syscall - system calls overview
System calls in the kernel are implemented through a set of
switch tables
for each emulation type. Each table is generated from the
``master''
file by sys/kern/makesyscalls.sh through the appropriate
rules in the
Makefile.
The ``master'' file is a text file consisting of a list of
lines for each
system call. Lines may be split by the means of back slashing the end of
the line. Each line is a set of fields separated by whitespace:
number type ...
Where:
number is the system call number;
type is one of:
STD always included;
OBSOL obsolete, not included in the system;
UNIMPL unimplemented, not included in the system;
NODEF included, but don't define the syscall
number;
NOARGS included, but don't define the syscall
args structure;
INDIR included, but don't define the syscall
args structure,
and allow it to be "really"
varargs;
COMPAT_XX a compatibility system call, only included if the
corresponding option is configured for
the kernel
(see options(4)).
The rest of the line for the STD, NODEF, NOARGS, and COMPAT_XX types is:
{ pseudo-proto } [alias]
pseudo-proto is a C-like prototype used to generate the system call argument
list, and alias is an optional name alias for the call.
The function
in the prototype has to be defined somewhere in the
kernel sources
as it will be used as an entry point for the corresponding
system call.
For other types the rest of the line is a comment.
To generate the header and code files from the ``master''
file a make(1)
command has to be run from the directory containing the
``master'' file.
Usage [Toc] [Back]
Entry from the user space for the system call is machine dependent. Typical
code to invoke a system call from the machine dependent
sources
might look like this:
const struct sysent *callp;
register_t code, args[8], rval[2];
struct proc *p = curproc;
int code, nsys;
...
/* ``code'' is the system call number passed from the
user space */
...
if (code < 0 || code >= nsys)
callp += p->p_emul->e_nosys; /* illegal */
else
callp += code;
/* copyin the arguments from the user space */
...
#ifdef SYSCALL_DEBUG
scdebug_call(p, code, args);
#endif
#ifdef KTRACE
if (KTRPOINT(p, KTR_SYSCALL))
ktrsyscall(p, code, argsize, args);
#endif
rval[0] = 0;
#if NSYSTRACE > 0
if (ISSET(p->p_flag, P_SYSTRACE))
error = systrace_redirect(code, p,
args, rval);
else
#endif
error = (*callp->sy_call)(p, args,
rval);
switch (error) {
case 0:
/* normal return */
...
break;
case ERESTART:
/*
* adjust PC to point before the system call
* in the user space in order for the
return
* back there we reenter the kernel to
repeat
* the same system call
*/
...
break;
case EJUSTRETURN:
/* just return */
break;
default:
/*
* an error returned:
* call an optional emulation errno mapping
* routine and return back to the
user.
*/
if (p->p_emul->e_errno)
error = p->p_emul->e_errno[error];
...
break;
}
#ifdef SYSCALL_DEBUG
scdebug_ret(p, code, orig_error, rval);
#endif
userret(p, frame.tf_eip, sticks);
#ifdef KTRACE
if (KTRPOINT(p, KTR_SYSRET))
ktrsysret(p, code, orig_error,
rval[0]);
#endif
The ``SYSCALL_DEBUG'' parts of the code are explained in the
section
Debugging later in the document. For the ``KTRACE'' portions of the code
refer to the ktrace(9) document for further explanations.
``NSYSTRACE''
is a system call tracing facility and is explained in the
systrace(9) and
systrace(4) documents.
Debugging [Toc] [Back]
For debugging purposes the line
option SYSCALL_DEBUG
should be included in the kernel configuration file (see options(4)).
This allows tracing for calls, returns, and arguments for
both implemented
and non-implemented system calls. A global integer variable contains
a mask for the desired logging events:
SCDEBUG_CALLS (0x0001) show calls;
SCDEBUG_RETURNS (0x0002) show returns;
SCDEBUG_ALL (0x0004) show even syscalls that are implemented;
SCDEBUG_SHOWARGS (0x0008) show arguments to calls.
Use ddb(4) to set the ``scdebug'' to a value desired.
sys/kern/makesyscalls.sh a sh(1) script for generating C
files out of
the syscall master file;
sys/{kern,compat/*}/syscalls.conf
a configuration file for the shell
script
above;
sys/{kern,compat/*}/syscalls.master
master files describing names and
numbers for
the system calls;
sys/{kern/,compat/*/*_}syscalls.c
system call names lists;
sys/{kern/init,compat/*/*}_sysent.c
system call switch tables;
sys/{sys/,compat/*/*_}syscallargs.h
system call argument lists;
sys/{sys/,compat/*/*_}syscall.h
system call numbers.
ktrace(2), syscall(2), systrace(4), ktrace(9), systrace(9)
The syscall section manual page appeared in OpenBSD 3.4.
OpenBSD 3.6 July 21, 2003
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