Cygnus implements the library functions to ensure that, whenever possible, these library functions are reentrant. However, there are some functions that can not be trivially made reentrant. Hooks have been provided to allow you to use these functions in a fully reentrant fashion. These hooks use the structure, _reent, defined in reent.h.

All functions which must manipulate global information are available in two versions.

• The first version has the usual name, using a single global instance of the reentrancy structure.
• The second has a different name, normally formed by prepending ‘_’ and appending ‘_r’ and taking a pointer to the particular reentrancy structure to use.

Each function which uses the global reentrancy structure uses the global variable, _impure_ptr, which points to a reentrancy structure.

This means that you have two ways to achieve reentrancy. Both require that each thread of execution control initialize a unique global variable of type, struct_reent.

• Use the reentrant versions of the library functions, after initializing a global reentrancy structure for each process. Use the pointer to this structure as the extra argument for all library functions.
• Ensure that each thread of execution control has a pointer to its own unique reentrancy structure in the global variable, _impure_ptr, and call the standard library subroutines.

Equivalent for errno variable:
 

_errno_r