You can use this module in multi threaded environment. In this case the module depend on the module thread.c which contains the thread and mutex interface functions that call the operating system thread and mutex functions on UNIX and on Windows NT.
In single thread environment there is no need to use the locking mechanism. To get a single-thread version either you can edit this file (myalloc.c) or compile is using the option -DMTHREAD=0 The default compilation is multi threaded.
Multi thread implementation has two levels. One is that the subroutines implemented in this module call the appropriate locking functions to ensure that no two concurrent threads access and modify the same data at a time and thus assure that the data of the module is correct. The other level is that you can tell the module that the underlying memory allocation and deallocation modules are mot thread safe. There are global variables implementing global mutexes that are locked and unlocked if you use the module that way. This can be useful in some environment where malloc and free are not thread safe.
Note that this should not be the case if you call malloc and free or you linked the wrong versio of libc. However you may use a non-thread safe debug layer for example the one that ScriptBasic uses.