Why are malloc() and printf() said as non-reentrant?

Question

In UNIX systems we know malloc() is a non-reentrant function (system call). Why is that?

Similarly, printf() also is said to be non-reent

Answer 1

Let's understand what we mean by re-entrant. A re-entrant function can be invoked before a previous invocation has finished. This might happen if

• a function is called in a signal handler (or more generally than Unix some interrupt handler) for a signal that was raised during execution of the function
• a function is called recursively

malloc isn't re-entrant because it is managing several global data structures that track free memory blocks.

printf isn't re-entrant because it modifies a global variable i.e. the content of the FILE* stout.