The function strncpy is a part of the C programming language's standard library, specifically included in the header file <string.h>. It is used to copy a specified number of characters from one string (source) to another (destination). This function helps prevent buffer overflows by allowing the programmer to limit the number of characters copied, making it safer than the simpler strcpy function. When using strncpy, if the source string is shorter than the specified number of characters, the destination string will be padded with null characters ('\0') until the limit is reached. However, if the source string is longer, it will be truncated, and the destination string may not be null-terminated, which can lead to issues if not handled properly.