Introduction to C — First Program, Compilation and Program Structure

C is one of the oldest popular languages (1972), and it's still everywhere: operating systems (Linux, Windows kernels), embedded devices, databases and the Python interpreter itself are written in C. Learning C teaches you how computers really work — memory, addresses, bytes.

1. Your first program

#include <stdio.h>

int main(void) {
    printf("Hello, Skills101!\n");
    return 0;
}

Hello, Skills101!

Every statement ends with ;. Blocks are inside { }. C is case-sensitive (Main ≠ main).

2. How a C program is compiled

1. Preprocessor — handles lines starting with #: pastes in header files, replaces macros.
2. Compiler — turns C into assembly and checks syntax and types.
3. Assembler — turns assembly into machine code (object file .o).
4. Linker — joins your object files with libraries (like the code for printf) into an executable.

With GCC: gcc hello.c -o hello then ./hello (Linux/Mac) or hello.exe (Windows). Beginners on Windows can install MinGW or use an online compiler — or our coding practice.

3. Printing numbers

#include <stdio.h>

int main(void) {
    int marks = 92;
    float cgpa = 8.75f;
    printf("Marks: %d\n", marks);
    printf("CGPA: %.2f\n", cgpa);
    printf("Sum: %d\n", 10 + 5);
    return 0;
}

Marks: 92
CGPA: 8.75
Sum: 15

%d, %f are format specifiers — placeholders replaced by the values after the comma. More in the next lesson.

4. Comments

// single-line comment (C99 and later)
/* multi-line
   comment */

5. Structure of a C program

/* documentation: what this program does */
#include <stdio.h>          ← header files
#define PI 3.14159          ← macros / constants
int count = 0;              ← global variables
int square(int n);          ← function declarations (prototypes)

int main(void) {            ← main function
    ...
    return 0;
}

int square(int n) {         ← other function definitions
    return n * n;
}

6. Why C is "middle level"

C has high-level features (functions, loops, structures) but also low-level access (pointers, memory addresses, bits). It is fast because it compiles to machine code and does very little checking at runtime — which also means you must be careful (no automatic bounds checks).

Common mistakes

• Forgetting #include <stdio.h> → warnings or errors for printf.
• Missing semicolons or the \n at the end of output.
• Writing void main() — the standard is int main(void) returning 0.

Quick recap

• C is fast, close to hardware, and used in OS and embedded systems.
• Preprocessor → compiler → assembler → linker.
• main is the entry point; printf with format specifiers prints values.
• Statements end with ;; C is case-sensitive.