Language FoundationsC Programming Deep DiveEasy

Implement memcpy, memset, strlen

Master the fundamental concepts of c programming deep dive through this focused micro-challenge.

~30 minutes C Programming Deep Dive Ready to learn

Topic Overview

Three Functions, Three Contracts

memcpy, memset, and strlen sit at the bottom of almost every C program. glibc's production memcpy picks among a dozen SIMD variants at load time; your version starts with byte loops that expose the contracts underneath.

memcpy: copy n bytes from src to dest; behavior is undefined if regions overlap (use memmove for that)
memset: write the low byte of value into n consecutive bytes; glibc uses this to zero pages before mmap returns them
strlen: scan until \0, return count excluding the terminator

Byte-by-Byte Implementation

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Cast to char* because void* forbids pointer arithmetic in standard C. For strlen, walk with const char *p and increment until *p == '\0'.

For this exercise, you will implement my_memcpy, my_memset, and my_strlen without #include <string.h>. This task asks you to match libc semantics exactly, because getting overlap, null handling, or the terminator rule wrong is how real buffer bugs start.

Keep the relevant man page, ABI doc, or Rust reference chapter open while you work. When your output disagrees with the reference implementation on the same machine, the mismatch is usually an alignment rule, an off-by-one terminator, or a register slot you misread in GDB.

Implementation Task

Your Task

Implement three libc building blocks from scratch, without calling <string.h> for them:

void *my_memcpy(void *dest, const void *src, size_t n)
void *my_memset(void *ptr, int value, size_t n)
size_t my_strlen(const char *s), which returns 0 for NULL

Each must work byte by byte through unsigned char pointers and do nothing when n is 0. Then run the check selected on stdin.

Input

One word: memcpy, memset, strlen or zero. Anything else (or empty input) runs all four in that order.

Output

mode	what to do	output
`memcpy`	copy `"Hello, World!"` (including its terminator) into a buffer	`memcpy: Hello, World!`
`memset`	fill 9 bytes with `'X'`, then terminate	`memset: XXXXXXXXX`
`strlen`	measure `"Hello, World!"`	`strlen: 13`
`zero`	call `my_memcpy` and `my_memset` with n = 0 (nothing may change)	`Zero size: OK`

Example

Input:

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Output:

c
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Requirements

Work through unsigned char * so that any byte value copies correctly.
Return dest/ptr like the real functions do.

Hidden tests cover memset, strlen and the zero-length calls.