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SYSCALL(2) BSD System Calls Manual SYSCALL(2)
syscall, __syscall -- indirect system call
Standard C Library (libc, -lc)
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>. 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.
The return values are defined by the system call being invoked. In general, a 0 return
value indicates success. A -1 return value indicates an error, and an error code is stored
The syscall() function call appeared in 4.0BSD.
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 registers, 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
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 documentation of the system call mentions otherwise. The conver-
sion between passing structs and unions is handled normally via userland stubs. The correct
arguments for the kernel entry points for each system call can be found in the header file
BSD August 7, 2009 BSD
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