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beginner15 min

Introduction to C and the Compile-Link-Run Model

What C is, why it still matters, and how source becomes a running program.

What you'll learn

  • Explain what kind of language C is and why it's still widely used
  • Describe the compile-link-run pipeline: .c source -> object file -> executable
  • Identify the parts of a minimal C program: #include, main, return

Explanation

C is a statically-typed, compiled language created in the early 1970s that still underpins most of the software world: operating system kernels, language runtimes, embedded firmware, and the C library that higher-level languages like Python and JavaScript are themselves implemented on top of. Learning C makes memory, pointers, and what a compiler actually does far less abstract.

A C program goes through three distinct stages before it runs. First, a compiler (commonly gcc or clang) translates each .c source file into an object file (.o), machine code with any references to other files left as placeholders. Second, a linker combines one or more object files (plus any libraries they need, like the standard C library) into a single executable. Third, you run that executable. The common command gcc main.c -o main actually does all three steps for you in one invocation for a simple program; ./main then runs the result.

Every C program needs a main function as its entry point: int main(void) { ... }. The int return type is a convention the operating system reads as an exit status -- return 0; conventionally means "success." Anything the program needs from the standard library, like printf, comes in through an #include directive, e.g. #include <stdio.h> for standard input/output functions.

This platform can't safely compile or execute real C code in your browser (see this course's guided-lab notice on every lesson), so instead of a live runner, each lesson gives you real C source code and asks you to work out what it does -- exactly the skill of reading code you'll need constantly as a working C developer, since C gives you very little safety net if you get it wrong.

Guided lab

Predict: A minimal C program

CNot executed
This lab does not run in your browser or on VisaSparkSchools's servers. Read the code, predict what it does, then reveal the real expected output.

Read this program and predict exactly what it prints when compiled and run.

#include <stdio.h>

int main(void) {
    printf("Hello, C!\n");
    printf("2 + 2 = %d\n", 2 + 2);
    return 0;
}

Stuck? Get a hint.

Common mistakes

  • Forgetting the `#include <stdio.h>` directive and then being confused why `printf` fails to compile.
  • Confusing compiling (translating source to an object file) with linking (combining object files and libraries into one executable) -- they're distinct steps even when one command does both.
  • Assuming a compiled C executable is portable across operating systems the way a script might be -- a Linux-compiled binary will not run on Windows or macOS without recompiling.

Knowledge check

Knowledge check

1. What is the entry-point function every C program must define?
2. Which step turns a `.c` source file into an object file?
3. What does the linker do that the compiler alone does not?

Takeaway

A C program's entry point is `int main(void)`; getting from source to a running program means compiling to object files, then linking them into an executable.

Summary

C is a compiled, statically-typed language still foundational to operating systems and other languages' runtimes; source goes through compiling and linking before it can run.

References

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Next: Variables, Types, and printf/scanf Format Specifiers →