A library function is simply one of the multitude of functions that constitutes the standard C library. (For brevity, when talking about a specific function in the rest of the book we’ll often just write function rather than library function.) The purposes of these functions are very diverse, including such tasks as opening a file, converting a time to a human-readable format, and comparing two character strings.
Many library functions don’t make any use of system calls (e.g., the string-manipulation functions). On the other hand, some library functions are layered on top of system calls. For example, the fopen() library function uses the open() system call to actually open a file. Often, library functions are designed to provide a more caller-friendly interface than the underlying system call. For example, the printf() function provides output formatting and data buffering, whereas the write() system call just outputs a block of bytes. Similarly, the malloc() and free() functions perform various bookkeeping tasks that make them a much easier way to allocate and free memory than the underlying brk() system call.