System Calls

What is a system call?

The operating system is responsible for

• Process Management (starting, running, stopping processes)
• File Management(creating, opening, closing, reading, writing, renaming files)
• Memory Management (allocating, deallocating memory)
• Other stuff (timing, scheduling, network management)

An application program makes a system call to get the operating system to perform a service for it, like reading from a file.

One nice thing about syscalls is that you don't have to link with a C library, so your executables can be much smaller.

System Calls in 32-bit Linux

• To make a system call in 32-bit Linux, place the system call number in eax, then its arguments, in order, in ebx, ecx, edx, esi, edi, and ebp, then invoke int 0x80.
• Some system calls return information, usually in eax.
• All registers are saved across the system call.

System Calls in 64-bit Linux

# ----------------------------------------------------------------------------------------
# Writes "Hello, World" to the console using only system calls. Runs on 64-bit Linux only.
# To assemble and run:
#
#     gcc -c hello.s && ld hello.o && ./a.out
#
# or
#
#     gcc -nostdlib hello.s && ./a.out
# ----------------------------------------------------------------------------------------

        .global _start

        .text
_start:
        # write(1, message, 13)
        mov     $1, %rax                # system call 1 is write
        mov     $1, %rdi                # file handle 1 is stdout
        mov     $message, %rsi          # address of string to output
        mov     $13, %rdx               # number of bytes
        syscall                         # invoke operating system to do the write

        # exit(0)
        mov     $60, %rax               # system call 60 is exit
        xor     %rdi, %rdi              # we want return code 0
        syscall                         # invoke operating system to exit
message:
        .ascii  "Hello, world\n"

• To make a system call in 64-bit Linux, place the system call number in rax, then its arguments, in order, in rdi, rsi, rdx, r10, r8, and r9, then invoke syscall.
• Some system calls return information, usually in rax. A value in the range between -4095 and -1 indicates an error, it is -errno.
• The system call destroys rcx and r11 but others registers are saved across the system call.
• Full details are in Section A.2.1 of the The AMD64 ABI.

Lists of Linux System Calls

There are hundreds of system calls in Linux. A good online source for 32-bit Linux is http://syscalls.kernelgrok.com/ . For 64-bit Linux, check out http://www.acsu.buffalo.edu/~charngda/linux_syscalls_64bit.html

Check those pages, and of course, the Linux source.

macOS System Calls

Windows System Calls