SYSCALL(9) | Kernel Developer's Manual | SYSCALL(9)

# 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:

`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 */
    ...
    	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

The `SYSCALL_DEBUG` parts of the code are explained in the Debugging section below. For the `KTRACE` portions of the code refer to the ktrace(9) document for further explanations.

## 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 scdebug to the desired value.

# 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.

December 13, 2023 | Debian