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

       intro - Introduction to system calls

       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.

       #include <linux/unistd.h>

       A _syscall macro

       desired system call

       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:


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

       #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,

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

       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 nega-
       tive  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.

       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

       Certain codes are used to indicate Unix variants and stan-
       dards to which calls in the section conform.  These are:

       SVr4   System  V Release 4 Unix, as described in the "Pro-
              grammer's Reference Manual:  Operating  System  API
              (Intel   processors)"   (Prentice-Hall  1992,  ISBN

       SVID   System V Interface Definition, as described in "The
              System  V  Interface  Definition,  Fourth Edition",
              available   at
              standards/svid in Postscript files.

              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.

              IEEE  Std 1003.1b-1993 (POSIX.1b standard) describ-
              ing 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

              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.


       Look  at  the  source  header  of  the manual page for the
       author(s) and copyright conditions.  Note that  these  can
       be different from page to page!

Linux 1.2.13               22 May 1996                          1