NAME

nsdispatch — name-service switch dispatcher routine

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <nsswitch.h>

int
nsdispatch(void *nsdrv, const ns_dtab dtab[], const char *database, const char *name, const ns_src defaults[], ...);

DESCRIPTION

The nsdispatch() function invokes the callback functions specified in dtab in the order given in /etc/nsswitch.conf for the database database until the action criteria for a source of that database is fulfilled.

nsdrv is passed to each callback function to use as necessary (to pass back to the caller of nsdispatch()).

dtab is an array of ns_dtab structures, which have the following format:

typedef struct { 
	const char *src; 
	nss_method cb; 
	void *cb_data; 
} ns_dtab;

The dtab array should consist of one entry for each source type that has a static implementation, with src as the name of the source, cb as a callback function which handles that source, and cb_data as a pointer to arbitrary data to be passed to the callback function. The last entry in dtab should contain NULL values for src, cb, and cb_data.
The callback function signature is described by the typedef:

typedef int (*nss_method)(void *cbrv, void *cbdata, va_list ap);;

cbrv

The nsdrv that nsdispatch() was invoked with.

cbdata

The cb_data member of the array entry for the source that this callback function implements in the dtab argument of nsdispatch().

ap

The ... arguments to nsdispatch(), converted to a va_list.

database and name are used to select methods from optional per-source dynamically-loaded modules. name is usually the name of the function calling nsdispatch(). Note that the callback functions provided by dtab take priority over those implemented in dynamically-loaded modules in the event of a conflict.

defaults contains a list of default sources to try in the case of a missing or corrupt nsswitch.conf(5), or if there isn't a relevant entry for database. It is an array of ns_src structures, which have the following format:

typedef struct { 
	const char *src; 
	uint32_t flags; 
} ns_src;

The defaults array should consist of one entry for each source to consult by default indicated by src, and flags set to the desired behavior (usually NS_SUCCESS; refer to Callback function return values for more information). The last entry in defaults should have src set to NULL and flags set to 0.
Some invokers of nsdispatch() (such as setgrent(3)) need to force all callback functions to be invoked, irrespective of the action criteria listed in nsswitch.conf(5). This can be achieved by adding NS_FORCEALL to defaults[0].flags before invoking nsdispatch(). The return value of nsdispatch() will be the result of the final callback function invoked.
For convenience, a global variable defined as:

extern const ns_src __nsdefaultsrc[];

exists which contains a single default entry for ‘files’ for use by callers which don't require complicated default rules.

... are optional extra arguments, which are passed to the appropriate callback function as a stdarg(3) variable argument list of the type va_list.

nsdispatch returns the value of the callback function that caused the dispatcher to finish, or NS_NOTFOUND otherwise.

Dynamically-loaded module interface

The nsdispatch() function loads callback functions from the run-time link-editor's search path using the following naming convention:

```
nss_<source>.so.<version>
```
<source>

The source that the module implements.

<version>

The nsdispatch module interface version, which is defined by the integer NSS_MODULE_INTERFACE_VERSION, which has the value 0.

When a module is loaded, nsdispatch() looks for and calls the following function in the module:

ns_mtab * nss_module_register(const char *source, u_int *nelems, nss_module_unregister_fn *unreg);;

The unregister function signature is described by the typedef:

typedef void (*nss_module_unregister_fn)(ns_mtab *mtab, u_int nelems);;

nss_module_register() returns an array of ns_mtab structures (with *nelems entries), or NULL if there was a failure. The ns_mtab structures have the following format:

typedef struct { 
	const char *database; 
	const char *name; 
	nss_method method; 
	void *mdata; 
} ns_mtab;

The mtab array should consist of one entry for each callback function (method) that is implemented, with database as the name of the database, name as the name of the callback function, method as the nss_method callback function that implements the method, and mdata as a pointer to arbitrary data to be passed to the callback function as its cbdata argument.

Valid source types

While there is support for arbitrary sources, the following #defines for commonly implemented sources are provided:

#define	Value
NSSRC_FILES	files
NSSRC_DNS	dns
NSSRC_NIS	nis
NSSRC_COMPAT	compat

Refer to nsswitch.conf(5) for a complete description of what each source type is.

Valid database types

While there is support for arbitrary databases, the following #defines for currently implemented system databases are provided:

#define	Value
NSDB_HOSTS	hosts
NSDB_GROUP	group
NSDB_GROUP_COMPAT	group_compat
NSDB_NETGROUP	netgroup
NSDB_NETWORKS	networks
NSDB_PASSWD	passwd
NSDB_PASSWD_COMPAT	passwd_compat
NSDB_SHELLS	shells

Refer to nsswitch.conf(5) for a complete description of what each database is.

Callback function return values

The callback functions should return one of the following values depending upon status of the lookup:

Return value	Status code
NS_SUCCESS	The requested entry was found.
NS_NOTFOUND	The entry is not present at this source.
NS_TRYAGAIN	The source is busy, and may respond to retries.
NS_UNAVAIL	The source is not responding, or entry is corrupt.

CALLBACK FUNCTION API FOR STANDARD DATABASES

The organization of the ap argument for an nss_method() callback function for a standard method in a standard database is:

Pointer to return value of the standard function.
First argument of the standard function.
(etc.)

For example, given the standard function getgrnam(3):

struct group * getgrnam(const char *name)

the ap organization used by the callback functions is:

struct group **
const char *

NOTE: Not all standard databases are using this calling convention yet; those that aren't are noted below. These will be changed in the future.

The callback function names and va_list organization for various standard database callback functions are:

Methods for hosts database

NOTE: The method APIs for this database will be changing in the near future.

Methods for group and group_compat databases

Methods for netgroup database

NOTE: The method APIs for this database will be changing in the near future.

Methods for networks database

Methods for passwd and passwd_compat databases

Methods for shells database

HISTORY

The nsdispatch routines first appeared in NetBSD 1.4. Support for dynamically-loaded modules first appeared in NetBSD 3.0.

AUTHORS

Luke Mewburn <lukem@NetBSD.org> wrote this freely distributable name-service switch implementation, using ideas from the ULTRIX svc.conf(5) and Solaris nsswitch.conf(4) manual pages. Support for dynamically-loaded modules was added by Jason Thorpe <thorpej@NetBSD.org>, based on code developed by the FreeBSD Project.

Manual browser: nsdispatch(3)