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I need to use a library that relies on FLASH_Unlock and functions following it, however upon calling that function I get an error saying the function is invalid in C99. The library I use has an example file which is written in mdk v4 and works just fine. Is there a way I can use FLASH_Unlock() function?
This is ST's code - nothing to do with ARM.
The message does not say anything about the function being invalid in C99!
"implicitly declared" means that you have failed to provide an explicit declaration - ie, either a prototype or definition of that function.
With all those "undefined" errors, you have probably omitted to include a required header file.
Or, perhaps, not correctly configured the project - so that the appropriate target-specific options are not defined.
You need to go back to the ST documentation on how to properly set up a project using this library.
ST should provide an example to get you started ...