C - Purpose of string.h

In C, string.h is the String Handling Header File.
It contains function prototypes and macros for manipulating C-style strings (null-terminated character arrays) and working with blocks of memory.

1. Purpose of `string.h`

When you include:

#include <string.h>

you gain access to:

String manipulation (copying, concatenation, comparison).
Searching within strings.
Measuring string lengths.
Memory block operations (memcpy, memmove, etc.).

These functions work with char arrays — not C++ string objects.

2. Commonly Used Functions in `string.h`

a) String Length

Function	Description
`strlen(s)`	Returns the length of string `s` (excluding `\0`).

b) Copying Strings

Function	Description
`strcpy(dest, src)`	Copies `src` to `dest` (until `\0`).
`strncpy(dest, src, n)`	Copies up to `n` characters from `src` to `dest`.

c) Concatenation

Function	Description
`strcat(dest, src)`	Appends `src` to end of `dest`.
`strncat(dest, src, n)`	Appends up to `n` characters from `src`.

d) Comparison

Function	Description
`strcmp(s1, s2)`	Compares strings lexicographically (`<0`, `0`, `>0`).
`strncmp(s1, s2, n)`	Compares up to `n` characters.

e) Searching

Function	Description
`strchr(s, c)`	Returns pointer to first occurrence of `c` in `s`.
`strrchr(s, c)`	Returns pointer to last occurrence of `c` in `s`.
`strstr(s1, s2)`	Finds first occurrence of substring `s2` in `s1`.

f) Memory Operations

Function	Description
`memcpy(dest, src, n)`	Copies `n` bytes from `src` to `dest` (no overlap).
`memmove(dest, src, n)`	Copies `n` bytes (safe for overlapping).
`memset(s, c, n)`	Sets `n` bytes in `s` to value `c`.
`memcmp(s1, s2, n)`	Compares `n` bytes of two memory blocks.

3. Example

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

int main() {
    char str1[20] = "Hello";
    char str2[] = "World";

    // Concatenate
    strcat(str1, " ");
    strcat(str1, str2);
    printf("After concatenation: %s\n", str1);

    // Length
    printf("Length: %zu\n", strlen(str1));

    // Copy
    char str3[20];
    strcpy(str3, str1);
    printf("Copy: %s\n", str3);

    // Compare
    if (strcmp(str1, str3) == 0) {
        printf("Strings are equal\n");
    }

    // Search
    char *pos = strstr(str1, "World");
    if (pos) {
        printf("Found 'World' at position: %ld\n", pos - str1);
    }

    return 0;
}

4. Important Notes

All strings in C are null-terminated (\0 at the end).
Functions like strcpy and strcat can cause buffer overflows if the destination is too small.
Safer versions like strncpy or platform-specific strlcpy are preferred in modern C.