Function without name

_malloc_message is a function pointer.

Somewhere in the code you will find the definition of a function whose prototype is like this:

void foo (const char* p1, const char* p2, const char* p3, const char* p4);

Then you assign the function to the function pointer like this:.

_malloc_message = foo;

and call it like this:

(*_malloc_message)(p1, p2, p3, p4);

The question is why you cannot call foo directly. One reason is that you know that foo needs to be called only at runtime.

_malloc_message is defined in malloc.c of jemalloc:

This is how you may use it:

extern void malloc_error_logger(const char *p1, const char *p2, const char *p3, const char *p4)
{
    syslog(LOG_ERR, "malloc error: %s %s %s %s", p1, p2, p3, p4);
}

//extern
_malloc_message = malloc_error_logger;

malloc_error_logger() would be called on various malloc library errors. malloc.c has more details.

It isn't a function. It's a declaration saying that _malloc_message is a pointer to a function, with return type void and the parameters as given.

In order to use it, you'd have to assign to it the address of a function with that arity, return type, and parameter types.

Then you'd use _malloc_message as if it were a function.