SYSCALL(2) BSD System Calls Manual SYSCALL(2)NAME
syscall, __syscall — indirect system call
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/syscall.h>
#include <unistd.h>
int
syscall(int number, ...);
quad_t
__syscall(quad_t number, ...);
DESCRIPTIONsyscall() 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⟩. 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. It should not be used in normal applications.
RETURN VALUES
The return values are defined by the system call being invoked. In gen‐
eral, a 0 return value indicates success. A -1 return value indicates an
error, and an error code is stored in errno.
HISTORY
The syscall() function call appeared in 4.0BSD.
BUGS
There is no way to simulate system calls that have multiple return values
such as pipe(2).
Since architectures return 32 bit and 64 bit results in different regis‐
ters, it may be impossible to portably convert the result of __syscall()
to a 32bit value. For instance sparc returns 32 bit values in %o0 and 64
bit values in %o0:%o1 (with %o0 containing the most significant part) so
a 32 bit right shift of the result is needed to get a correct 32 bit
result.
Many architectures mask off the unwanted high bits of the syscall number,
rather than returning an error.
Due to ABI implementation differences in passing struct or union type
arguments to system calls between different processors, all system calls
pass instead pointers to such structs or unions, even when the documenta‐
tion of the system call mentions otherwise. The conversion between pass‐
ing structs and unions is handled normally via userland stubs. The cor‐
rect arguments for the kernel entry points for each system call can be
found in the header file ⟨sys/syscallargs.h⟩
BSD August 7, 2009 BSD
