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



NAME
       intro, _syscall - 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).

   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.

   Synopsis
       #include 

       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 specify.  Once you
       include the _syscall() in your source file, you call the system call by name.

EXAMPLE
       #include 
       #include 
       #include      /* for _syscallX macros/related stuff */
       #include      /* 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);
            return(0);
       }

Sample Output
       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 create 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).

       The  preferred  way  to  invoke  system  calls  that glibc does not know about yet, is via
       syscall(2).

CONFORMING TO
       Certain codes are used to indicate Unix variants and standards to which calls in the  sec-
       tion conform.  These are:

       SVr4   System  V Release 4 Unix, as described in the "Programmer's Reference Manual: Oper-
              ating System API (Intel processors)" (Prentice-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

SEE ALSO
       syscall(2), errno(3)



Linux 1.2.13                                1996-05-22                                   INTRO(2)