UNIX Filesystems: Evolution, Design, and Implementation

Library Functions versus System Calls

System calls are functions that transfer control from the user process to the operating system kernel. Functions such as read() and write() are system calls. The process invokes them with the appropriate arguments, control transfers to the kernel where the system call is executed, results are passed back to the calling process, and finally, control is passed back to the user process.

Library functions typically provide a richer set of features. For example, the fread() library function reads a number of elements of data of specified size from a file. While presenting this formatted data to the user, internally it will call the read() system call to actually read data from the file.

Library functions are implemented on top of system calls. The decision whether to use system calls or library functions is largely dependent on the application being written. Applications wishing to have much more control over how they perform I/O in order to optimize for performance may well invoke system calls directly. If an application writer wishes to use many of the features that are available at the library level, this could save a fair amount of programming effort. System calls can consume more time than invoking library functions because they involve transferring control of the process from user mode to kernel mode. However, the implementation of different library functions may not meet the needs of the particular application. In other words, whether to ...