Why nested functions in C are against C standards

Question

Nested functions(function declarations in block scope) are not allowed in C standards(ANSI[C89], C99, C11).

But I couldn't find stating it in C standards.

Edit :

Why a function definition cannot be in a function definition(compound statement)?

Answer 1

There is a difference between a function declaration and a function definition. A declaration merely declares the existence of a function and a definition defines the function.

int f(void) { /* ... */ } // function definition
int f(void);              // function declaration

In 6.9.1 the syntax for a function is defined as

function-definition: declaration-specifiers declarator declaration-list_opt compound-statment

In 6.8.2, the things you can put in a compound statement are defined as a declaration or a statement. A function definition isn't considered to be either of these syntactically.

So yes, a function declaration is legal in a function but a function definition is not e.g.

int main(int argc, char*argv[])
{
    int f(void);                // legal
    int g(void) { return 1; } ; // ILLEGAL

    // blah blah
}

Answer 2

It may not be stated directly but if you work through the grammar for function-definition you'll find they're not accepted within the grammar.

Why? According to Dennis Richie (who is a bit of an authority on the matter) it appears they were excluded from the start:

"Procedures can be nested in BCPL, but may not refer to non-static objects defined in containing procedures. B and C avoid this restriction by imposing a more severe one: no nested procedures at all."

https://www.bell-labs.com/usr/dmr/www/chist.html

It's humor to avoid a restriction by imposing a more severe one. I read this as it being a simplifying maneuver. Nested procedures add complexity to the compiler (which Ritchie was very keen to limit on machines of the time) and add little value.

The standardization process was (wisely) never seen as an opportunity to extend C willy-nilly and (from the same document):

"From the beginning, the X3J11 committee took a cautious, conservative view of language extensions."

It's difficult to put a case that nested functions offer significant benefits so it's not surprising that even if some implementations were supporting them they weren't adopted as standard.

In general the standards efforts ever since have been at least equally conservative and again it's difficult to see a lot of support amongst implementers to add such a feature.

At the end of the day if you're worried that some function be used outside its intended purpose and is (logically) a sub-function of exactly one given function then give it static linkage and introduce another source file or even whole translation unit.

Answer 3

Nested or private functions are something that many C compilers used to allow, but are not part of the C standard, and now it's quite rare to find compilers that support them, certainly by default.

The standard is determined by a committee, and nested functions will be something that they have discussed, and there will be a rationale, but I don't know what it is offhand, nor do most C programmers. Nested functions aren't inherently a bad idea, but you can achieve virtually all of the benefits by writing a static file scope function, which is the method of creating private functions which was standardised.