man2/_syscall.2

   1 .\" Copyright (c) 1993 Michael Haardt (michael@moria.de),
   2 .\"   Fri Apr  2 11:32:09 MET DST 1993
   3 .\"
   4 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
   5 .\" This is free documentation; you can redistribute it and/or
   6 .\" modify it under the terms of the GNU General Public License as
   7 .\" published by the Free Software Foundation; either version 2 of
   8 .\" the License, or (at your option) any later version.
   9 .\"
  10 .\" The GNU General Public License's references to "object code"
  11 .\" and "executables" are to be interpreted as the output of any
  12 .\" document formatting or typesetting system, including
  13 .\" intermediate and printed output.
  14 .\"
  15 .\" This manual is distributed in the hope that it will be useful,
  16 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
  17 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  18 .\" GNU General Public License for more details.
  19 .\"
  20 .\" You should have received a copy of the GNU General Public
  21 .\" License along with this manual; if not, see
  22 .\" <http://www.gnu.org/licenses/>.
  23 .\" %%%LICENSE_END
  24 .\"
  25 .\" Tue Jul  6 12:42:46 MDT 1993 <dminer@nyx.cs.du.edu>
  26 .\" Added "Calling Directly" and supporting paragraphs
  27 .\"
  28 .\" Modified Sat Jul 24 15:19:12 1993 by Rik Faith <faith@cs.unc.edu>
  29 .\"
  30 .\" Modified 21 Aug 1994 by Michael Chastain <mec@shell.portal.com>:
  31 .\"   Added explanation of arg stacking when 6 or more args.
  32 .\"
  33 .\" Modified 10 June 1995 by Andries Brouwer <aeb@cwi.nl>
  34 .\"
  35 .\" 2007-10-23 mtk: created as a new page, by taking the content
  36 .\" specific to the _syscall() macros from intro(2).
  37 .\"
  38 .TH _SYSCALL 2 2019-03-06 "Linux" "Linux Programmer's Manual"
  39 .SH NAME
  40 _syscall \- invoking a system call without library support (OBSOLETE)
  41 .SH SYNOPSIS
  42 .B #include <linux/unistd.h>
  43 .PP
  44 A _syscall macro
  45 .PP
  46 desired system call
  47 .SH DESCRIPTION
  48 The important thing to know about a system call is its prototype.
  49 You need to know how many arguments, their types,
  50 and the function return type.
  51 There are seven macros that make the actual call into the system easier.
  52 They have the form:
  53 .PP
  54 .in +4n
  55 .EX
  56 .RI _syscall X ( type , name , type1 , arg1 , type2 , arg2 ,...)
  57 .EE
  58 .in
  59 .PP
  60 where
  61 .IP
  62 .I X
  63 is 0\(en6, which are the number of arguments taken by the
  64 system call
  65 .IP
  66 .I type
  67 is the return type of the system call
  68 .IP
  69 .I name
  70 is the name of the system call
  71 .IP
  72 .I typeN
  73 is the Nth argument's type
  74 .IP
  75 .I argN
  76 is the name of the Nth argument
  77 .PP
  78 These macros create a function called
  79 .I name
  80 with the arguments you
  81 specify.
  82 Once you include the _syscall() in your source file,
  83 you call the system call by
  84 .IR name .
  85 .SH FILES
  86 .I /usr/include/linux/unistd.h
  87 .SH CONFORMING TO
  88 The use of these macros is Linux-specific, and deprecated.
  89 .SH NOTES
  90 Starting around kernel 2.6.18, the _syscall macros were removed
  91 from header files supplied to user space.
  92 Use
  93 .BR syscall (2)
  94 instead.
  95 (Some architectures, notably ia64, never provided the _syscall macros;
  96 on those architectures,
  97 .BR syscall (2)
  98 was always required.)
  99 .PP
 100 The _syscall() macros
 101 .I "do not"
 102 produce a prototype.
 103 You may have to
 104 create one, especially for C++ users.
 105 .PP
 106 System calls are not required to return only positive or negative error
 107 codes.
 108 You need to read the source to be sure how it will return errors.
 109 Usually, it is the negative of a standard error code,
 110 for example,
 111 .RI \- EPERM .
 112 The _syscall() macros will return the result
 113 .I r
 114 of the system call
 115 when
 116 .I r
 117 is nonnegative, but will return \-1 and set the variable
 118 .I errno
 119 to
 120 .RI \- r
 121 when
 122 .I r
 123 is negative.
 124 For the error codes, see
 125 .BR errno (3).
 126 .PP
 127 When defining a system call, the argument types
 128 .I must
 129 be
 130 passed by-value or by-pointer (for aggregates like structs).
 131 .\" The preferred way to invoke system calls that glibc does not know
 132 .\" about yet is via
 133 .\" .BR syscall (2).
 134 .\" However, this mechanism can be used only if using a libc
 135 .\" (such as glibc) that supports
 136 .\" .BR syscall (2),
 137 .\" and if the
 138 .\" .I <sys/syscall.h>
 139 .\" header file contains the required SYS_foo definition.
 140 .\" Otherwise, the use of a _syscall macro is required.
 141 .\"
 142 .SH EXAMPLE
 143 .EX
 144 #include <stdio.h>
 145 #include <stdlib.h>
 146 #include <errno.h>
 147 #include <linux/unistd.h>       /* for _syscallX macros/related stuff */
 148 #include <linux/kernel.h>       /* for struct sysinfo */
 149
 150 _syscall1(int, sysinfo, struct sysinfo *, info);
 151
 152 int
 153 main(void)
 154 {
 155     struct sysinfo s_info;
 156     int error;
 157
 158     error = sysinfo(&s_info);
 159     printf("code error = %d\en", error);
 160     printf("Uptime = %lds\enLoad: 1 min %lu / 5 min %lu / 15 min %lu\en"
 161            "RAM: total %lu / free %lu / shared %lu\en"
 162            "Memory in buffers = %lu\enSwap: total %lu / free %lu\en"
 163            "Number of processes = %d\en",
 164            s_info.uptime, s_info.loads[0],
 165            s_info.loads[1], s_info.loads[2],
 166            s_info.totalram, s_info.freeram,
 167            s_info.sharedram, s_info.bufferram,
 168            s_info.totalswap, s_info.freeswap,
 169            s_info.procs);
 170     exit(EXIT_SUCCESS);
 171 }
 172 .EE
 173 .SS Sample output
 174 .EX
 175 code error = 0
 176 uptime = 502034s
 177 Load: 1 min 13376 / 5 min 5504 / 15 min 1152
 178 RAM: total 15343616 / free 827392 / shared 8237056
 179 Memory in buffers = 5066752
 180 Swap: total 27881472 / free 24698880
 181 Number of processes = 40
 182 .EE
 183 .SH SEE ALSO
 184 .BR intro (2),
 185 .BR syscall (2),
 186 .BR errno (3)