Section: C Library Functions (3)
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pmDiscoverServices - discover PCP services on the network


#include < pcp/pmapi.h >

              int pmDiscoverServices(const char *service, const char *mechanism, char ***urls);

#include < pcp/impl.h >

              int __pmDiscoverServicesWithOptions(const char *service, const char *mechanism, const char *optionsString, const volatile unsigned *flags, char ***urls);

cc ... -lpcp


Given a PCP service name, as identified by service , and using the type of discovery optionally specified in mechanism , pmDiscoverServices returns, via urls , a list of URLs representing the services discovered on the network.

The internal function __pmDiscoverServicesWithOptions performs the same function and adds arguments for global options and a mechanism for interrupting the discovery process.

The pmfind (1) utility provides command line access to both of these interfaces.


pmDiscoverServices( service , mechanism , urls )

is equivalent to calling

__pmDiscoverServicesWithOptions( service , mechanism , NULL, NULL, urls );

service specifies the PCP service to be discovered. Currently supported services are PM_SERVER_SERVICE_SPEC, PM_PROXY_SERVICE_SPEC and PM_WEBD_SERVICE_SPEC, which search for pmcd (1), pmproxy (1), and pmwebd (1), servers respectively.

mechanism specifies the style of discovery to be used.

The currently supported mechanisms are:

This searches for services which are broadcasting using mDNS via avahi-daemon (8). An optional suffix ",timeout=N" may be added to specify the discovery timeout in floating-point multiples of one second. The default timeout is 0.5 seconds, which may be overridden by the AVAHI_DISCOVERY_TIMEOUT environment variable, also specified in floating-point multiples of one second. If both are specified, then the value specified in the environment variable takes precedence.
Actively probes the given subnet for the requested PCP service(s). <net-address> is an inet or ipv6 network address and <mask-bits> is the number of bits used to define the subnet. For example, defines an 8 bit subnet consisting of the addresses through An optional suffix ",maxThreads=N" may be added to limit the number of threads used while probing. The default is no fixed limit, which is to say that the process' rlimits for the number of threads and open file descriptors will be respected. An optional suffix ",timeout=N" may be added to specify the timeout for each connection attempt in floating-point multiples of one second. The default timeout is 20 milliseconds.

mechanism may also be NULL, which means to use all available discovery mechanisms.

For __pmDiscoverServicesWithOptions, optionsString specifies global options to be applied to the discovery process. Options are comma-separated and may be one or more of the following:

This requests that DNS name resolution be attempted for the addresses of any discovered services.
This specifies a timeout period after which the discovery process will be interrupted. N is a floating point number representing the number of seconds before timing out.

optionsString may also be NULL, which means that no global options are specified.

For __pmDiscoverServicesWithOptions, flags specifies a pointer to an object of type unsigned which is a bit mask of options/status flags for the discovery process. The supported flags are:

Specifying this flag is equivalent to specifying resolve in the optionsString
This flag must be unset when calling __pmDiscoverServicesWithOptions but may be set asynchronously (by an interrupt handler, for example) in order to interrupt the service discovery process.

flags may also be NULL, which indicates that no flags are set.

pmDiscoverServices and __pmDiscoverServicesWithOptions will return the number of services discovered, else a value less than zero for an error. The value zero indicates that no services were discovered.

The resulting list of pointers, urls , and the values (the URLs) that the pointers reference will have been allocated by pmDiscoverServices or __pmDiscoverServicesWithOptions with a single call to malloc (3), and it is the responsibility of the pmDiscoverServices or __pmDiscoverServicesWithOptions caller to free ( urls ) to release the space when it is no longer required.

When an error occurs, or no services are discovered, urls is undefined (no space will have been allocated, and so calling free (3) is a singularly bad idea).


Environment variables with the prefix PCP_ are used to parameterize the file and directory names used by PCP. On each installation, the file /etc/pcp.conf contains the local values for these variables. The $PCP_CONF variable may be used to specify an alternative configuration file, as described in pcp.conf (5). Values for these variables may be obtained programmatically using the pmGetConfig (3) function.


PMAPI (3), pmcd (1), pmproxy (1), pmwebd (1), pmfind (1), pmGetConfig (3), pcp.conf (5), pcp.env (5) and avahi-daemon (8).


An invalid argument has been specified.
Unable to allocate memory required to process the request.
The specified mechanism is not supported.