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, ...);
DESCRIPTION
syscall() performs the system call whose assembly language interface has the specified number with the specified arguments. Symbolic con-
stants 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 parame-
ters 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 applica-
tions.
RETURN VALUES
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 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 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 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 documentation of the system call mentions otherwise. The conversion
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 <sys/syscallargs.h>
BSD August 7, 2009 BSD
Check Out this Related Man Page
_SYSCALL(2) Linux Programmer's Manual _SYSCALL(2)NAME
_syscall - invoking a system call without library support (OBSOLETE)
SYNOPSIS
#include <linux/unistd.h>
A _syscall macro
desired system call
DESCRIPTION
The important thing to know about a system call is its prototype. You need to know how many arguments, their types, and the function
return type. There are seven macros that make the actual call into the system easier. They have the form:
_syscallX(type,name,type1,arg1,type2,arg2,...)
where
X is 0-6, which are the number of arguments taken by the system call
type is the return type of the system call
name is the name of the system call
typeN is the Nth argument's type
argN is the name of the Nth argument
These macros create a function called name with the arguments you specify. Once you include the _syscall() in your source file, you call
the system call by name.
FILES
/usr/include/linux/unistd.h
CONFORMING TO
The use of these macros is Linux-specific, and deprecated.
NOTES
Starting around kernel 2.6.18, the _syscall macros were removed from header files supplied to user space. Use syscall(2) instead. (Some
architectures, notably ia64, never provided the _syscall macros; on those architectures, syscall(2) was always required.)
The _syscall() macros do not produce a prototype. You may have to create one, especially for C++ users.
System calls are not required to return only positive or negative error codes. You need to read the source to be sure how it will return
errors. Usually, it is the negative of a standard error code, for example, -EPERM. The _syscall() macros will return the result r of the
system call when r is nonnegative, but will return -1 and set the variable errno to -r when r is negative. For the error codes, see
errno(3).
When defining a system call, the argument types must be passed by-value or by-pointer (for aggregates like structs).
EXAMPLE
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <linux/unistd.h> /* for _syscallX macros/related stuff */
#include <linux/kernel.h> /* for struct sysinfo */
_syscall1(int, sysinfo, struct sysinfo *, info);
/* Note: if you copy directly from the nroff source, remember to
REMOVE the extra backslashes in the printf statement. */
int
main(void)
{
struct sysinfo s_info;
int error;
error = sysinfo(&s_info);
printf("code error = %d
", error);
printf("Uptime = %lds
Load: 1 min %lu / 5 min %lu / 15 min %lu
"
"RAM: total %lu / free %lu / shared %lu
"
"Memory in buffers = %lu
Swap: total %lu / free %lu
"
"Number of processes = %d
",
s_info.uptime, s_info.loads[0],
s_info.loads[1], s_info.loads[2],
s_info.totalram, s_info.freeram,
s_info.sharedram, s_info.bufferram,
s_info.totalswap, s_info.freeswap,
s_info.procs);
exit(EXIT_SUCCESS);
}
Sample Output
code error = 0
uptime = 502034s
Load: 1 min 13376 / 5 min 5504 / 15 min 1152
RAM: total 15343616 / free 827392 / shared 8237056
Memory in buffers = 5066752
Swap: total 27881472 / free 24698880
Number of processes = 40
SEE ALSO intro(2), syscall(2), errno(3)COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2007-12-19 _SYSCALL(2)