.C

C source code

Introduction

A .c file holds a program written in the C programming language, as plain text. A compiler turns each .c file into an object file, and a linker joins those into a program or library.

A .h file is a header: declarations of functions, types and constants that several .c files share by including it. C++ and Objective-C also use .h headers. On systems where file names are case sensitive, a capital .C usually means C++ rather than C.

Software

Structure

A C source file is plain text, today usually UTF-8 or plain ASCII, with no header or signature. Before compiling, the preprocessor handles the lines that start with #: #include pastes in a header, #define makes a macro, and #if keeps or drops blocks of text. What remains is a series of declarations and function definitions, one of which is usually main.

#include <stdio.h>

int main(void)
{
    printf("Hello, world\n");
    return 0;
}
A small C program: one include and one function.