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

       _syscall - invoking a system call without library support (OBSOLETE)

       #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  seven
       macros that make the actual call into the system easier.  They have the form:



              X is 0-6, 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.


       The use of these macros is Linux-specific, and deprecated.

       Starting around kernel 2.6.18, the _syscall macros were removed from  header  files
       supplied  to  user  space.   Use  syscall(2) instead.  (Some architectures, notably
       ia64, never provided the _syscall macros; on those  architectures,  syscall(2)  was
       always required.)

       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, for example, -EPERM.  The _syscall() macros will
       return  the  result r of the system call when r is non-negative, but will return -1
       and set the variable errno to -r when r is negative.   For  the  error  codes,  see

       When  defining  a  system  call,  the argument types must be passed by-value or by-
       pointer (for aggregates like structs).

       #include <stdio.h>
       #include <stdlib.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. */

           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

       intro(2), syscall(2), errno(3)

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

Linux                             2007-12-19                       _SYSCALL(2)

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