strlcat

strlcat is a C library function that concatenates the string src to the end of dst with a size limit. It was designed to be a safer alternative to strcat by requiring a known total size for the destination buffer and by guaranteeing NUL termination when possible. The function signature is strlcat(char *dst, const char *src, size_t dstsize). It appends at most dstsize-1 - strlen(dst) bytes from src to dst and always terminates dst with a NUL if dstsize is greater than zero.

The function returns a size_t value that represents the length of the string it tried to create, namely the initial length of dst plus the length of src. If this value is greater than or equal to dstsize, truncation occurred. If the initial length of dst is already at or beyond dstsize, strlcat writes nothing and returns dstsize + strlen(src). If dstsize is zero, nothing is written and the return value is strlen(src).

Originating in BSD Unix, strlcat is not part of the C standard. It is common on BSD-derived systems such as OpenBSD, NetBSD, and macOS, and is often available on Linux via libbsd or compatibility libraries. Because it is not universally standardized, portability concerns may arise on platforms lacking a native strlcat implementation.

Usage considerations include checking the return value to detect truncation and ensuring that the destination buffer is sized appropriately for the combined length of dst and src. If portability is a concern, developers may use snprintf or rely on platform-specific safe string functions, such as strlcpy/strlcat where available.