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  pro-
     totype.   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


               where X is 05, 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 negative
     of a standard error code, e.g., -EPERM.  The _syscall() mac-
     ros 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 structs).


     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  Reference  Manual:  Operating System API
          (Intel processors)"  (Prentice-Hall  1992,  ISBN  0-13-

     SVID System V Interface Definition,  as  described  in  "The
          System  V Interface Definition, Fourth Edition", avail-
          able       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)  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).

          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.




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     author(s)  and copyright conditions.  Note that these can be
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