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C Programming Cheat Sheet

C Programming Cheat Sheet










C Programming Cheat Sheet

Structs, Pointers, Memory & File I/O






Structs & Memory Alignment

The size of a struct isn't just the sum of its parts. The compiler adds invisible bytes called **padding** to align members in memory for faster access.


Calculating Struct Size

Consider this struct on a 32-bit system:

struct contact {
char name[30];
int phone;
char email[30];
};


Memory Breakdown


name[30]: 30 bytes

Padding: 2 bytes (to align int on a 4-byte boundary)

phone: 4 bytes

email[30]: 30 bytes

Padding: 2 bytes (to make total size a multiple of 4)


Total Size = 30 + 2 + 4 + 30 + 2 = 68 bytes



Factors that can change a struct's size include:


Computer architecture (32-bit vs. 64-bit).

Adding/removing members.

Changing the order of members.







Pointers & Functions

How you pass variables to functions determines if the original value can be changed.


Pass-by-Value vs. Pass-by-Reference

Pass-by-Value: The function gets a copy of the data. Changes inside the function do not affect the original variable. (Used for y below).

Pass-by-Reference: The function gets the memory address (a pointer) of the data. Changes made via the pointer do affect the original variable. (Used for x below).


// x is passed by reference (a pointer)
// y is passed by value (a copy)
void func(int *x, int y) {
// This changes the ORIGINAL x in main()
*x = *x + y;

// This only changes the LOCAL copy of y
y = 2;
}

void main() {
int x = 10, y = 10;

// Pass the address of x, and a copy of y
func(&x, y);

// Output: x: 20, y: 10
printf("x: %d, y: %d", x, y);
}


Pointer Pitfall!

Inside func, be careful about the difference:


*x = 20; — Changes the value at the original address. (Affects main's x).

x = 20; — Changes where the local pointer `x` is pointing.…