System Calls vs. Library Functions¶

System Calls ¶

The kernel is not a library ⟶ no direct function calls, but rather System Calls.

Entry points into the kernel
Every system call has a unique number and a fixed set of parameters and registers (ABI)
Changes context from user mode to kernel mode
Implementation is CPU specific (software interrupt …)
Numbers, parameters, etc. are Linux specific
⟶ “kernel acts on behalf of a process”

System calls are never used directly by a program …

syscall wrapper: write()¶

#include <unistd.h>
int main() {
    write(1, "hallo\n", 6);
    return 0;
}

Distinction is not always clear ⟶ manual pages

System calls (`man -s 2 ...`)	No system calls (`man -s 3 ...`)
`write()` (man -s 2 write) `read()` (man -s 2 read) `connect()` (man -s 2 connect) …	`malloc()` (man -s 3 malloc) `printf()` (man -s 3 printf) `getaddrinfo()` (man -s 3 getaddrinfo …

man [section] name

For example: man man ⟶