Your First C Program

C is a compact language created in the early 1970s. It gives you direct control over memory and produces very fast programs, which is why it is still used for operating systems, microcontrollers and performance-critical libraries.

Unlike Python, C is compiled: a program called a compiler turns your source file into a machine-code executable, and then you run that executable.

Setting up a compiler

• Linux: install gcc from your package manager (for example sudo apt install build-essential).
• macOS: run xcode-select --install to get clang, which works the same way.
• Windows: install MSYS2 or MinGW-w64 to get gcc, or use an IDE such as Code::Blocks.

Hello, world

Save this as hello.c:

#include <stdio.h>

int main(void) {
    printf("Hello, world!\n");
    return 0;
}

Hello, world!

Compile and run it from a terminal:

gcc hello.c -o hello
./hello

What each line does

• #include <stdio.h> brings in the standard input/output library, which provides printf.
• int main(void) is where every C program starts. It returns an int to the operating system.
• printf(...) prints text. The \n at the end means "new line"; C does not add one for you.
• return 0; tells the system the program finished without errors.
• Every statement ends with a semicolon ;, and curly braces { } group statements together.

Printing numbers

printf uses format specifiers to insert values into text: %d for whole numbers and %f for decimals.

#include <stdio.h>

int main(void) {
    printf("%d + %d = %d\n", 7, 5, 7 + 5);
    printf("Half of 9 is %.1f\n", 9 / 2.0);
    return 0;
}

7 + 5 = 12
Half of 9 is 4.5

GATE-style practice questions

#include <stdio.h>
int main(void) {
    printf("GATE");
    printf(" %d\n", printf("CS"));
    return 0;
}