_syscall(2)

1_syscall(2)                   System Calls Manual                  _syscall(2)
2
3
4

NAME

6       _syscall - invoking a system call without library support (OBSOLETE)
7

SYNOPSIS

9       #include <linux/unistd.h>
10
11       A _syscall macro
12
13       desired system call
14

DESCRIPTION

16       The  important thing to know about a system call is its prototype.  You
17       need to know how many arguments, their types, and the  function  return
18       type.  There are seven macros that make the actual call into the system
19       easier.  They have the form:
20
21           _syscallX(type,name,type1,arg1,type2,arg2,...)
22
23       where
24
25              X is 0–6, which are the number of arguments taken by the  system
26              call
27
28              type is the return type of the system call
29
30              name is the name of the system call
31
32              typeN is the Nth argument's type
33
34              argN is the name of the Nth argument
35
36       These macros create a function called name with the arguments you spec‐
37       ify.  Once you include the _syscall() in your source file, you call the
38       system call by name.
39

FILES

41       /usr/include/linux/unistd.h
42

STANDARDS

44       Linux.
45

HISTORY

47       Starting  around  Linux  2.6.18,  the _syscall macros were removed from
48       header files supplied to user space.  Use  syscall(2)  instead.   (Some
49       architectures,  notably  ia64,  never  provided the _syscall macros; on
50       those architectures, syscall(2) was always required.)
51

NOTES

53       The _syscall() macros do not produce a prototype.  You may have to cre‐
54       ate one, especially for C++ users.
55
56       System calls are not required to return only positive or negative error
57       codes.  You need to read the source to be sure how it will  return  er‐
58       rors.   Usually, it is the negative of a standard error code, for exam‐
59       ple, -EPERM.  The _syscall() macros will return the  result  r  of  the
60       system call when r is nonnegative, but will return -1 and set the vari‐
61       able errno to -r when r is negative.  For  the  error  codes,  see  er‐
62       rno(3).
63
64       When defining a system call, the argument types must be passed by-value
65       or by-pointer (for aggregates like structs).
66

EXAMPLES

68       #include <stdio.h>
69       #include <stdlib.h>
70       #include <errno.h>
71       #include <linux/unistd.h>       /* for _syscallX macros/related stuff */
72       #include <linux/kernel.h>       /* for struct sysinfo */
73
74       _syscall1(int, sysinfo, struct sysinfo *, info);
75
76       int
77       main(void)
78       {
79           struct sysinfo s_info;
80           int error;
81
82           error = sysinfo(&s_info);
83           printf("code error = %d\n", error);
84           printf("Uptime = %lds\nLoad: 1 min %lu / 5 min %lu / 15 min %lu\n"
85                  "RAM: total %lu / free %lu / shared %lu\n"
86                  "Memory in buffers = %lu\nSwap: total %lu / free %lu\n"
87                  "Number of processes = %d\n",
88                  s_info.uptime, s_info.loads[0],
89                  s_info.loads[1], s_info.loads[2],
90                  s_info.totalram, s_info.freeram,
91                  s_info.sharedram, s_info.bufferram,
92                  s_info.totalswap, s_info.freeswap,
93                  s_info.procs);
94           exit(EXIT_SUCCESS);
95       }
96
97   Sample output
98       code error = 0
99       uptime = 502034s
100       Load: 1 min 13376 / 5 min 5504 / 15 min 1152
101       RAM: total 15343616 / free 827392 / shared 8237056
102       Memory in buffers = 5066752
103       Swap: total 27881472 / free 24698880
104       Number of processes = 40
105