I've come across a macro in my company's legacy code that looks like this:
`#define BW8(buf, data) *((buf)++) = (uint8_t)(data)`
This macro is frequently used to allocate buffers for storing or reading data by keeping track of the current position in the buffer. The purpose is to write one byte at the current position and then move the pointer forward by one byte. I've tested this code and confirmed it works as intended. However, I'm a bit confused. I expected that the parentheses around `buf++` would mean the post-increment would apply before dereferencing, which would store the data one byte ahead of the intended location. Can anyone explain why that doesn't happen?
2 Answers
It seems like you might have misrepresented the macro in your initial question. Have you verified that the parentheses are correct? It looks like your macro is actually structured as it should be based on what you've shared! The post-increment happens after the original value of `buf` is used, so the actual dereference happens at the correct location before `buf` is incremented.
You’re right to be cautious about the parentheses, but generally in C, the post-increment operator returns the value before incrementing. So `buf++` gives you the original value first, which is dereferenced to store your data. After the value is accessed, `buf` is incremented. That's why your data is written correctly and doesn't go out of bounds.
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