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


       #include <stdio.h>
       #include <errno.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 = %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,

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() 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).


       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 0-13-951294-2)

       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) 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 .)

              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.




       syscall(2), errno(3)