Multithreaded network programming

Thread-specific error variables

Each thread has its own instance of each library error variable, such as t_errno. Typically, each library error variable is actually defined as a macro such as:

   #define t_errno 	(*(_t_errno()))

Unlike storage for the thread-specific errno, which is allocated when a thread is created, storage for each thread-specific library error variable is allocated by the relevant library at the first reference to the variable. In conditions of low memory, the allocation will fail. In the case of t_errno, the function in the error variable macro will return the address of a global variable shared by all threads (a kind of overflow location). In the case of all other library error variables, the function will return NULL. If you try to assign a value to t_errno when the macro function has returned the address of the common global location, the value may later be overwritten by another thread. If you try to assign a value to an error variable when the macro function has returned NULL, you will cause a fatal error (a segmentation violation). For these reasons, the networking libraries do not access library error variables directly. Instead, they use a family of access functions. These access functions are also available for your use in multithreaded applications and applications that you may multithread later.

For example, the function get_t_errno(3sock) returns the value of t_errno for the current thread. A corresponding function, set_t_errno, sets the value of t_errno for the current thread. (In general, you should avoid setting library error variables, since they are intended to be set only by library functions.)

NOTE: The functions get_t_errno and set_t_errno are not part of the X/Open Transport Interface (XTI), so using them may make your applications less portable. However, if you are developing critical or long-running applications, you may wish to use these access functions anyway.

The library error variables access functions are listed in ``Routines for accessing library error variables''.

Routines for accessing library error variables

Error variable Get function Set function

h_errno get_h_errno set_h_errno

_nderror get_nderror set_nderror

rpc_createerr get_rpc_createerr set_rpc_createerr

Rx_errno get_Rx_errno set_Rx_errno

Rx_cserrno get_Rx_cserrno set_Rx_cserrno

t_errno get_t_errno set_t_errno

Error variable	Get function	Set function
h_errno	get_h_errno	set_h_errno
_nderror	get_nderror	set_nderror
rpc_createerr	get_rpc_createerr	set_rpc_createerr
Rx_errno	get_Rx_errno	set_Rx_errno
Rx_cserrno	get_Rx_cserrno	set_Rx_cserrno
t_errno	get_t_errno	set_t_errno