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(libc.info)System Calls

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System Calls
============

   A system call is a request for service that a program makes of the
kernel.  The service is generally something that only the kernel has
the privilege to do, such as doing I/O.  Programmers don't normally
need to be concerned with system calls because there are functions in
the GNU C library to do virtually everything that system calls do.
These functions work by making system calls themselves.  For example,
there is a system call that changes the permissions of a file, but you
don't need to know about it because you can just use the GNU C
library's `chmod' function.

   System calls are sometimes called kernel calls.

   However, there are times when you want to make a system call
explicitly, and for that, the GNU C library provides the `syscall'
function.  `syscall' is harder to use and less portable than functions
like `chmod', but easier and more portable than coding the system call
in assembler instructions.

   `syscall' is most useful when you are working with a system call
which is special to your system or is newer than the GNU C library you
are using.  `syscall' is implemented in an entirely generic way; the
function does not know anything about what a particular system call
does or even if it is valid.

   The description of `syscall' in this section assumes a certain
protocol for system calls on the various platforms on which the GNU C
library runs.  That protocol is not defined by any strong authority, but
we won't describe it here either because anyone who is coding `syscall'
probably won't accept anything less than kernel and C library source
code as a specification of the interface between them anyway.

   `syscall' is declared in `unistd.h'.

 - Function: long int syscall (long int SYSNO, ...)
     `syscall' performs a generic system call.

     SYSNO is the system call number.  Each kind of system call is
     identified by a number.  Macros for all the possible system call
     numbers are defined in `sys/syscall.h'

     The remaining arguments are the arguments for the system call, in
     order, and their meanings depend on the kind of system call.  Each
     kind of system call has a definite number of arguments, from zero
     to five.  If you code more arguments than the system call takes,
     the extra ones to the right are ignored.

     The return value is the return value from the system call, unless
     the system call failed.  In that case, `syscall' returns `-1' and
     sets `errno' to an error code that the system call returned.  Note
     that system calls do not return `-1' when they succeed.

     If you specify an invalid SYSNO, `syscall' returns `-1' with
     `errno' = `ENOSYS'.

     Example:


          #include <unistd.h>
          #include <sys/syscall.h>
          #include <errno.h>
          
          ...
          
          int rc;
          
          rc = syscall(SYS_chmod, "/etc/passwd", 0444);
          
          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

     This, if all the compatibility stars are aligned, is equivalent to
     the following preferable code:


          #include <sys/types.h>
          #include <sys/stat.h>
          #include <errno.h>
          
          ...
          
          int rc;
          
          rc = chmod("/etc/passwd", 0444);
          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);