Linux syscall - dklibc/dklibc GitHub Wiki

POSIX declares API. Libc implements POSIX API. But libc cannot do this without help from kernel. Libc uses kernel functions ("syscalls") How to do a syscall depends on arch. For x86 Linux syscalls are done by int 0x80. The most functions from the POSIX API have a corresponding syscall. One-by-one: one call of POSIX function leads to the single syscall. But there are exceptions from this rule: wrapper libc function can make a great job by preparing args and doing different syscalls.

For the most of the Linux syscalls there is the corresponding function in glibc. If you haven't a new version of glibc but you need to test a new syscall -- use glibc wrapper for doing syscall by its number: long syscall(long number, ...) Symbolic constants of syscalls numbers (SYS_bla_bla) you can get here -- sys/syscall.h

For asm-masters:

  1. Store syscall number in EAX
  2. Store the rest of syscall args in (by order): EBX, ECX, EDX, ESI, EDI, EBP
  3. int 0x80
  4. Result will be in EAX
Example: 
mov eax, 1	; Exit syscall
xor ebx, ebx	; exit code = 0
int 0x80	; Do it!

Compare: BSD kernel gets syscall args from the stack.

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