---
name: "snmp_alarm_register"
section: "3"
platform: "linux"
package: "net-snmp"
version: "5.9.5.2-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/snmp_alarm"
---

# snmp\_alarm\_register(3)

alarm functions

Part of [snmp\_alarm(3)](/snmp_alarm/3.md).

## NAME

snmp\_alarm\_register, snmp\_alarm\_register\_hr, snmp\_alarm\_unregister - alarm functions

## SYNOPSIS

**#include \<net-snmp/utilities.h\>**

**unsigned int**\
 **snmp\_alarm\_register(unsigned int** *seconds***,**\
                     **unsigned int** *flags***,**\
                     **SNMPAlarmCallback \****f\_callback***,**\
                     **void \****clientarg***);**

**unsigned int**\
 **snmp\_alarm\_register\_hr(struct timeval** *t***,**\
                        **unsigned int** *flags***,**\
                        **SNMPAlarmCallback \****f\_callback***,**\
                        **void \****clientarg***);**

**void**\
 **snmp\_alarm\_unregister(unsigned int** *reg***);**

## DESCRIPTION

These functions implement support for a generic timer handling mechanism for multiple parts of an application to register function callbacks to happen at a particular time in the future.

## USAGE

The usage is fairly simple and straight-forward:  Simply create a function you want called back at some point in the future.  The function definition should be similar to:

**void my\_callback(unsigned int** *reg***, void \****clientarg***);**

Then, call **snmp\_alarm\_register()** to register your callback to be called *seconds* from now.  The *flags* field should either be  *SA\_REPEAT* or *NULL.* If flags is set with *SA\_REPEAT,* then the registered callback function will be called every *seconds.* If *flags* is *NULL* then the function will only be called once and then removed from the alarm system registration.

The *clientarg* parameter in the registration function is used only by the client function and is stored and passed back directly to them on every call to the system.

The  **snmp\_alarm\_register()** function returns a unique *unsigned int* (which is also passed as the first argument of each callback), which can then be used to remove the callback from the queue at a later point in the future using the **snmp\_alarm\_unregister()** function. If the **snmp\_alarm\_register()** call fails it returns zero.  In particular, note that it is entirely permissible for an alarm function to unregister itself.

The **snmp\_alarm\_register\_hr()** function is identical in operation to the **snmp\_alarm\_register()** function, but takes a *struct timeval* as a first parameter, and schedules the callback after the period represented by *t* (the letters **hr** stand for "high resolution").  The operation of this function is dependent on the provision of the [**setitimer**(2)](/setitimer/2.md) system call by the operating system.  If this system call is not available, the alarm will be scheduled as if **snmp\_alarm\_register()** had been called with a first argument equal to the value of the *tv\_sec* member of *t*. See, however, the notes below.

## INITIALIZATION

The **init\_snmp()** function initialises the snmp\_alarm subsystem by calling  **init\_snmp\_alarm()** and then  **init\_alarm\_post\_config()** to set up the first timer to initialise the callback function.  These two functions should not be used directly by applications.

## NOTES

The default behaviour of the snmp\_alarm subsystem is to request *SIGALRM* signals from the operating system via the [**alarm**(2)](/alarm/2.md) or [**setitimer**(2)](/setitimer/2.md) system calls.  This has the disadvantage, however, that no other part of the application can use the *SIGLARM* functionality (or, if some other part of the application *does* use the *SIGALRM* functionality, the snmp\_alarm subsystem will not work correctly).

If your application runs a  [**select**(2)](/select/2.md)-based event loop, however, there is no need to use *SIGALRM* for the snmp\_alarm subsystem, leaving it available for other parts of the application.  This is done by making the following call:

```
netsnmp_ds_set_boolean(NETSNMP_DS_LIBRARY_ID,
                       NETSNMP_DS_LIB_ALARM_DONT_USE_SIG, 1);
```

before calling **init\_snmp()**. Then,  **snmp\_select\_info()** takes alarms into account when calculating the timeout value to be used for [**select**(2)](/select/2.md). All you need to do is call **run\_alarms()** when [**select**(2)](/select/2.md) times out (return value of zero).  This is the approach taken in the agent; see *snmpd.c*. Furthermore, when using this method, high resolution alarms do not depend on the presence of the [**setitimer**(2)](/setitimer/2.md) system call, although overall precision is of course still determined by the underlying operating system.  Recommended.

## SEE ALSO

[**netsnmp\_session\_api**(3)](/netsnmp_session_api/3.md), [**default\_store**(3)](/default_store/3.md),  [**alarm**(2)](/alarm/2.md), [**setitimer**(2)](/setitimer/2.md), [**select**(2)](/select/2.md)