Append text to a file.

Use append mode in C without replacing the existing file.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <errno.h>
#include <limits.h>

int main(void) {
    FILE *file = fopen("notes.txt", "a");
    if (!file) return 1;
    int failed = fputs("second\n", file) == EOF;
    if (fclose(file) == EOF) failed = 1;
    if (failed) return 1;
    printf("Appended\n");
    return 0;
}

Expected output EXAMPLE

Appended

Details

Save as snippet.c. Compile with gcc -std=c17 -Wall -Wextra snippet.c -o snippet, then run ./snippet. Uses the C standard library.

✓ Verified

Tested on Ubuntu 24.04.5 LTS · 2026-11-02 18:00:47 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: 9eb7d52306325d8b436e8ff07b1d26c871ed0965b68f700d9894471e6f8ba5c7
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.

printf 'first\n' > notes.txt

Observed standard output

Appended

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

Additional assertions that passed

Checks recorded by the test harness. Where Python assertions are shown, browser cases use Playwright and simulator cases may inspect pin transitions: result.stdout is captured output, work is the temporary directory, and Path is pathlib.Path. The harness also imports json and csv.

assert Path('notes.txt').read_text() == 'first\nsecond\n'

# C # append