man-pages/man2/intro.2

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.\" Copyright (c) 1993 Michael Haardt (michael@moria.de),
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.\" Tue Jul  6 12:42:46 MDT 1993 <dminer@nyx.cs.du.edu>
.\" Added "Calling Directly" and supporting paragraphs
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.\" Modified Sat Jul 24 15:19:12 1993 by Rik Faith <faith@cs.unc.edu>
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.\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
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.\" Modified 10 June 1995 by Andries Brouwer <aeb@cwi.nl>
.\"
.TH INTRO 2 1996-05-22 "Linux" "Linux Programmer's Manual"
.SH NAME
intro, _syscall \- Introduction to system calls
.SH DESCRIPTION
This chapter describes the Linux system calls.
For a list of the Linux system calls, see syscalls(2).
.SS "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
.BR syscall (2).
Historically, this was also possible using one of the _syscall macros
described below.
.SS "Synopsis"
.B #include <linux/unistd.h>

A _syscall macro

desired system call
.SS Set-up
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:
.sp
.RS
.RI _syscall X ( type , name , type1 , arg1 , type2 , arg2 ,...)
.RS
.HP
where \fIX\fP is 0\(en5, which are the number of arguments taken by the
system call
.HP
\fItype\fP is the return type of the system call
.HP
\fIname\fP is the name of the system call
.HP
\fItypeN\fP is the Nth argument's type
.HP
\fIargN\fP is the name of the Nth argument
.RE
.RE
.sp
These macros create a function called \fIname\fP with the arguments you
specify.
Once you include the _syscall() in your source file,
you call the system call by \fIname\fP.
.SH FILES
.I /usr/include/linux/unistd.h
.SH "CONFORMING TO"
Certain codes are used to indicate Unix variants and standards to
which calls in the section conform.
See
.BR standards (7).
.SH NOTES
Starting around kernel 2.6.18, the _syscall macros were removed
from header files supplied to user space.
Use
.BR syscall (2)
instead.
(Some architectures, notably ia64, never provided the _syscall macros;
on those architectures,
.BR syscall (2)
was always required.)

The _syscall() macros \fI do not\fP produce a prototype.
You may have to
create one, especially for C++ users.
.sp
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, \-\fBEPERM\fP.
The _syscall() macros will return the result \fIr\fP of the system call
when \fIr\fP is non-negative, but will return \-1 and set the variable
.I errno
to \-\fIr\fP when \fIr\fP is negative.
For the error codes, see
.BR errno (3).

Some system calls, such as
.BR 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 \fImust\fP 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
.\" .BR syscall (2).
.\" However, this mechanism can only be used if using a libc
.\" (such as glibc) that supports
.\" .BR syscall (2),
.\" and if the
.\" .I <sys/syscall.h>
.\" header file contains the required SYS_foo definition.
.\" Otherwise, the use of a _syscall macro is required.
.\"
.SH EXAMPLE
.sp
.nf
#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);
}
.fi
.SS "Sample Output"
.nf
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
.fi
.SH "SEE ALSO"
.BR syscall (2),
.BR errno (3),
.BR feature_test_macros (7),
.BR standards (7)