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
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 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
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
syscall is declared in unistd.h.
syscallperforms a generic system call.
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,
errnoto an error code that the system call returned. Note that system calls do not return
-1when they succeed. If you specify an invalid sysno,
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);