Learn to use CRunner

CRunner is a free online C compiler and personal program manager. Write C code, compile it, and interact with a running program in your browser. There is no account to create or desktop compiler to install. The online C compiler workspace keeps your editor, saved programs, and terminal together.

How to run a C program

1. Start with hello_world.c or create a new program. Enter your C source in the editor.
2. Select Run code, or press Ctrl + Enter. The first run downloads the compiler assets and can take longer than later runs.
3. Read output and compiler messages in the terminal. If your program requests input, type it there and press Enter.
4. Edit and run again to check your changes. Use Stop if you want to end a running program.

For example, a program can print a question with printf("Number: "); and read your reply with scanf("%d", &number);. Declare number as an integer first and include stdio.h. The value you type appears after the prompt, even when the prompt has no newline.

Interactive input with scanf and getchar

The terminal sends your input to the actual running C program. You can answer several scanf() requests in succession without clicking the terminal between replies. getchar() also reads from stdin; input is submitted a line at a time when you press Enter. This is an interactive program terminal, not a command shell.

Time spent waiting for stdin does not consume the active execution budget. Separate idle and session limits end abandoned programs. Compilation errors, nonzero exit codes, and detectable WebAssembly runtime failures appear in the same terminal.

A workspace for small C projects

Syntax highlighting, editor tabs, keyboard shortcuts, and a resizable terminal help you work through a program. You can create, rename, duplicate, delete, and search saved programs, or return to recent work. Autosave keeps edits in your browser. Download a single .c file or export your full library as a backup that you can import later.

Use CRunner to practice loops and arrays, trace pointer operations, experiment with structures and functions, or prepare a small classroom example. Try changing one part of a program and comparing its output. It is also useful for checking a short algorithm before moving it into a larger project.

Real C compilation inside your browser

CRunner uses LLVM Clang and a WebAssembly runtime. Compilation and execution happen in a Web Worker on your device, so a busy program does not run on the editor's main thread. Source code and terminal input are not sent to a remote compilation service.

The compiler targets C17 with WASI libraries. Common standard C features work, including functions, arrays, structures, pointers, and dynamic memory allocation. Operating-system-specific headers, native processes, and a full Linux environment are outside its scope. Resource limits keep runs bounded; an older or low-memory device may have difficulty loading the compiler.

Your programs stay in local browser storage

Your program library is stored in IndexedDB on this device and browser profile. It can persist after you close the website, but it does not sync between devices. Clearing site data, using private browsing, or browser storage eviction can remove it. Export a backup before changing browsers or clearing storage.

The website still downloads its static files over the internet, and the hosting provider can process ordinary connection data. Read the Privacy Policy for details about local storage, hosting, and any future advertising. Learn more about CRunner or visit Contact for reporting guidance.