I thought the shared memory they were referring to would be read only memory or something. So yes, the memory is shared, but your process doesn't have any access to overwrite any of it. Likewise, when you call library functions, when it mallocs something, it uses memory that your process has write access to, not the shared library memory.

In the context of the question, I assume the asker was mostly interested in reading some kind of sensitive data from a previous process, not reading the same librsry-code-only memory or something.

Note: all of this could be wrong, it was just my understanding

Edit: looks like this answers that: https://stackoverflow.com/questions/20857134/memory-write-pr...