C-Preprocessor: Calculate number of elements from an array of structures

Question

Let's say, I have in a C application a structure:

typedef struct {
    int aParameter;
    int anotherParameter;
} Structure_t;

and an array of Structure_t elements:

const Structure_t arrayOfStructres[] = {
    {
        .aParameter = 1,
        .anotherParameter = 2
    },
    {
        .aParameter = 3,
        .anotherParameter = 4
    },
};

The arrayOfStructures is constant, so no changes during runtime. This means, that theoretically the number of elements in the array is known during compilation time. I know I could calculate the number of elements with

sizeof(arrayOfStructures) / sizeof(arrayOfStructes[0]);

during runtime, but how to get the number of elements during compilation time with the preprocessor? As the number of elements may be much more than just the two in the example, counting every element when some are added or removed is not very fast and also error prone.

sizeof runs at compile time

lost_in_the_source
– lost_in_the_source

2016-08-29 12:51:30 +00:00
Commented Aug 29, 2016 at 12:51 — lost_in_the_source
– lost_in_the_source, Commented Aug 29, 2016 at 12:51
@stackptr, except for VLA's

David Ranieri
– David Ranieri

2016-08-29 12:53:52 +00:00
Commented Aug 29, 2016 at 12:53 — David Ranieri
– David Ranieri, Commented Aug 29, 2016 at 12:53
Maybe see equivalents-to-msvcs-countof-in-other-compilers

NGI
– NGI

2016-08-29 14:28:29 +00:00
Commented Aug 29, 2016 at 14:28 — NGI
– NGI, Commented Aug 29, 2016 at 14:28

Lundin · Accepted Answer · 2016-08-29 13:21:16Z

2

I know I could calculate the number of elements with sizeof(arrayOfStructures)/sizeof(arrayOfStructes[0]); during runtime

No, that calculation is done at compile-time, so it is what you need.

The only calculation of sizeof that is done in run-time is the special case of VLAs, which doesn't apply here.

answered Aug 29, 2016 at 13:21

Lundin

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Grzegorz Szpetkowski · Accepted Answer · 2016-08-29 13:09:52Z

2

The const qualifier has nothing to do with the number of elements in given array. The arrays in C are of fixed size (well, with notable exception to flexible array member, but it is diffrent thing), with no possibility of extending their elements at runtime.

The sizeof operator will (indirectly) give you the number of elements at compile-time. The only exception is with VLA (variable length) arrays, for which sizeof is calculated are runtime, but even with them, you cannot add more elements after declaration (because of that "variable length" is somewhat unfortunate name).

edited Aug 29, 2016 at 13:09

answered Aug 29, 2016 at 12:59

Grzegorz Szpetkowski

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1 Comment

LPs Over a year ago

To be pedantic: sizeof returns number of bytes. sizeof(array)/sizeof(type_of_element) returns number of elements. ;)

Ashouri · Accepted Answer · 2016-08-29 12:57:02Z

0

I think you can use _countof Macro which operates at compile time, for more information take a look at here

answered Aug 29, 2016 at 12:57

Ashouri

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