intro

intro, _syscall - Introduction to system calls

This chapter describes the Linux system calls. For a list of the Linux system calls, 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 syscall(2). Historically, this was also possible using one of the _syscall macros described below.

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:

where X is 05, 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.

/usr/include/linux/unistd.h

Certain codes are used to indicate Unix variants and standards to which calls in the section conform. See standards(7).

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 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).

#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

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