Function without name

Question

I would like to know how to call this function? And where can i find it's implementation if it doesn't have name?

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

This is a pointer on function... – Jarod42 Sep 03 '18 at 14:08 — Jarod42, Sep 03 '18 at 14:08

score 22 · Accepted Answer · edited Sep 03 '18 at 23:46

22

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.

edited Sep 03 '18 at 23:46

Eevee

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answered Sep 03 '18 at 14:08

Bathsheba

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What if there would be two functions which return type void and the parameters given? – Filip Falcon Sep 03 '18 at 14:19
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Well you'd hope that `_malloc_message` would point to one of them, depending on conditions apparent by inspecting the program. – Bathsheba Sep 03 '18 at 14:20

P.W · Answer 2 · 2018-09-03T14:37:34.043

3

_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.

edited Sep 03 '18 at 14:37

answered Sep 03 '18 at 14:28

P.W

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It is not my code. I'm trying to learn advanced C++ by reading somebody else code. There are all those steps except a calling one. That is interesting, maybe the function is no longer used in that code and author forgot to delete it. – Filip Falcon Sep 03 '18 at 14:33
1

@Chyu • there are several good books on learning advanced C++. Here is a curated list of books: https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list – Eljay Sep 03 '18 at 14:52
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@Chyu The call might be hidden in a macro. Also, since it's declared `extern`, it might be called from a different compilation unit. – Barmar Sep 03 '18 at 22:14

score 1 · Answer 3 · answered May 16 '19 at 21:23

_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.

Function without name

3 Answers3