库函数 vs 系统调用

  quote 'Expert C programming' written by Peter Van Der Linden

library call	system call
the C library is the same on every ANSI C implementation	the system calls are different in each OS
is a call to a routine in a library	is a call to the kernel for a service
linked with the user address space	is an entry point to the OS
executes in the user address space	executes in the kernel address space
counts as part of the “user” time	counts as part of the “system” time
has the lower overhead of a procedure call	has high overhead context switch to kernel and back
there are about 300 routines in the C library libc	There are about 90 system calls in Unix
Documented in Section 3 of the UNIX manual	Documented in Section 2 of the UNIX manual
typical C library calls: system, fprintf, malloc	typical system calls: chdir, fork, write, brk