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In file "cerberOS_BSP.h" I have the following:

extern char cmp_ids[][];
extern UInt8 periph_list[];

In file "BSP_unpnp.c", I have:

UInt8 periph_list[AMOUNT_OF_PERIPH] = {0};
char cmp_ids[MAX_CMPS][4] = {0};

This gives no errors for periph_list but gives the following for cmp_ids:

../../uJ/cerberOS_BSP.h:55:13: error: array type has incomplete element type
 extern char cmp_ids[][];

Unsure on how to solve this since I don't fully understand the issue, any ideas?

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2 Answers 2

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In the case of …

char cmp_ids[][];

… you have two dimensions with open (unspecified) size. Since the location of an element is calculated by start + index * sizeofelements, it is necessary to know the size of the elements.

The element of the outer array is the inner array char[]. The size is not known.

You can only omit the most outer size. All other sizes have to be specified.

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Assuming by "omit the most outer size" you mean the indice that comes first. myarray[ ][123] is valid, myarray[123][ ] is invalid.
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An array must be declared with a size, for instance:

extern char cmp_ids[MAX_CMPS][4];
extern UInt8 periph_list[AMOUNT_OF_PERIPH];

Therefor, in the header file you need to add (or include) the definitions of MAX_CMPS and AMOUNT_OF_PERIPH. If the sizes need to be calculated at run-time you can instead use pointers:

extern char **cmp_ids;
extern UInt8 *periph_list;
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2 Comments

The declaration can omit the size of the most outer dimension. Therefore there is no error for periph_list.
@AminNegm-Awad That's correct. However, without the size of the first dimension there is no way to tell the length of the array in the client module; sizeof is not applicable to incomplete types like UInt8[].

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