Store a variable-length payload after a structure header.

C: A flexible array member lets one allocation hold metadata and trailing bytes.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <stdint.h>
#include <stdbool.h>
#include <errno.h>
#include <limits.h>
#include <math.h>
#include <time.h>

int main(void) {
    struct Packet { size_t length; unsigned char data[]; };
    size_t count=3;
    struct Packet *packet=malloc(sizeof *packet+count);
    if (packet==NULL) return 1;
    packet->length=count;
    memcpy(packet->data,"abc",count);
    printf("%zu %c\n",packet->length,packet->data[2]);
    free(packet);
    return 0;
}

Expected output EXAMPLE

3 c

Details

Save as snippet.c; compile with gcc -std=c17 -Wall -Wextra snippet.c -lm -o snippet, then run ./snippet. A flexible array member lets one allocation hold metadata and trailing bytes.

Keep in mind

Check allocation-size addition for overflow when count comes from external input.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:02:34 UTC · gcc (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0 · gcc 4:13.2.0-7ubuntu1

Test inputs, results and environment

The exact published code ran in a dedicated Linux test container with prepared inputs. Unprivileged snippet process, external networking disabled, read-only root filesystem, and a fresh temporary working directory. Fixtures and output records are isolated from the application database. Testing covers this example and its stated checks, not every input, operating system, or runtime version. The Linux container shares its host's kernel; this is not a native Windows test.

Architecture / host kernel: x86_64 / 6.18.54.1-microsoft-standard-WSL2
Snippet identity / locale: uid=65534, gid=65534 (unprivileged) / C.UTF-8
Interpreter invocation: gcc -std=c17 -Wall -Wextra -Werror snippet.c -lm -o snippet && ./snippet
Container image ID: sha256:32dc2d998158b6554c99b73b533f423696a0f3e31e8cbea9db10b5eee9a2fadd
Code SHA-256: 30d636a8962ef26bbde2bf671a51382eb514bae6e6302ab2b11a3cef4510041c
Dependencies: gcc 4:13.2.0-7ubuntu1

Test input setup

These are the prepared inputs used by the test harness. See Details above for the prerequisites and how to run this example.

# No input files required.

Observed standard output

3 c

Exit code: 0. Standard error: empty. Validation: Exact stdout comparison. System-dependent values above are one recorded run.