SYSCALL(9) OpenBSD Kernel Manual SYSCALL(9)NAMEsyscall - system calls overview
DESCRIPTION
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
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
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
``scdebug'' 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.
CODE REFERENCES
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.
SEE ALSOktrace(2), syscall(2), systrace(4), ktrace(9), sysctl_int(9), systrace(9)HISTORY
The syscall section manual page appeared in OpenBSD 3.4.
OpenBSD 4.9 September 19, 2010 OpenBSD 4.9
