File Handling in Memory in C

File Handling

Write and read a temporary runtime file while understanding Circuit’s filesystem boundary.

Complete program

#include <stdio.h>

int main(void) {
    FILE *file = fopen("note.txt", "w");
    if (!file) { perror("open for writing"); return 1; }
    if (fputs("Hello from a file\n", file) == EOF) { fclose(file); return 1; }
    if (fclose(file) != 0) return 1;
    file = fopen("note.txt", "r");
    if (!file) { perror("open for reading"); return 1; }
    char line[64];
    if (fgets(line, sizeof line, file)) printf("Read: %s", line);
    int failed = ferror(file);
    if (fclose(file) != 0) failed = 1;
    return failed ? 1 : 0;
    return 0;
}

Try in Circuit →

Opens a new program without replacing your saved code. Press Run when you are ready.

How it works

Opening with w creates or truncates the runtime file. Closing it finishes that write before the second open reads from the beginning. Each operation can fail, so the example checks the important return values.

Circuit provides an in-memory WASI directory for each execution. This file does not appear on your computer, enter your saved program library, or survive another run. For persistent files and full operating-system APIs, use a local toolchain.

Expected output

Read: Hello from a file

Concepts to practice

• FILE streams
• fopen / fclose
• Error checking
• Ephemeral filesystem

Predict a different input or change one value, then compare the result with your prediction. Keep changes within the documented bounds and read warnings before trusting an answer.