1_SYSCALL(2)                Linux Programmer's Manual               _SYSCALL(2)
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NAME

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

9       #include <linux/unistd.h>
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11       A _syscall macro
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13       desired system call
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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:
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21           _syscallX(type,name,type1,arg1,type2,arg2,...)
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23       where
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25              X is 0–6, which are the number of arguments taken by the  system
26              call
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28              type is the return type of the system call
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30              name is the name of the system call
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32              typeN is the Nth argument's type
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34              argN is the name of the Nth argument
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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.
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FILES

41       /usr/include/linux/unistd.h
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CONFORMING TO

44       The use of these macros is Linux-specific, and deprecated.
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NOTES

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

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

106       intro(2), syscall(2), errno(3)
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COLOPHON

109       This page is part of release 5.10 of the Linux  man-pages  project.   A
110       description  of  the project, information about reporting bugs, and the
111       latest    version    of    this    page,    can     be     found     at
112       https://www.kernel.org/doc/man-pages/.
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116Linux                             2020-06-09                       _SYSCALL(2)
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