system call & library functions in unix(APUE study 1)

 1  Both system call and library functions are in the form of C functions,for user to invoke. We can consider the system calls as being C functions.
2 From an implementor 's point of view, the distinction between a system call and  a library function is fundamental.
3 From a user's perspective, the difference is not as critical.
eg. sbrk(2) is a system call, however,malloc is a library function:the system call in the kernel allocates an additonla chunk of space on behalf of the process. The malloc library function manages this space from user level.
4 Another difference between system calls or library functions is that system calls usually provide a minimal interface,whereas library functions often provide more elaborate functionality.