To use the NIS name services, the client process must be "bound" to an NIS server that serves the appropriate domain using yp_bind(). Binding need not be done explicitly by user code. Binding is done automatically whenever an NIS lookup function is called. The yp_bind() function can be called directly for processes that make use of a backup strategy, for example, a local file in cases when NIS services are not available. A process should call yp_unbind() when it is finished using NIS in order to free up resources.

Each binding allocates or uses up one client process socket descriptor. Each bound domain costs one socket descriptor. However, multiple requests to the same domain use that same descriptor. The yp_unbind() function is available at the client interface for processes that explicitly manage their socket descriptors while accessing multiple domains. The call to yp_unbind() makes the domain unbound, and frees all per-process and per-node resources used to bind it.

If an RPC failure results upon use of a binding, that domain will be unbound automatically. At that point, the ypclnt() layer will retry a few more times or until the operation succeeds, provided that rpcbind(8) and ypbind(8) are running, and either:

Under the following circumstances, the ypclnt layer will return control to the user code, with either an error or success code and the results:

NIS lookup calls require a map name and a domain name, at minimum. The client process should know the name of the map of interest. Client processes fetch the node's default domain by calling yp_get_default_domain() and use the returned outdomain as the indomain parameter to successive NIS name service calls. The domain returned is the same as that returned using the SI_SRPC_DOMAIN command to the sysinfo(2) system call. The value returned in outdomain should not be freed.

The yp_match() function returns the value associated with a passed key. This key must be exact because no pattern matching is available. yp_match() requires a full YP map name, such as hosts.byname, instead of the nickname hosts.

The yp_first() function returns the first key-value pair from the named map in the named domain.

The yp_next() function returns the next key-value pair in a named map. The inkey parameter must be the outkey returned from an initial call to yp_first() (to get the second key-value pair) or the one returned from the nth call to yp_next() (to get the nth + second key-value pair). Similarly, the inkeylen parameter must be the outkeylen returned from the earlier yp_first() or yp_next() call.

The concept of first and next is particular to the structure of the NIS map being processed. Retrieval order is not related to either the lexical order within any original (non-NIS name service) data base, or to any obvious numerical sorting order on the keys, values, or key-value pairs. The only ordering guarantee is that if the yp_first() function is called on a particular map, and then the yp_next() function is repeatedly called on the same map at the same server until the call fails with a reason of YPERR_NOMORE, every entry in the data base is seen exactly once. Further, if the same sequence of operations is performed on the same map at the same server, the entries are seen in the same order.

Under conditions of heavy server load or server failure, the domain can become unbound, then bound once again (perhaps to a different server) while a client is running. This binding can cause a break in one of the enumeration rules. Specific entries may be seen twice by the client, or not at all. This approach protects the client from error messages that would otherwise be returned in the midst of the enumeration. For a better solution to enumerating all entries in a map, see yp_all().

The yp_all() function provides a way to transfer an entire map from server to client in a single request using TCP (rather than UDP as with other functions in this package). The entire transaction takes place as a single RPC request and response. The yp_all() function can be used just like any other NIS name service procedure to identify the map in the normal manner and to supply the name of a function that will be called to process each key-value pair within the map. The call to yp_all() returns only when the transaction is completed (successfully or unsuccessfully), or the foreach() function decides that it does not want to see any more key-value pairs.

The third parameter to yp_all() is:

struct ypall_callback *incallback {
	int (*foreach)();
	char *data;
};

The function foreach() is called:

foreach(int instatus, char *inkey,
int inkeylen, char *inval,
int invallen, char *indata);

The instatus parameter holds one of the return status values defined in <rpcsvc/yp_prot.h>, either YP_TRUE or an error code. See ypprot_err(), for a function that converts an NIS name service protocol error code to a ypclnt layer error code.

The key and value parameters are somewhat different than defined in the synopsis section above. First, the memory pointed to by the inkey and inval parameters is private to the yp_all() function, and is overwritten with the arrival of each new key-value pair. The foreach() function must do something useful with the contents of that memory, but it does not own the memory itself. Key and value objects presented to the foreach() function look exactly as they do in the server's map. If they were not NEWLINE-terminated or null-terminated in the map, they would not be here either.

The indata parameter is the contents of the incallback->data element passed to yp_all(). The data element of the callback structure can be used to share state information between the foreach() function and the mainline code. Its use is optional, and no part of the NIS client package inspects its contents; cast it to something useful, or ignore it. The foreach() function is Boolean. It should return 0 to indicate that it wants to be called again for further received key-value pairs, or non-zero to stop the flow of key-value pairs. If foreach() returns a non-zero value, it is not called again. The functional value of yp_all() is then 0.

The yp_order() function returns the order number for a map.

The yp_master() function returns the machine name of the master NIS server for a map.

The yperr_string() function returns a pointer to an error message string that is null-terminated but contains no period or NEWLINE.

The ypprot_err() function takes an NIS name service protocol error code as input, and returns a ypclnt() layer error code, which can be used as an input to yperr_string().