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Is va_start (etc.) reentrant?

When editing for a class with a long history, I was driven into a dead end by the architect’s special habit of wrapping it in va_start → va_end in a mutex. The list of changes for this addition (which was made about 15 years ago and has not been revised since) noted that this was because va_start et. everything was not reentrant.

I was not aware of such problems with va_start, since I always thought it was just a macro for some math with a stack pointer. Is there something here that I don't know about? I do not want to change this code if there are side effects.

In particular, the function in question looks something like this:

void write(const char *format, ...)
{
    mutex.Lock();
    va_list args;
    va_start(args, format);
    _write(format, args);
    va_end(args);
    mutex.Unlock();
}

This is called from multiple threads.

+5
3

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