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Online C compiler

Write C, type its input, press Run. Mistakes that a real C program hides, such as reading past an array or using a NULL pointer, are stopped and explained.

Opening the compiler…

About the C compiler

How to use the C compiler

Type or paste your program in the editor, or pick a starter program from Templates: scanf and printf, arrays, recursion, sorting, a linked list and files. Press Run (or Ctrl+Enter). The output appears on the right, or in the Output tab on a phone, with how long the program took and the exit code that main returned.

Your code, input and tests are saved on this device as you type. Share makes a link with the code and input inside it; nothing is uploaded.

How input works

Everything in the Input tab is given to the program as its standard input, as if you had typed it in a terminal before it started. scanf, getchar and fgets(..., stdin) read from it in order, across lines. Put one value for each %d your scanf reads; values may be separated by spaces or new lines. If a read finds no input left, scanf returns EOF, and the compiler tells you the program wanted more.

What runs your code

C runs in your browser on StepVioz's own C interpreter, not on a server. It follows the C standard closely and checks every step: an index outside an array, an uninitialized variable, integer overflow, a NULL or dangling pointer, use after free and printf formats that do not match their arguments all stop the program with the line and a plain explanation, where a real compiled program would crash later or print garbage. Files written with fopen live in memory and disappear when the run ends.

Each run may take 5 seconds; then it is stopped. Output is kept up to 10,000 lines or 1 MB, and the rest is cut with a notice. Programs cannot reach the network, and no packages or libraries can be installed: what a language brings with it is what you can use. An interpreter is slower than compiled code: a few million loop steps per second, which is plenty for course work but not for contest programs that run a hundred million steps.

See it step by step

Visualize this run opens the same code and input in the C code visualizer: it runs one line at a time and draws the call stack, every variable, arrays and the heap, with pointers as arrows. It is the clearest way to see why a pointer or a loop does what it does. The visualizer follows the first 1,000 steps, so keep the input small.

Questions

Is this a real C compiler like gcc?
No. It is StepVioz's C interpreter, which runs in your browser. It accepts standard C and gives the same output as gcc for correct programs, but it also stops and explains undefined behaviour that gcc would let through.
Why does my program say it wants more input?
scanf read past the end of the Input tab. Add one value for each value the program reads, then run again.
Can I use files?
Yes: fopen, fprintf, fscanf, fgets, fputc and the rest work on files kept in memory. A file exists only during the run that wrote it.
Which headers are available?
stdio.h, stdlib.h, string.h, math.h, ctype.h, limits.h, stdbool.h, stdint.h, time.h and assert.h, with the functions courses use.
Does my code leave my computer?
No. It runs in your browser in a sandbox with no network access, and it is saved only on this device.

See also