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. */
            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.  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


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

              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

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

              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.