C Playground Guide

Open the C Playground to compile and run small C programs without installing a toolchain.

Read the default result

The initial program sums {2, 4, 6, 8} and prints sum: 20. When you press Run, the compiler messages appear first if there are warnings or errors, then the program output. Compiler diagnostics are shown, so you can fix a typo and Run again without losing the source.

Find prime numbers

Choose Find prime numbers. It prints:

Primes: 2 3 5 7 11 13 17 19 23 29

The predicate rejects integers below 2, then tests divisors up to d * d <= n. Any larger factor would have a matching smaller factor. Change n <= 30 to extend the range.

Structs and functions

OOP: bank account defines an Account struct and passes it by pointer to deposit and withdraw. Depositing 100 and withdrawing 30 prints Balance: 70. C has no methods, so the functions take Account * and update the field through ->.

Prepare input and read it

The Read standard input example fills the input field with Ilham:

#include <stdio.h>

int main(void) {
    char name[64];
    if (fgets(name, sizeof(name), stdin) == NULL) {
        puts("No input provided.");
        return 1;
    }
    for (int i = 0; name[i] != '\0'; i++) {
        if (name[i] == '\n') name[i] = '\0';
    }
    printf("Hello, %s!\n", name);
    return 0;
}

It prints Hello, Ilham!. fgets stops after sizeof(name) - 1 characters and leaves the newline, which the loop removes. Provide another line for each additional read. If the program reports no input, add the missing answer and Run again.

Star patterns and errors

Centered pyramid prints:

    *
   ***
  *****
 *******
*********

It prints height - row spaces and 2 * row - 1 stars for each row. Copy output preserves the spacing. Other examples cover arrays, sorting, binary search, letter frequency and hollow or inverted pyramids.

Syntax errors, missing headers and type warnings come from Clang and appear in the output. Stop terminates a stuck program. Runtime assets are fetched from jsDelivr; an unavailable CDN prevents execution, not editing. The plain editor works without Monaco, and no C formatter is bundled.

References

Questions you might have

How do I read input?: Fill Standard input before Run, one answer per line, then call fgets or scanf. fgets keeps the trailing newline, so strip it before printing. Missing input returns NULL from fgets. This is prepared input, not a live terminal.
Why does the first Run take longer?: The Clang compiler, linker and sysroot download from jsDelivr on the first Run — roughly 26 MiB compressed. Runtime loading has a 90-second timeout, followed by a separate 30-second execution limit. Later Runs in the same page reuse the loaded toolchain.
Can I use exceptions, threads, fork or sockets?: This is a WASI build of Clang, not a desktop toolchain. Exceptions are unavailable in C, and threads, fork, and native OS processes are not supported. The standard C library works for console programs.
Does selecting an example erase my code?: Yes. C examples are complete programs with their own main, so clicking one replaces the editor and pauses Auto-run. Copy anything you want to keep first.
Is this a security sandbox?: No. A Worker prevents direct DOM access and can be terminated, but compiled code can make network requests through WASI. The tool itself does not upload or persist code or input. Each Run starts fresh; output is limited to 500 rows.