1. System Calls System calls are the user API to the OS
CSCE 410/611
System calls are the user API to the OS
System calls are not function calls!
How are system calls invoked?
Why are system calls implemented this way?
System Calls

2. Operating System Interfaces: System Calls
CSCE 410/611
Operating System Interfaces: System Calls
applications programs/
processes
system call
system call
interface
scheduler
file system
kernel
memory manager
etc.
device drivers
hardware
System Calls

3. Types of System Calls File Management Device Management
CSCE 410/611
Types of System Calls
File Management
create file, delete file
open, close
read, write, reposition
get/set file attributes
Process Control
load
execute
end, abort
create process
terminate process
get/set process attributes
wait for time, wait event, signal event
allocate, free memory
Device Management
request device, release device
read, write, reposition
get/set device attributes
logically attach or detach devices
Information Maintenance
get/set time or date
get/set system data
get/set process, file, or device attributes
Communication
create, delete communication connection
send, receive messages
transfer status information
attach or detach remote devices
System Calls

4. System Calls – They look like Functions ...
CSCE 410/611
System Calls – They look like Functions ...
#include<unistd.h>
int main(void) {
write(1, "Hello, world\n", 13);
return 0; }
System Calls

5. ... but they are not! Making System Calls in 32-bit Linux
CSCE 410/611
... but they are not!
Making System Calls in 32-bit Linux
load system call number in register eax.
load arguments to system call in registers ebx, exc, edx, esi, edi, ebp
invoke software interrupt: int 0x80
Returned values are stored in eax.
64-bit Linux?
same but different
different registers
different numbers
syscall instruction
System Calls

6. CSCE 410/611
System Call Numbers
syscalls.kernelgrok.com
System Calls

7. System Calls - Example
Using the Unix Standard Library

8. System Calls - Example
Using 32-bit Linux System Calls

9. System Calls - Example
Using 64-bit Linux System Calls

10. System Calls are Expensive!
CSCE 410/611
System Calls are Expensive!
Software interrupts are expensive!
Cost of context switch (saving/restoring registers)
Caches are stale
TLBs
CPU pipelines
Compiler optimization not possible.
System Calls

11. Why Interrupts or syscall?
CSCE 410/611
Why Interrupts or syscall?
Reason 1: Can load user program into memory without knowing exact address of system functions.
Reason 2: Separation of address space, including stacks: user stack and kernel stack.
Reason 3: Automatic change to supervisor mode.
Reason 4: Can control access to kernel by masking interrupts.
System Calls

12. Reason 4: Mutual Exclusion in Kernel
CSCE 410/611
Reason 4: Mutual Exclusion in Kernel
user space
Thread 2
Thread 1
Thread 1
Thread 1
interrupt!
system call
kernel space
Thread 1 executing in kernel.
Interrupts are masked.
Thread 2 can not enter
kernel because of
masked interrupts
unmasks interrupt
and returns
rti
System Calls

13. Summary: System Calls System calls are the OS API
CSCE 410/611
System calls are the OS API
They look like function calls...
... but they are not!
They are implemented using software interrupts (or variations thereof)
Why are they implemented this way?
System Calls