strncat

strncat is a function in the C standard library used to concatenate a limited number of characters from one string (src) onto the end of another string (dest). It is declared in <string.h> in C and in <cstring> in C++.

Prototype: char *strncat(char *dest, const char *src, size_t n); It appends at most n characters from src to the end of dest, stopping early if a null terminator is encountered in src. After the copy, strncat terminates dest with a null character.

Usage considerations: The destination buffer must have enough space for the original contents of dest, plus up to n characters from src (or fewer if src is shorter), plus the terminating null. If there is insufficient space, a buffer overflow can occur. The function does not check for overlap between dest and src; behavior is undefined if they overlap. Dest must point to a modifiable array or allocated memory, not to a string literal.

Return value: A pointer to dest.

Example: char s[20] = "Hello"; strncat(s, "World", 3); // s becomes "HelloWor"

Related notes: strncat is similar to strcat but with a length limit. In portable code that requires safer concatenation, consider safer bounds-checked approaches or higher-level abstractions such as C++'s std::string. Some environments provide strlcat, but it is not part of the C standard.