C Mastery / string.h: String and Memory Functions in Depth
Part 3 — The Standard Library

string.h: String and Memory Functions in Depth

This chapter is the authoritative reference for <string.h>: signatures, semantics, failure modes, and portability notes for every major string and memory function.

Why This Matters

These functions are the source of a large fraction of memory-safety bugs in C. Knowing the exact contract of strncpy, memcpy, memmove, and the rest is the difference between safe and unsafe code.

Prerequisites

Core Concept

<string.h> provides two families:

1. String functions (str*): operate on null-terminated strings, scanning for '\0'. 2. Memory functions (mem*): operate on explicit byte counts, no null termination assumed.

Function Reference

strlen

size_t strlen(const char *s);

Returns the number of characters before the first null. UB if s is not null-terminated.

strcpy

char *strcpy(char *dest, const char *src);

Copies src (including null) to dest. UB if dest is too small or buffers overlap.

strncpy

char *strncpy(char *dest, const char *src, size_t n);

Copies up to n characters; pads with zeros if src is shorter. Does not null-terminate if src is at least n characters.

strcat / strncat

char *strcat(char *dest, const char *src);
char *strncat(char *dest, const char *src, size_t n);

Appends src to dest. strcat needs dest large enough and non-overlapping. strncat appends at most n chars and always null-terminates (writes at most n+1 bytes total).

strcmp / strncmp

int strcmp(const char *a, const char *b);
int strncmp(const char *a, const char *b, size_t n);

Lexicographic comparison; returns negative, zero, or positive.

strchr / strrchr

char *strchr(const char *s, int c);
char *strrchr(const char *s, int c);

First/last occurrence of c (converted to char), or NULL.

strstr

char *strstr(const char *haystack, const char *needle);

First occurrence of needle in haystack, or NULL.

strtok

char *strtok(char *str, const char *delim);

Tokenizes a string, modifying it (replaces delimiters with null). Uses static state, so it is not reentrant and not thread-safe.

strspn / strcspn / strpbrk

size_t strspn(const char *s, const char *accept);
size_t strcspn(const char *s, const char *reject);
char *strpbrk(const char *s, const char *accept);

Span over characters in/not in a set; strpbrk finds the first character from a set.

memcpy

void *memcpy(void *dest, const void *src, size_t n);

Copies exactly n bytes. UB if buffers overlap.

memmove

void *memmove(void *dest, const void *src, size_t n);

Copies exactly n bytes, correctly handling overlap.

memset

void *memset(void *s, int c, size_t n);

Sets n bytes to (unsigned char)c.

memcmp

int memcmp(const void *a, const void *b, size_t n);

Compares n bytes as unsigned char.

memchr

void *memchr(const void *s, int c, size_t n);

Finds first occurrence of (unsigned char)c in n bytes, or NULL.

Examples

memmove for overlapping buffers

#include <string.h>

int main(void)
{
    char buf[] = "abcdef";
    memmove(buf + 2, buf, 4);   /* safe overlap */
    /* buf becomes "ababcd" */
    return 0;
}

strtok tokenization

#include <string.h>
#include <stdio.h>

int main(void)
{
    char s[] = "one,two,three";
    for (char *p = strtok(s, ","); p != NULL; p = strtok(NULL, ","))
        printf("%s\n", p);
    return 0;
}

Expected output: one, two, three (one per line).

Common Mistakes

Undefined Behavior

Portability

provides strtok_r as a reentrant alternative (not ISO C).

Under the Hood

Compilers often replace memcpy/memset/strlen with built-in, optimized versions (vectorized or specialized) when the size is known.

Practical Usage

Exercises

1. Implement strlen, strcpy, strcmp, memcpy, and memmove from scratch. 2. Demonstrate strncpy non-termination and the correct fix. 3. Show memmove correctly handling overlap and memcpy's UB with overlap. 4. Parse a comma-separated string with strtok.

Deep Challenge

Write a safe string-builder that grows a dynamic buffer and appends formatted content using vsnprintf (or manual formatting), with correct overlap and allocation handling. Explain every memory-safety decision.

References

Verification

verified.`

Progress

Concept checkboxes

Mastery levels

ConceptCurrent level (0–8)Target level
c.lib.string06
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