C Sandbox

C Sandbox - Run C Code Online Free

C is the lingua franca of systems programming — the Linux kernel, the Python and Node.js runtimes, every embedded firmware, every database engine, every cryptocurrency wallet, and roughly half of all 'high-level' language interpreters are written in it. Yet setting up a local C toolchain (installing GCC/Clang, configuring PATH, learning Makefile basics) is the first wall every new programmer hits. This sandbox removes that wall: paste a snippet, press Run, and your code compiles and executes against GCC 10.2.0 with full C11 standard support in under a second. The execution happens server-side via the open-source Piston API (engineer-man/piston on GitHub) in an isolated Docker container with no network access and a hard 3-second runtime limit (so untrusted code cannot harm anything, and heavy benchmarks may be cut off). You get the program's stdout, any compilation warnings/errors with line numbers, execution time, and exit code. A C Standard picker (C89/ANSI, C99, C11, C17) and a one-click -Werror toggle let you verify portability and conformance, while custom compiler arguments (-Wall, -Wextra, -O0..-O3, -lm, -lpthread) let you experiment with optimization levels and library linking. The stdin field feeds standard input to scanf/getchar/fgets-based programs without rewriting them. Six categorised starter examples — Hello World, for-loop, function definition, array iteration, pointer dereferencing, struct usage — cover the syntax you'd see in week one of any C course.

What is C Sandbox?

C Sandbox is an online C compiler that allows you to run C code directly in your browser without installing anything. Using the Piston API and GCC compiler, it provides:

- Instant code compilation and execution
- Support for standard C library
- Custom compiler flags and arguments
- Standard input (stdin) support
- Error messages and debugging info
- Code examples to learn from

Perfect for students, developers, and anyone learning C programming.

How do I use this C Sandbox?

Using the C Sandbox is simple:

1. Write or paste your C code in the editor
2. (Optional) Add compiler arguments like -Wall -O2
3. (Optional) Provide stdin input if your program needs it
4. Click 'Run Code' to compile and execute
5. View the output or compilation errors
6. Try example codes to learn C basics

You can also download your code as a .c file for later use.

What C features are supported?

The sandbox supports full C programming language features:

- All C11 standard features
- Standard library (stdio.h, stdlib.h, string.h, math.h, etc.)
- Dynamic memory allocation (malloc, calloc, free)
- File I/O operations
- Pointers and pointer arithmetic
- Structures and unions
- Arrays and strings
- Function pointers
- Preprocessor directives

Compiler: GCC 10.2.0 with C11 standard support.

Can I use compiler flags?

Yes! You can add custom compiler arguments in the 'Compiler Arguments' field. Common examples:

- `-Wall` - Enable all warnings
- `-Wextra` - Enable extra warnings
- `-O2` - Optimization level 2
- `-std=c11` - Use C11 standard
- `-lm` - Link math library
- `-g` - Include debugging symbols

Example: `-Wall -Wextra -O2 -std=c11`

How does stdin input work?

If your program uses scanf(), gets(), fgets(), or other input functions, you can provide the input in the 'Standard Input' field.

Example program:
```c
#include <stdio.h>
int main() {
int num;
scanf("%d", &num);
printf("You entered: %d\n", num);
return 0;
}
```

In the stdin field, enter: `42`

The program will read this value and output: `You entered: 42`

Why does my program with malloc/free seem to leak memory?

It probably doesn't — but the sandbox can't tell you for sure because Valgrind isn't available server-side. The illusion of a memory leak in this environment usually comes from one of three things. (1) You allocated memory and forgot to free() before the program exited — strictly speaking this isn't a leak, since the OS reclaims all process memory on exit, but it's still a bug that would matter in long-running code. (2) You're confusing 'used heap memory' with 'leaked memory': allocating an array of a million ints uses ~4 MB legitimately. (3) Your free() is in an unreachable code path — for example after an early return or inside an if branch that didn't fire. To check this offline, install GCC plus Valgrind locally and run 'valgrind --leak-check=full ./yourprogram' — Valgrind reports every malloc without a matching free along with the exact source line of the allocation. Modern compilers also support AddressSanitizer ('gcc -fsanitize=address') which catches leaks at runtime with much lower overhead than Valgrind.

Why does my code crash with 'Segmentation fault'?

Segfault means your program tried to access a memory address it doesn't own. The five most common causes in beginner C:

(1) Dereferencing a NULL pointer: int *p = NULL; *p = 5; — crash. Always check pointers before dereferencing.

(2) Dereferencing an UNINITIALIZED pointer: int *p; *p = 5; — p contains random stack garbage that almost certainly isn't a valid address.

(3) Buffer overflow: char buf[10]; strcpy(buf, "this string is way too long"); — writes past the array's end into adjacent stack memory.

(4) Use-after-free: int *p = malloc(4); free(p); *p = 5; — p still holds the old address but the memory is no longer yours.

(5) Stack overflow from infinite recursion: int f(int n) { return f(n+1); } — main calls f, f calls f, eventually the call stack hits its size limit.

The sandbox prints 'Segmentation fault (core dumped)' and an exit code of 139 (128+SIGSEGV) when this happens. Compile with -g and use a local debugger (gdb) or AddressSanitizer to pinpoint the exact line.

Which C standard should I choose (C89, C99, C11, C17)?

Use the 'C Standard' dropdown to compile against a specific version of the language — it sets -std= for you. Pick by target, not by 'newest is best':

- C89 (ANSI C): the most portable. Many embedded toolchains and legacy codebases require it. C89 forbids // line comments, declarations after statements, and for-loop-scoped variables (for (int i...) is an error). Selecting C89 is the fastest way to check whether your code is truly portable.

- C99: adds // comments, mixed declarations, designated initializers, variable-length arrays, long long, stdint.h, and bool via stdbool.h. A sensible baseline for most modern code.

- C11: adds _Static_assert, anonymous structs/unions, _Generic, and the optional <threads.h>/<stdatomic.h> for portable concurrency. Default here.

- C17 (a.k.a. C18): no new features — it is C11 with defect fixes. Choose it when you want C11 semantics with the latest bug corrections.

Tip: enable -Werror and compile under C89 to surface every non-portable construct as a hard error.

What does -O0 vs -O3 change, and why might output differ?

The -O flags control optimization, which can dramatically change generated code (and sometimes reveal bugs):

- -O0 (default if you set nothing): no optimization. Fastest to compile, easiest to debug, variables stay in memory so a debugger shows true values. Best while developing.

- -O1 / -O2: progressively more aggressive — inlining, dead-code elimination, loop optimizations, register allocation. -O2 is the usual release setting.

- -O3: adds aggressive inlining and vectorization. Larger binaries; not always faster.

- -Os / -Ofast: optimize for size / for raw speed (relaxing IEEE math). Avoid -Ofast unless you understand the floating-point tradeoffs.

Why results can differ between levels: if your program has UNDEFINED BEHAVIOR (signed overflow, reading uninitialized memory, out-of-bounds access, strict-aliasing violations), the optimizer is allowed to assume it never happens and may delete or reorder code accordingly. Code that 'works' at -O0 but breaks at -O2 is almost always hiding undefined behavior — compile with -Wall -Wextra and try -fsanitize=undefined locally to catch it.

Is my code safe and private?

Your code privacy depends on the execution method:

- Code is sent to Piston API (emkc.org) for compilation
- Piston is open-source and maintained by the community
- Code is not stored or logged by Piston
- Execution happens in isolated containers
- No access to your local system

For maximum privacy, you can self-host Piston using Docker.

Key Features

• Run C code online with GCC compiler
• No installation or signup required
• Instant compilation and execution
• Switch C standard: C89, C99, C11, C17
• One-click -Werror (warnings as errors)
• Custom compiler flags and arguments
• Standard input (stdin) support
• Clear error messages and line numbers
• Code examples for learning
• Download code as .c file
• Execution time tracking
• Dark mode support
• Mobile-friendly responsive design
• Free forever - powered by Piston API
• Open source and community-driven