C Syntax

beginner

Part of C Fundamentals

Theory

Every C program follows a specific structure. Understanding C syntax is the first step to writing correct and efficient C code.

Structure of a C Program

A C program consists of preprocessor directives, global declarations, function definitions, and the mandatory main() function. The compiler processes the program from top to bottom.

#include <stdio.h>   // Preprocessor directive
#define PI 3.14159   // Macro definition
 
int globalVar = 0;   // Global variable
 
int main() {         // Entry point
    printf("Hello!\n");
    return 0;
}

The main() Function

main() is the entry point. It returns an int — return 0 means success, non-zero means error. It can accept command-line arguments via argc and argv[].

printf and scanf

printf() — formatted output: printf("Value: %d\n", x)
scanf() — formatted input: scanf("%d", &x) (note the & address-of operator)

Common format specifiers: %d (int), %f (float), %c (char), %s (string), %p (pointer).

Header Files and Preprocessor

The preprocessor runs before compilation. #include inserts file contents. #define creates macros. The preprocessor also handles conditional compilation with #ifdef, #ifndef, #endif.

Practical Examples

Example 1: Full Program Structure

Example 2: Preprocessor Directives

Exercises

Temperature Converter

easy

Write a C program that asks the user for a temperature in Celsius using scanf(), converts it to Fahrenheit using the formula F = C * 9/5 + 32, and prints the result using printf().

Starter Code:

#include <stdio.h>\n\nint main() {\n  float celsius;\n  // Your code here\n  return 0;\n}

Expected Output:

Enter temperature in Celsius: 25\n25.00°C = 77.00°F