register_premib_handler (3) - Linux Manuals

register_premib_handler: read_config functions


register_config_handler, register_premib_handler unregister_config_handler, register_mib_handlers, read_configs, read_premib_configs, config_perror, config_pwarn - read_config functions


#include <net-snmp/config_api.h>

struct config_line *
register_config_handler(const char *filePrefix,
const char *token,
void (*parser)(const char *, char *),
void (*releaser)(void),
const char *usageLine);

struct config_line *
register_premib_handler(const char *filePrefix,
const char *token,
void (*parser)(const char *, char *),
void (*releaser)(void),
const char *usageLine);

void unregister_config_handler(const char *filePrefix,
const char *token);

struct config_line *
register_app_config_handler(const char *token,
void (*parser)(const char *, char *),
void (*releaser)(void),
const char *usageLine);

struct config_line *
register_app_premib_handler(const char *token,
void (*parser)(const char *, char *),
void (*releaser)(void),
const char *usageLine);

void unregister_app_config_handler(const char *token);

void read_config_print_usage(char *lead);

void read_configs(void);

void read_premib_configs(void);

void config_pwarn(const char *string);
void config_perror(const char *string);


The functions are a fairly extensible system of parsing various configuration files at the run time of an application. The configuration file flow is broken into the following phases:
Registration of handlers.
Reading of the configuration files for pre-MIB parsing requirements.
Reading and parsing of the textual MIB files.
Reading of the configuration files for configuration directives.
Optionally re-reading the configuration files at a future date.

The idea is that the calling application is able to register handlers for certain tokens specified in certain types of files. The read_configs() function can then be called to look for all the files that it has registrations for, find the first word on each line, and pass the remainder to the appropriately registered handler.


Handler functions should have the following signature:

void handler(const char *token, char *line);

The function will be called with two arguments, the first being the token that triggered the call to this function (which would be one of the tokens that the function had been registered for), and the second being the remainder of the configuration file line beyond the white space following the token.


If the parameter releaser passed to register_config_handler is non-NULL, then the function specified is called if and when the configuration files are re-read. This function should free any resources allocated by the token handler function and reset its notion of the configuration to its default. The token handler function will then be called again. No arguments are passed to the resource freeing handler.


The handler() function above could be registered for the configuration file snmp.conf, with the token genericToken and the help string (discussed later) ARG1 [ARG2] using the following call to the register_config_handler() function:

register_config_handler("snmp", "genericToken", handler, NULL, "ARG1 [ARG2]");
This would register the handler() function so that it will get called every time the first word of a line in the snmp.conf configuration file(s) matches "genericToken" (see read_configs() below).
The register_premib_handler() function works identically to the register_config_handler() function but is intended for config file tokens that need to be read in before the textual MIBs are read in, probably because they will be used to configure the MIB parser. It is rarely the case that anything but the SNMP library itself should need to use this function.
Removes the registered configuration handler for the filePrefix and token.

These functions are analagous to register_config_handler(), register_premib_handler() and unregister_config_handler() but don't require the file type argument (which is filled in by the application). It is intended that MIB modules written for the agent use these functions to allow the agent to have more control over which configuration files are read (typically the snmpd.conf files).


The usageLine parameter passed to register_config_handler() and similar calls, is used to display help information when the read_config_print_usage() function is called. This function is used by all of the applications when the -H flag is passed on the command line. It prints a summary of all of the configuration file lines, and the associated files, that the configuration system understands. The usageLine parameter should be a list of arguments expected after the token, and not a lengthy description (which should go into a manual page instead). The lead prefix will be prepended to each line that the function prints to stderr, where it displays its output.

The init_snmp() function should be called before the read_config_print_usage() function is called, so that the library can register its configuration file directives as well for the read_config_print_usage() function to display.


Once the relevant configuration token parsers have been registered, init_snmp() should be called. It will parse the configuration file tokens registered with register_premib_handler(), read in the textual MIB files using init_mib(), and finally parse the configuration file tokens registered with register_config_handler().

If the init_snmp() function is used, none of the following functions need to be called by the application:

The SNMP library's routine to register its configuration file handlers.
The routine that parses the configuration files for tokens registered to be dealt with before the textual MIBs are read in. See read_configs() below.
Reads all the configuration files it can find in the SNMPCONFPATH environment variable (or its default value) for tokens and appropriately calls the handlers registered to it, or prints a "Unknown token" warning message. It looks for any file that it has previously received a registration request for.


The configuration files read are found by using the colon separated SNMPCONFPATH environment variable (or its default value, which will be /etc/snmp, followed by /usr/share/snmp, followed by /usr/lib/snmp (or /usr/lib64/snmp), followed by $HOME/.snmp) and reading in the files found that match both the prefix registered and the two suffixes .conf and .local.conf. The idea behind the two different suffixes is that the first file can be shared (via rdist or an NFS mount) across a large number of machines and the second file can be used to configure local settings for one particular machine. They do not need to be present, and will only be read if found.


The two functions config_pwarn() and config_perror() both take an error string as an argument and print it to stderr along with the file and line number that caused the error. A call to the second function will also force read_configs() to eventually return with an error code indicating to it's calling function that it should abort the operation of the application.


A colon separated list of directories to search for configuration files in. Default: /etc/snmp:/usr/share/snmp:/usr/lib(64)/snmp:$HOME/.snmp


mib_api(3), snmp_api(3)