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What is the difference between Thread safety vs Re entrancy

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Can you please explain with examples. Also like to know if all re-entrant code is thread safe.

posted Jun 21, 2014 by Amit Kumar Pandey

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1 Answer

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Lets first understand the meaning of these two -
Re-entrancy: no global only local variables.
Thread-Safety: it is not possible for more then one threads to conflict with each other.

Now coming to the difference:
A function is "thread-safe" is its critical section is accessed/modified by only one thread at a time and rest of the other thread would wait to get a release before using it. While a function is re-entrant if it can be processed while the function is already in progressing.
Check the following example:

int length = 0;
char *s = NULL;

void AddToString(int ch)
{
  StartCriticalSection(&CriticalSection);
  char *newString = realloc(s, (length+1) * sizeof(char));
  if (newString) {
    if (ch == '\0' || ch == '\\') {
      AddToString('\\'); // escape prefix
    }
    newString[length++] = ch;
    newString[length] = '\0';
    s = newString;
  }
  StopCriticalSection(&CriticalSection);
}

Above function is thread safe as critical section is guarded and only accessed by one thread at a time but not re-entrant because use of global variable.

answer Jun 21, 2014 by Salil Agrawal
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