Declare function pointer with variable args

Question

How do I declare a pointer to a function with variable args?

e.g int (*my_printf) (FILE *stream, const char *format, ..., void *data) = NULL;

The error from clang was:

a.c:8:56: error: expected ')'
int (*my_printf) (FILE *stream, const char *format, ..., char *data) = NULL;
                                                       ^
a.c:8:18: note: to match this '('
int (*my_printf) (FILE *stream, const char *format, ..., char *data) = NULL;
                 ^
1 error generated.

Of course, I could simple place the data parameter as the last one. But I still want a general solution

@Jim:

So, what do you think about execle function?

(From man execle I see this)

 int execle(const char *path, const char *arg,
              ..., char * const envp[]);

enter image description here

The ellipsis must be last, whether it's a function pointer or a function. Read the varargs documentation. — Jim Balter
– Jim Balter, Commented Dec 17, 2013 at 1:01
@warl0ck that's not the declaration found in the code though. That's the definition used for documentation. — randomusername
– randomusername, Commented Dec 17, 2013 at 1:10
cat /usr/include/unistd.h |grep execle => int execle(const char *, const char *, ...); (OSX 10.9) — Till
– Till, Commented Dec 17, 2013 at 1:13
@Guido you mean, envp is part of ..., but should be the last one in the list? — daisy
– daisy, Commented Dec 17, 2013 at 1:17
A man page is not a program and need not be syntactically correct. See manpagez.com/man/3/execle which goes out of its way to avoid being syntactically incorrect by using a comment, /*, (char *)0, char *const envp[] */ — Jim Balter
– Jim Balter, Commented Dec 17, 2013 at 1:33

Fiddling Bits · Accepted Answer · 2013-12-17 01:12:09Z

4

Ellipsis (...) must always be the last formal argument.

answered Dec 17, 2013 at 1:12

Fiddling Bits

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Community · Accepted Answer · 2020-06-20 09:12:55Z

2

The ellipsis notation must be in the end, or it's undefined behavior.

C11 §6.9.1 Function definitions Subsection 8

If a function that accepts a variable number of arguments is defined without a parameter type list that ends with the ellipsis notation, the behavior is undefined.

As for the prototype of execle, what you quote is incorrect, it should be:

int execle(const char *path, const char *arg0, ... /*,
   (char *)0, char *const envp[]*/);

Note that envp etc. are inside comments /* */.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 17, 2013 at 1:13

Yu Hao

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1 Comment

Guido Over a year ago

@warl0ck That prototype specifies how you should use it, but it's not defined that way in the code. You can run grep -rn execle /usr/include/ to see it.

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