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INTRO(2)                   Linux Programmer's Manual                  INTRO(2)



NAME
       intro - Introduction to system calls

DESCRIPTION
       This  chapter  describes the Linux system calls.  For a list of the 164
       syscalls present in Linux 2.0, see syscalls(2).

   CallingDirectly
       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.

   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:

              _syscallX(type,name,type1,arg1,type2,arg2,...)

                     where  X  is 0-5, 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 spec-
       ify.  Once you include the _syscall() in your source file, you call the
       system call by name.

EXAMPLE
       #include <stdio.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 = %ds\nLoad: 1 min %d / 5 min %d / 15 min %d\n"
                       "RAM: total %d / free %d / shared %d\n"
                       "Memory in buffers = %d\nSwap: total %d / free %d\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);
            return(0);
       }

SampleOutput
       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

NOTES
       The _syscall() macros DO NOT produce a prototype.  You may have to cre-
       ate 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, e.g.,
       -EPERM.  The _syscall() macros will return the result r of  the  system
       call  when  r  is  nonnegative, 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,  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).

CONFORMINGTO
       Certain codes are used to indicate Unix variants and standards to which
       calls in the section conform.  These are:

       SVr4   System V Release 4 Unix, as described in the "Programmer's  Ref-
              erence  Manual:  Operating System API (Intel processors)" (Pren-
              tice-Hall 1992, ISBN 0-13-951294-2)

       SVID   System V Interface Definition, as described  in  "The  System  V
              Interface Definition, Fourth Edition".

       POSIX.1
              IEEE  1003.1-1990  part  1,  aka ISO/IEC 9945-1:1990s, aka "IEEE
              Portable Operating System Interface for Computing Environments",
              as  elucidated  in  Donald  Lewine's  "POSIX Programmer's Guide"
              (O'Reilly & Associates, Inc., 1991, ISBN 0-937175-73-0.

       POSIX.1b
              IEEE Std 1003.1b-1993 (POSIX.1b standard)  describing  real-time
              facilities   for   portable   operating   systems,  aka  ISO/IEC
              9945-1:1996, as elucidated in "Programming for the real world  -
              POSIX.4"  by  Bill  O.  Gallmeister (O'Reilly & Associates, Inc.
              ISBN 1-56592-074-0).

       SUS, SUSv2
              Single Unix Specification.  (Developed by X/Open  and  The  Open
              Group. See also http://www.UNIX-systems.org/version2/ .)

       4.3BSD/4.4BSD
              The  4.3  and  4.4  distributions  of Berkeley Unix.  4.4BSD was
              upward-compatible from 4.3.

       V7     Version 7, the ancestral Unix from Bell Labs.

FILES
       /usr/include/linux/unistd.h

SEEALSO
       errno(3)



Linux 1.2.13                      1996-05-22                          INTRO(2)