snmp_sess_init, snmp_sess_open, snmp_sess_session, snmp_sess_send, snmp_sess_async_send, snmp_sess_select_info, snmp_sess_read, snmp_sess_timeout, snmp_sess_synch_response, snmp_sess_close, snmp_sess_error - session functions


#include <net-snmp/session_api.h>

void snmp_sess_init(struct snmp_session *session);

void *snmp_sess_open(struct snmp_session *session);

struct snmp_session *snmp_sess_session(void *handle);

int snmp_sess_send(void *handle, struct snmp_pdu *pdu);

int snmp_sess_async_send(void *handle,
struct snmp_pdu *
void *

int snmp_sess_select_info(void *handle,
int *
numfds, fd_set *fdset,
struct timeval *
int *

int snmp_sess_read(void *handle, fd_set *fdset);

void snmp_sess_timeout(void *handle);

int snmp_sess_synch_response ( void *handle,

netsnmp_pdu *pdu,
netsnmp_pdu **

int snmp_sess_close(void *handle);

void snmp_sess_error(void *handle, int *pcliberr,
int *
psnmperr, char **pperrstring);


These functions define a subset of the API that can be used to manage single SNMP sessions in a multi-threaded application. Except for snmp_sess_session(), these functions are single session versions of the traditional SNMP library API.

Note that these functions use an opaque pointer (handle in the above prototypes) to identify a single session in lieu of a session pointer (as in the traditional API).

snmp_sess_init() prepares a struct snmp_session that sources transport characteristics and common information that will be used for a set of SNMP transactions. After this structure is passed to snmp_sess_open() to create an SNMP session, the structure is no longer used. Instead the opaque pointer returned by snmp_sess_open() is used to refer to that session henceforth.

SNMP sessions that are created with snmp_sess_open() are not affected by, and SHOULD NOT BE USED WITH, snmp_select_info(), snmp_read(), snmp_timeout() nor snmp_close(). Rather the equivalent single session functions described here should be used.

snmp_sess_init() and snmp_sess_open() each take as input a pointer to a struct snmp_session object. This structure contains information for a set of transactions that will share similar transport characteristics. snmp_sess_session() takes the opaque session handle and returns a pointer to its associated struct snmp_session.

snmp_sess_send() and snmp_sess_async_send() each take a pdu parameter, which points to a struct snmp_pdu object containing information that describes a transaction that will be performed over an open session.

Consult snmp_api.h for the definitions of these structures.

With the snmp_sess_async_send() call, snmp_sess_read will invoke the specified callback when the response is received.

snmp_sess_select_info(), snmp_sess_read() and snmp_sess_timeout() provide an interface for the use of the select(2) system call so that SNMP transactions for a single session can occur asynchronously.

snmp_sess_select_info() is passed the information that would have been passed to select(2) in the absence of SNMP. For example, this might include file descriptors associated with the main loop of a graphical application. This information is modified so that SNMP will get the service it requires from the call to select(2). In this case, numfds, fdset and timeout correspond to the nfds, readfds and timeout arguments to select(2) respectively. The only exception is that timeout must ALWAYS point to an allocated (but perhaps uninitialized) struct timeval (it cannot be NULL as for select(2)). If timeout would have been passed as NULL, block is instead set to true, and timeout is treated as undefined. This same rule applies upon return from snmp_select_info().

After calling snmp_sess_select_info() , select(2) should be called with the returned data. When it returns, snmp_sess_read() should then be called with the fd_set returned from select(2). This will read any input from this session’s SNMP socket. If select(2) times out (that is, it returns zero), snmp_sess_timeout() should be called to see if a timeout has occurred on the SNMP session.

snmp_sess_synch_response is a convenience routine that will send the request, wait for the response and process it before returning. See the descriptions of snmp_sess_send , snmp_sess_read etc for details.


Error return status from snmp_sess_open() is indicated by return of a NULL pointer. Error return status from snmp_sess_close() and snmp_sess_send() is indicated by a return value of 0. A successful status will return 1.

Further information can be obtained by using snmp_sess_error() to see what type of error has occurred. This function returns the SNMP snmp_errno variable, the value of the system errno variable, and a string interpretation of both variables. The string must be freed after use by the caller.

For errors returned by snmp_sess_open(), use the corresponding function snmp_error() instead of snmp_sess_error().

Consult snmp_api.h for the complete set of SNMP library error values. The SNMP library error value snmperr can be one of the following values:


A generic error occurred.


The local port was bad because it had already been allocated or permission was denied.


The host name or address given was not useable.


The specified session was not open.


















select(2), snmp_api(3), snmp_api.h

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