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I'm currently learning about operating systems the use of traps to facilitate system calls within the Linux kernel. I've located the table of the traps in traps.c and the implementation of many of the traps within entry.S.
However, I'm instructed to find an implementation of two system calls in the Linux kernel which utilize traps to implement a system call. Although I can find the definition of the traps themselves, I'm not sure what a "call" to one of these traps within the kernel would look like. Therefore, I'm struggling to find an example of this behavior.
Before anyone asks, yes, this is homework.
As a note, I'm using Github to browse the kernel source, since kernel.org is down: https://github.com/torvalds/linux/
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The trap handler in the kernel knows, from the type of the trap, that it is a user-initiated trap asking for a system call, finds the name of the systems call, and calls the appropriate kernel procedure to handle the call passing it the arguments stored on the stack.
A system call is implemented by a ``software interrupt'' that transfers control to kernel code; in Linux/i386 this is ``interrupt 0x80''. The specific system call being invoked is stored in the EAX register, abd its arguments are held in the other processor registers.
A system call is a function that allows a process to communicate with the Linux kernel. It's just a programmatic way for a computer program to order a facility from the operating system's kernel. System calls expose the operating system's resources to user programs through an API (Application Programming Interface).
A system call is a way for programs to interact with the operating system. A computer program makes a system call when it makes a request to the operating system's kernel. System call provides the services of the operating system to the user programs via Application Program Interface(API).
For the x86 architecture the SYCALL_VECTOR (0x80) interrupt is used only for 32bit kernels. You can see the interrupt vector layout in arch/x86/include/asm/irq_vectors.h. The trap_init() function from traps.c is the one that sets the trap handler defined in entry_32.S:
set_system_trap_gate(SYSCALL_VECTOR, &system_call);
For the 64bit kernels, the new SYSENTER (Intel) or SYSCALL (AMD) intructions are used for performance reasons. The syscall_init() function from arch/x86/kernel/cpu/common.c sets up the "handler" defined in entry_64.S and bearing the same name (system_call).
For the user-space perspetive you might want to take a look at this page (a bit outdated for the function/file names).
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