intro

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NAME

       intro - Introduction to system calls


DESCRIPTION

       This chapter describes the Linux system calls.

   Calling Directly
       In  most  cases, it is unnecessary to invoke a system call
       directly, but there are times with the Standard C  library
       does not implement a nice function call for you.

   Synopsis
       #include <linux/unistd.h>

       A _syscall macro

       desired system call


   Setup
       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:

              _syscallX(type,name,type1,arg1,type2,arg2,...)

                     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.


EXAMPLE

       #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,
                 s_info.procs);
            return(0);
       }


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


NOTES

       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
       structs).


FILES

       /usr/include/linux/unistd.h


AUTHORS

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