System Calls

System Call Flow

User Space Program -> Interrupt Handlers Table -> System Call Handler -> System Call Routines

Interrupt Handler Table located at  /usr/include/i386-linux-gnu/asm/unistd_32.h

To Envoke a System Call
	int 0x80
	SYSENTER
	VDSO(Virtual Dynamic Shared Object)


To view the Man page to see the required flags and args
	man 2 <call name, ex write, read>


File Descriptor
	STDIN = 0
	STDOUT = 1
	STDERROR = 2


EX. Writing to the screen with write syscall
	ssize_T write(int fd, const void *buf, size_t count);
			1	message		length of string


Invoking a System Call in the Registers with 0x80
	EAX - System Call Number (EX 4 for write) - This will typically contain the call's return value
	EBX - 1st Arg (EX 1 for fd)
	ECX - 2nd Arg (EX message the data to be printed out)
	EDX - 3rd Arg (EX length of the string)
	ESI - 4th Arg
	EDI - 5th Arg