CONFIG_API(3) Net-SNMP CONFIG_API(3)
NAME
register_config_handler, register_premib_handler unregister_config_handler, register_mib_handlers, read_configs, read_premib_configs, con-
fig_perror, config_pwarn - config_api functions
SYNOPSIS
#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 register_mib_handlers(void);
void unregister_config_handler(const char *filePrefix,
const char *token);
void unregister_all_config_handlers(void);
Application Handlers
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);
Reading Configuration Files
void read_premib_configs(void);
void read_configs(void);
Help Strings and Errors
void read_config_print_usage(char *lead);
void config_pwarn(const char *string);
void config_perror(const char *string);
DESCRIPTION
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:
1. Registration of handlers.
2. Reading of the configuration files for pre-MIB parsing requirements.
3. Reading and parsing of the textual MIB files.
4. Reading of the configuration files for configuration directives.
5. 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 named configuration files.
The read_configs() function can then be called to look for all relevant configuration files, match the first word on each line against the
list of registered tokens and pass the remainder of the line to the appropriate registered handler.
REGISTERING A HANDLER
register_config_handler()
Registers a configuration handler routine, which should be called to process configuration directives starting with the specified
token. For example:
register_config_handler("snmp", "exampleToken", example_handler, NULL, "ARG1 [ARG2]");
would register the example_handler() function so that it will get called every time the first word of a line in the snmp.conf con-
figuration file(s) matches "exampleToken".
Calling the appropriate handlers to process the configuration file directives is the responsibility of read_configs() (see below).
register_premib_handler()
Similar to the register_config_handler() function, but the registered handler routine will be called before the textual MIBs are
read in. This is typically used for tokens that will affect the configuration of the MIB parser, and will normally only be used
within the SNMP library itself.
register_mib_handlers()
Initialisation routine to register the internal SNMP library configuration handlers.
unregister_config_handler()
Removes the registered configuration handler for the specified filePrefix and token.
unregister_all_config_handlers()
Removes all registered configuration handlers.
Token Handlers
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 (i.e. the token used
when registering the handler), and the second being the remainder of the configuration file line (i.e. everything following the white space
following the matched token).
Freeing Handlers
If the token handler function dynamically allocates resources when processing a configuration entry, then these may need to be released
before re-reading the configuration files. If the fourth parameter ( releaser ) passed to register_config_handler is non-NULL, then this
specifies a function to be called before re-reading the configuration files. 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 can then safely be called
again. No arguments are passed to the resource freeing handler.
Note that this function is not called when the handler is unregistered individually (but is called as part of unregister_all_config_han-
dlers() ).
Application Handlers
register_app_config_handler()
register_app_premib_handler()
unregister_app_config_handler()
These functions are analagous to register_config_handler(), register_premib_handler() and unregister_config_handler() but do not
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).
READING CONFIGURATION FILES
read_premib_configs()
read_configs()
These routines process the configuration files found in the configuration search path (see below). For each entry, the handler reg-
istered for that configuration token is called.
read_premib_configs() is run before the MIB files are read in, and processes those configuration tokens registered using register_pre-
mib_handler() (or register_app_premib_handler() ). All other entries are ignored.
read_configs() is run after the MIB files have been read in, and processes those configuration tokens registered using register_config_han-
dler() (or register_app_config_handler() ). If it encounters a configuration token for which no handler has been registered (either pre-
or post-mib), then it will display a warning message, and continue processing with the next line of the configuration file.
Configuration Search Path
The configuration files to be read are found by searching a list of configuration directories for appropriately named files. In each such
directory, the library will look for files named
snmp.conf,
snmp.local.conf, app.conf, app.local.conf, (where app is the appication-specific filePrefix used to register configuration handlers). It
is not necessary for any or all of these files to be present in each directory. Missing files will be silently skipped.
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.
The default list of directories to search is /etc/snmp, followed by /usr/share/snmp, followed by /usr/lib64/snmp, followed by $HOME/.snmp.
This list can be changed by setting the environmental variable SNMPCONFPATH to be a (colon separated) list of directories to search.
init_snmp()
The normal mode of operation would be to register the application-specific configuration handlers, and then invoke init_snmp(). This would
call the routines listed above to register the internal library configuration handlers, process any configuration 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 these functions need to be explicitly called by the application.
HELP STRINGS AND ERRORS
The usageLine parameter passed to register_config_handler() and similar calls, is used to display help information when the read_con-
fig_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 usage-
Line 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.
Error Handling Functions
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.
ENVIRONMENT VARIABLES
SNMPCONFPATH
A colon separated list of directories to search for configuration files in. Default:
/etc/snmp:/usr/share/snmp:/usr/lib64/snmp:$HOME/.snmp
SEE ALSO
mib_api(3), snmp_api(3)
4.2 Berkeley Distribution 07 Mar 2002 CONFIG_API(3)