Question about declaration of two dimension array

[fl]

Hi,

I read the following code snippet. I am not clear about the usage of 'sampleInstInitConfig[][EDMA3_MAX_REGIONS]'.

When I replace the declaration with:
sampleInstInitConfig[NUM_EDMA3_INSTANCES][EDMA3_MAX_REGIONS], it generates compiling error.

In fact, EDMA3_MAX_REGIONS is defined external too.

Could you explain the rule about the external, two dimension array?

Thanks,



--------------------
main.c file:

extern EDMA3_DRV_InstanceInitConfig sampleInstInitConfig[][EDMA3_MAX_REGIONS];

int maio()
{
....
}

--------------------
Another .c file:

EDMA3_RM_InstanceInitConfig sampleInstInitConfig[NUM_EDMA3_INSTANCES][EDMA3_MAX_REGIONS] =
{
/* EDMA3 INSTANCE# 0 */
.....

 

[Eric Sosman]

Hi,

I read the following code snippet. I am not clear about the usage of 'sampleInstInitConfig[][EDMA3_MAX_REGIONS]'.

When I replace the declaration with:
sampleInstInitConfig[NUM_EDMA3_INSTANCES][EDMA3_MAX_REGIONS], it generates compiling error.

For future reference: Telling people that "it generates compiling
error" without mentioning *what* error is like telling your doctor
"it hurts" without mentioning whether it's your elbow or your eyeball.
Next time, copy and paste the actual error message.

In fact, EDMA3_MAX_REGIONS is defined external too.

Do you mean NUM_EDMA3_INSTANCES?

Could you explain the rule about the external, two dimension array?

When you declare an array that is actually defined elsewhere
you don't need to specify how many elements it has: A declaration
like `extern Something list[];' is enough to go on, saying that
some other module defines `list' as an array each of whose elements
is a `Something'. You can make use of that array without stating
how many `Something's there are -- the module that actually defines
`list' must specify the count, but it isn't needed here.

However, this ability to omit the element count only works
for the leftmost dimension of a multi-dimensional array. That's
because C's multi-dimensional arrays aren't quite like those of
some other languages: They're actually one-dimensional arrays
whose elements are themselves one-dimensional arrays. C needs
to know the size of each array element in order to navigate from
one to another, so if the elements are subordinate arrays C needs
to know how many elements each has. All the remaining dimensions
must be specified explicitly.

The declaration that puzzles you means

`sampleInstInitConfig' is an array of some unspecified
number of elements. Each of those elements is itself
an array of `EDMA3_MAX_REGIONS' elements, and each of
*those* elements is an 'EDMA3_RM_InstanceInitConfig'.

You *can* omit the leftmost array dimension -- but of
course it's all right if you supply it (correctly), too. Why
does the compiler "generates compiling error?"

Because for an `extern' declaration (or any declaration at
"file scope," outside any function), all the dimensions that
you choose to specify must be given as compile-time constants.
A variable is not a constant (not even if it has `const' in its
declaration), so if you specify a variable name as a file-scope
dimension the compiler will complain. (That's why I think you
meant that NUM_EDMA3_INSTANCES rather than EDMA3_MAX_REGIONS is
"defined external too," because if the latter were an `extern'
variable then the compiler would complain about *both* versions.)