Reentrant and Thread-Safe

Writing Reentrant and Thread-Safe Code
http://www.navo.hpc.mil/cgi-bin/search/search.pl?q=index&showurl=%2Fusersupport%2FIBM%2Faixprggd%2Fgenprogc%2Fwriting_reentrant_thread_safe_code.htm
http://publib.boulder.ibm.com/infocenter/systems/index.jsp?topic=/com.ibm.aix.genprogc/doc/genprogc/writing_reentrant_thread_safe_code.htm
http://blog.csdn.net/lovekatherine/archive/2007/03/28/1544585.aspx

Reentrance

A reentrant function does not hold static data over successive calls, nor does it return a pointer to static data. All data is provided by the caller of the function. A reentrant function must not call non-reentrant functions.

Making a function reentrant

• returning dynamically allocated data
• use caller-provided storage
  

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Thread-Safety

A thread-safe function protects shared resources from concurrent access by locks. Thread-safety concerns only the implementation of a function and does not affect its external interface.

In C, local variables are dynamically allocated on the stack. Therefore, any function that does not use static data or other shared resources is trivially thread-safe.

Making a function thread-safe

• locking shared resources
• workaround
• use a global lock on the library
• use a lock for each library component

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Converting Libraries

• exported global variables
  should be encapsulated, make private (static), and accessed by subroutines.
  
• static variables
  protect shared resource by locks.
  
• make non-reentrant function reentrant
  
• make thread-unsafe function thread-safe
  

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My Understanding
if Reetrant ==> thread-safe
if thread-safe 不一定是 reetrant