_syscall — invoking a system call without library support (OBSOLETE)
#include <linux/unistd.h> A _syscall macro desired system call
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:
_syscall
X(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.
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).
#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\n", error);
printf("Uptime = %lds\nLoad: 1 min %lu / 5 min %lu / 15 min %lu\n"
"RAM: total %lu / free %lu / shared %lu\n"
"Memory in buffers = %lu\nSwap: total %lu / free %lu\n"
"Number of processes = %d\n",
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);
}