OpenBSD manual page server

NAME

syscall — 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:

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 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.

	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

CODE REFERENCES

sys/kern/makesyscalls.sh

a sh(1) script for generating C files out of the syscall master file;

sys/kern/syscalls.conf

a configuration file for the shell script above;

sys/kern/syscalls.master

master files describing names and numbers for the system calls;

sys/kern/syscalls.c

system call names lists;

sys/kern/init_sysent.c

system call switch tables;

sys/sys/syscallargs.h

system call argument lists;

sys/sys/syscall.h

system call numbers;

sys/sys/syscall_mi.h

Machine-independent syscall entry end return handling.

SEE ALSO

ktrace(2), ktrace(9), sysctl_int(9)

HISTORY

The syscall section manual page appeared in OpenBSD 3.4.