Chapter 50. Other portmapper functionality


Advertising executable's pathname

The portmapper object has a dereg() method that undoes a reg() - deregisters a specific port or ports with the portmapper daemon, while keeping any other registered ports.

reg(), dereg(), and connect() methods also take an additional optional parameter, a terminator file descriptor. If the terminator file descriptor becomes readable, an exception gets thrown, and the connection to the portmapper daemon is closed, all registered ports get deregistered.

Advertising executable's pathname

On Linux, the /proc/pid/exe symlink gives the name of the executable that started the process. On Linux, reading a process's executable pathname succeeds only if both processes have the same userid. The portmapper provides a way for cooperating processes to make their executable's name visible to other processes.

The third parameter to reg() is a set of bitwise-or flags one of which is x::httportmap::base::pm_public. If set, the process's executable pathname gets included as part of the registered service. A low-level portmapper method, list(), returns a list of services registered with the portmapper, include the executable pathname, if made public.

This is the low-level approach. The high level approach uses regpid2exe():

#include <x/httportmap.H>

x::httportmap portmapper(x::httportmap::create());


// ...
pid_t pid;

// ...

std::string path=portmapper->pid2exe(pid);

regpid2exe() advertises this process's executable pathname through the portmapper. pid2exe() returns another process's executable pathname, if the other process called regpid2exe() previously. An empty string gets returned if the process could not be found, or did not call regpid2exe().

There's also a deregpid2exe() that deregisters the reserved service that announces the executable pathname. Like other portmapper methods, these three functions also take an optional timeout file descriptor parameter; although that since they always connect to a portmapper instance running on the same machine, the timeout file descriptor offers little practical purpose.


regpid2exe() counts towards the limit on the maximum number of registered services, with the portmapper, from the same process.


Advertised executable pathnames are considered to be supplementary metadata, and should not be exclusively relied upon, for authentication. For example, after reading an executable's process id, the original process can fork, with the parent exiting, and subsequently the process id gets recycled for another process. When a process creates a connection with the portmapper, if the portmapper has a registered entry ostensibly from the same process id, but a different executable name, the portmapper removes the other process's executable name.

Announcing and advertising a pathname provides a means for identifying and connecting to applications. In situations that involve some kind of authentication, it should be done by other means, like sending userid/groupid credentials over filesystem domain sockets.

If a process's executable binary gets deleted, but the process still runs, its executable pathname no longer exists. Long-running server daemons should connect to the portmapper and advertise their pathname promptly after starting, on systems that use prelink, which tends to overwrite binaries, periodically.