From msg to csv or vcard?

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How to convert msg to csv or vcard?

Answer 1

Can n be greater than zero if we do count = 0 first?

static unsigned long count = 0;
Declares and initializes a variable. A variable is initialized only once during its lifetime. In the case of a static variable, before the first run of the function.
That is, the initial value of n is 0, after which it is no longer reset to zero.

why is mutex here

Think what will happen. if two threads stumble upon an increment at the same time without synchronization. Especially with the existence of a cache, the presence of optimizations that consolidate writing to memory, and the like. In fact, it's better to use atomic here, but oh well.