Library Function Vs System Calls

Standard functions can be classified into two major categories:

1. Library function calls
2. System function calls

Library Functions	System Calls
They are part of standard C library and executes in user space. Eg: strcmp(), malloc()	These functions change the execution mode of the program from user mode to kernel mode. Eg: open(), read()
Library functions may or may not make system calls.	They are called by library functions and executed by kernel.
Library functions are portable i.e. an application using library functions will run on every system.	An application relying on system call may not run on every system as system call interface may vary from system to system.
Library functions can be debugged using a debugger.	System calls cannot be debugged as they are executed by kernel.

How does system calls switch modes ?

Traditionally, the mechanism of raising an interrupt of ‘int $0x80′ to kernel was used. After trapping the interrupt, kernel processes it and changes the execution mode from user to kernel mode. Today, the systenter/sysexit instructions are used for switching the execution mode.