NSDISPATCH(3) FreeBSD Library Functions Manual NSDISPATCH(3)


nsdispatchname-service switch dispatcher routine


Standard C Library (libc, -lc)


#include < sys/types.h>
#include < stdarg.h>
#include < nsswitch.h>

nsdispatch( void *retval, const ns_dtab dtab[], const char *database, const char *method_name, const ns_src defaults[], ...);


The nsdispatch() function invokes the methods specified in dtab in the order given by nsswitch.conf(5) for the database database until a successful entry is found.

retval is passed to each method to modify as necessary, to pass back results to the caller of nsdispatch().

Each method has the function signature described by the typedef:

typedef int (*nss_method)( void *retval, void *mdata, va_list *ap);

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

typedef struct _ns_dtab { 
 const char *src; 
 nss_method  method; 
 void  *mdata; 
} ns_dtab;

The dtab array should consist of one entry for each source type that is implemented, with src as the name of the source, method as a function which handles that source, and mdata as a handle on arbitrary data to be passed to the method. The last entry in dtab should contain NULL values for src, method, and mdata.

Additionally, methods may be implemented in NSS modules, in which case they are selected using the database and method_name arguments along with the configured source. (The methods supplied via dtab take priority over those implemented in NSS modules in the event of a conflict.)

defaults contains a list of default sources to try if nsswitch.conf(5) is missing or corrupted, or if there is no relevant entry for database. It is an array of ns_src structures, which have the following format:

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

The defaults array should consist of one entry for each source to be configured by default indicated by src, and flags set to the criterion desired (usually NS_SUCCESS; refer to Method return values for more information). The last entry in defaults should have src set to NULL and flags set to 0.

For convenience, a global variable defined as:

extern const ns_src __nsdefaultsrc[];

exists which contains a single default entry for the source ‘files’ that may be used by callers which do not require complicated default rules.

...’ are optional extra arguments, which are passed to the appropriate method as a variable argument list of the type va_list.

Valid source types

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

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

Method return values

The nss_method functions must return one of the following values depending upon status of the lookup:
Return value Status code
NS_SUCCESS success
NS_NOTFOUND notfound
NS_UNAVAIL unavail
NS_TRYAGAIN tryagain
NS_RETURN -none-

Refer to nsswitch.conf(5) for a complete description of each status code.

The nsdispatch() function returns the value of the method that caused the dispatcher to terminate, or NS_NOTFOUND otherwise.


FreeBSD's Standard C Library (libc, -lc) provides stubs for compatibility with NSS modules written for the GNU C Library nsswitch interface. However, these stubs only support the use of the “ passwd” and “ group” databases.


The nsdispatch() function first appeared in FreeBSD 5.0. It was imported from the NetBSD Project, where it appeared first in NetBSD 1.4. Support for NSS modules first appeared in FreeBSD 5.1.


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. The FreeBSD Project added the support for threads and NSS modules, and normalized the uses of nsdispatch() within the standard C library.
April 4, 2010 FreeBSD