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I will be continually splitting strings in a multithreaded application, I've read that strtok is not suitable for this, but why?
Should I consider using a semaphore around the portion of my code that calls strtok?
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Every call to the strtok() function, returns a refrence to a NULL terminated string and it uses a static buffer during parsing. Any subsequent call to the function will refer to that buffer only, and it gets altered.! It is independent of who called it, and thats is the reason for it is not thread safe.
The identity of the delimiting character is lost. The strtok() function uses a static buffer while parsing, so it's not thread safe.
The C function strtok() is a string tokenization function that takes two arguments: an initial string to be parsed and a const -qualified character delimiter. It returns a pointer to the first character of a token or to a null pointer if there is no token.
A [portion of code] is thread-safe if it behaves correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.
You should consider not using strtok or strtok_r at all. It's trivial to write your own functions similar to these but better-tailored to the exact way you want to use them, and of course have the caller store all the state and pass a pointer to the state for thread-safety/reentrancy.
As for your question about using a semaphore (or other locking primitive) around calls to strtok, that will not help if you just put it around the actual call. You'd have to hold the lock during the whole process of parsing the string to protect the internal state of strtok. I believe this is what many people refer to as locking code in place of data and it's generally considered A Bad Thing.
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Use strtok_r() for thread safety.
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