Why operator sizeof( function ) in C is outputting 1 byte

Question

Possible Duplicate:
Why is the size of a function in C always 1 byte?

Consider the below code snippet in C:

double fn()
{
    return 1.1 ;
}

int main()
{
    printf( "%d", sizeof( fn ) );
    return 0;
}

The output is:

Why the sizeof() operator is outputting 1 byte?

[`sizeof` returns `size_t` which must be printed out using `%zu`](http://stackoverflow.com/q/940087/995714) — phuclv, Feb 25 '17 at 12:46

ronalchn · Answer 1 · 2012-09-08T00:23:03.380

4

The sizeof operator should not be applied to a function. It is meaningless and its behaviour is not defined.

The size operator shall not be applied to an expression that has function or incomplete type,...

Reference: ISO/IEC 14882 (section 5.3.3)

The sizeof operator shall not be applied to an expression that has function type or an incomplete type, to the parenthesized name of such a type, or to an expression that designates a bit-field member.

Reference: ISO/IEC 9899:1999 (section 6.5.3.4)

The sizeof operator may not be applied to:

A bit field

A function type

An undefined structure or class

An incomplete type (such as void)

Reference: http://publib.boulder.ibm.com/

edited Sep 08 '12 at 00:23

answered Sep 08 '12 at 00:11

ronalchn

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That's only a reference for one compiler however, I wouldn't trust it as the defining source for all compilers. – Richard J. Ross III Sep 08 '12 at 00:12
That's only because it takes me a while to find all the references and that was the first I found. I already knew the answer, just wanted to publish it earlier before all information was found. – ronalchn Sep 08 '12 at 00:19

score 2 · Answer 2 · edited May 23 '17 at 10:29

2

When you use sizeof on a function, it's value is meaningless, as described here Why is the size of a function in C always 1 byte?

To get the size of the return value, try using sizeof(fn()), because sizeof is a compile-time constant (in most scenarios), fn will not really be called, however.

To get the size that the function takes up in bytes, however, is not possible to do portably, unfortunately. If you told us what you really wanted to accomplish, we could probably help you do more.

edited May 23 '17 at 10:29

Community

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1

answered Sep 08 '12 at 00:11

Richard J. Ross III

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Obviously, i am not expecting the size to be as the size of return type. I was expecting the size of a pointer which is 32 bits on 32 bit architecture. Then why 1 byte only? – Green goblin Sep 08 '12 at 00:16
3

Try `sizeof(&fn)`. Functions do not decay to pointers in `sizeof` context. – R.. GitHub STOP HELPING ICE Sep 08 '12 at 00:46

Why operator sizeof( function ) in C is outputting 1 byte

2 Answers2