xref: /openbsd-src/share/man/man9/syscall.9 (revision 307a624eaecbd09c2301d08683aff3db9a3e8bb2)

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.\" Copyright (c) 2003 Michael Shalayeff
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.Dd $Mdocdate: December 13 2023 $
.Dt SYSCALL 9
.Os
.Sh NAME
.Nm syscall
.Nd system calls overview
.Sh DESCRIPTION
System calls in the kernel are implemented through a set of
switch tables for each emulation type.
Each table is generated from the
.Dq master
file by
.Pa sys/kern/makesyscalls.sh
through the appropriate rules in the
.Pa Makefile .
.Pp
The
.Dq 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:
.Pp
.D1 Cd number type ...
.Pp
Where:
.Bl -tag -width number -compact
.It number
is the system call number;
.It type
is one of:
.Bl -tag -width COMPAT_XXX -compact
.It STD
always included;
.It OBSOL
obsolete, not included in the system;
.It UNIMPL
unimplemented, not included in the system;
.It NODEF
included, but don't define the syscall number;
.It NOARGS
included, but don't define the syscall args structure;
.It INDIR
included, but don't define the syscall args structure,
and allow it to be "really" varargs;
.It COMPAT_XX
a compatibility system call, only included if the corresponding
option is configured for the kernel (see
.Xr options 4 ) .
.El
.El
.Pp
The rest of the line for the STD, NODEF, NOARGS, and COMPAT_XX
types is:
.Pp
.D1 Cd { pseudo-proto } [alias]
.Pp
.Nm 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.
.Pp
For other types the rest of the line is a comment.
.Pp
To generate the header and code files from the
.Dq master
file a
.Xr make 1
command has to be run from the directory containing the
.Dq master
file.
.Ss 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:
.Bd -literal -offset indent

	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 */
\&...
	rval[0] = 0;

/* the following steps are now performed using mi_syscall() */
#ifdef SYSCALL_DEBUG
	scdebug_call(p, code, args);
#endif
#ifdef KTRACE
	if (KTRPOINT(p, KTR_SYSCALL))
		ktrsyscall(p, code, argsize, args);
#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;
	}

/* the following steps are now performed using mi_syscall_return() */
#ifdef SYSCALL_DEBUG
	scdebug_ret(p, code, orig_error, rval);
#endif
	userret(p);
#ifdef KTRACE
	if (KTRPOINT(p, KTR_SYSRET))
		ktrsysret(p, code, orig_error, rval[0]);
#endif

.Ed
.Pp
The
.Dv SYSCALL_DEBUG
parts of the code are explained in the
.Sx Debugging
section below.
For the
.Dv KTRACE
portions of the code refer to the
.Xr ktrace 9
document for further explanations.
.Ss Debugging
For debugging purposes the line
.Pp
.D1 Cd option SYSCALL_DEBUG
.Pp
should be included in the kernel configuration file (see
.Xr options 4 ) .
This allows tracing for calls, returns, and arguments for both
implemented and non-implemented system calls.
A global integer variable
.Va scdebug
contains a mask for the desired logging events:
.Pp
.Bl -tag -width SCDEBUG_SHOWARGS__ -compact
.It SCDEBUG_CALLS
(0x0001) show calls;
.It SCDEBUG_RETURNS
(0x0002) show returns;
.It SCDEBUG_ALL
(0x0004) show even syscalls that are implemented;
.It SCDEBUG_SHOWARGS
(0x0008) show arguments to calls.
.El
.Pp
Use
.Xr ddb 4
to set
.Va scdebug
to the desired value.
.Sh CODE REFERENCES
.Bl -tag -width sys/kern/syscalls.master -compact
.It Pa sys/kern/makesyscalls.sh
a
.Xr sh 1
script for generating C files out of the syscall master file;
.It Pa sys/kern/syscalls.conf
a configuration file for the shell script above;
.It Pa sys/kern/syscalls.master
master files describing names and numbers for the system calls;
.It Pa sys/kern/syscalls.c
system call names lists;
.It Pa sys/kern/init_sysent.c
system call switch tables;
.It Pa sys/sys/syscallargs.h
system call argument lists;
.It Pa sys/sys/syscall.h
system call numbers;
.It Pa sys/sys/syscall_mi.h
Machine-independent syscall entry end return handling.
.El
.Sh SEE ALSO
.Xr ktrace 2 ,
.Xr ktrace 9 ,
.Xr sysctl_int 9
.Sh HISTORY
The
.Nm
section manual page appeared in
.Ox 3.4 .