INTRO(2)

NAME
intro, _syscall - Introduction to system calls

DESCRIPTION
This chapter describes the Linux system calls. For a list of the 164 syscalls present in Linux 2.0, see syscalls(2).

  Calling Directly
In most cases, it is unnecessary to invoke a system call directly, but there are times when the Standard C library does not implement a nice function call for you. In this case, the programmer must manually invoke the system call using either one of the _syscall macros, or syscall(). The latter technique is described in syscall(2). This page describes the _syscall macros, and includes some notes on when to use one or other mechanism.

  Synopsis
#include <linux/unistd.h> A _syscall macro desired system call

  Setup
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 six macros that make the actual call into the system easier. They have the form:

    _syscallX(type,name,type1,arg1,type2,arg2,...)

        where X is 0–5, 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
Certain codes are used to indicate Unix variants and standards to which calls in the section conform. See standards(7).

NOTES
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, e.g., -EPERM. The _syscall() macros will return the result r of the system call when r is non-negative, but will return -1 and set the variable errno to -r when r is negative. For the error codes, see errno(3).

Some system calls, such as mmap(2), require more than five arguments. These are handled by pushing the arguments on the stack and passing a pointer to the block of arguments.

When defining a system call, the argument types MUST be passed by-value or by-pointer (for aggregates like structs).

The preferred way to invoke system calls that glibc does not know about yet is via syscall(2). However, this mechanism can only be used if using a libc (such as glibc) that supports syscall(2), and if the <sys/syscall.h> header file contains the required SYS_foo definition. Otherwise, the use of a _syscall macro is required. Some architectures, notably ia64, do not provide the _syscall macros. On these architectures, syscall(2) must be used.

EXAMPLE

#include <stdio.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);
}

  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
syscall(2), errno(3), feature_test_macros(7), standards(7)