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SYSCALL(2) System Calls Manual SYSCALL(2)

syscall, __syscallindirect system call

#include <sys/syscall.h>
#include <unistd.h>

int
syscall(int number, ...);

__syscall(quad_t number, ...);

syscall() performs the system call whose assembly language interface has the specified number with the specified arguments. Symbolic constants for system calls can be found in the header file <sys/syscall.h>.

Since different system calls have different return types, a prototype of __syscall specifying the correct return type should be declared locally. This is especially important for system calls returning larger-than-int results.

The __syscall form should be used when one or more of the parameters is a 64-bit argument to ensure that argument alignment is correct. This system call is useful for testing new system calls that do not have entries in the C library.

The return values are defined by the system call being invoked. In general, for system calls returning int, a 0 return value indicates success. A -1 return value indicates an error, and an error code is stored in errno.

The predecessor of these functions, the former indir() system call, first appeared in Version 4 AT&T UNIX. The syscall() function first appeared in 3BSD.

This would all be easier if the layout of structures with long long members matched how long long arguments were aligned on the stack for syscall arguments. They don't match for some ILP32 archs so explicit padding is necessary for consistent handling.

May 30, 2016 OpenBSD-current