const keyword before a function in C

Question

I want to know what a "const" keyword before a function in 'C' does.

For example:

extern const int func1(int x[], const struct dummy_s* dummy)

Thanks in advance

possible duplicate of [What's the point of const pointers?](http://stackoverflow.com/questions/7715371/whats-the-point-of-const-pointers) — n0p, Oct 17 '14 at 13:37
the type is missing, so this is certainly what you found somewhere, can you give use the exact line? — Jens Gustedt, Oct 17 '14 at 13:42
What are you struggling with? Why `const` is used, or that the return type is implicitly taken to be `int`? — David Heffernan, Oct 17 '14 at 13:43

score 4 · Accepted Answer · answered Oct 17 '14 at 13:54

4

If you turn on warnings you will have two:

warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]

which easily allows you to conclude that this:

extern const func1(int x[], const struct dummy_s* dummy)

is basically the same as:

extern int func1(int x[], const struct dummy_s* dummy)

answered Oct 17 '14 at 13:54

Drax

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Don't just turn your warnings on, compile your C code as standard C. `gcc -std=c11 -pedantic-errors -Wall -Wextra`. You will now get errors, not just warnings. – Lundin Oct 17 '14 at 14:26
@Lundin that depends entirely on which standard the user is using. If they are using C89 it should be perfectly valid code. You cannot assume every program has been made since 2011 :) – Vality Oct 17 '14 at 14:57

Vlad from Moscow · Answer 2 · 2014-10-17T13:53:41.997

2

It has no any sense. It seems that it is some old code that was valid when C allowed implicit return type int if the return type is not specified explicitly. But in any case the return value is not an lvalue and can not be changed. So the const qualifier is superfluous.

edited Oct 17 '14 at 13:53

answered Oct 17 '14 at 13:45

Vlad from Moscow

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Lundin · Answer 3 · 2014-10-17T14:37:08.517

2

In standard C, your code will not compile. You are not allowed to omit the return type of a C function.

In an older, obsolete version of C known as C90, you were allowed to omit the return type, in which case it would default to int. In that old version of C, your code would be equal to:

extern const int func1(int x[], const struct dummy_s* dummy);

The next question then is, does it ever make sense to return const int from a function, rather than just int? No, it doesn't... because the returned variable is always a hard copy placed on the stack. It is not a lvalue and the function is executed in runtime anyhow, there is no reason why this value needs to be const.

edited Oct 17 '14 at 14:37

answered Oct 17 '14 at 14:26

Lundin

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"you can always assign a non-const variable to a const one" - you mean _from_ a const one, I think. – davmac Oct 17 '14 at 14:28
@davmac Tried to clarify a bit. – Lundin Oct 17 '14 at 14:37

const keyword before a function in C

3 Answers3