondary pointer dereferencing overhead compared to using a separate heap pointer member.

snippet.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
typedef struct {
    size_t length;
    char payload[]; /* C99 Flexible Array Member */
} DynamicBuffer;
 
DynamicBuffer* create_buffer(const char *data) {
    size_t len = strlen(data);
    /* Allocate header and variable payload in a single contiguous memory block */
    DynamicBuffer *buf = malloc(sizeof(DynamicBuffer) + len + 1);
    if (!buf) return NULL;
    buf->length = len;
    memcpy(buf->payload, data, len + 1);
    return buf;
}
 
int main(void) {
    DynamicBuffer *buf = create_buffer("Expert C Memory Layout");
    if (buf) {
        printf("Len: %zu, Content: %s\n", buf->length, buf->payload);
        free(buf);
    }
    return 0;
}

Breakdown

char payload[]; /* C99 Flexible Array Member */

Declares an unsized trailing array inside the struct, contributing zero bytes to sizeof(DynamicBuffer).

DynamicBuffer *buf = malloc(sizeof(DynamicBuffer) + len + 1);

Allocates a single contiguous memory region housing both struct header metadata and the string payload.

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Flexible Array Member Allocation and Memory Layout Optimization

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