Passing multi-dimensional arrays to functions in C

Question

Why is it necessary to specify the number of elements of a C-array when it is passed as a parameter to a function (10 in the following example)?

void myFun(int arr[][10]) {}

Is it so because the number of elements is needed to determine the address of the cell being accessed?

score 3 · Accepted Answer · answered Mar 30 '12 at 12:41

3

Yes. It's because arr[i][j] means ((int *)arr)[i * N + j] if arr is an int [][N]: the pointer-arithmetic requires the length of a row.

answered Mar 30 '12 at 12:41

ruakh

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score 1 · Answer 2 · answered Mar 30 '12 at 12:44

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The compiler needs to have an idea when the next row starts in memory (as a 2D array is just a continuous chunk of memory, one row after the other). The compiler is not psyche!

answered Mar 30 '12 at 12:44

Ed Heal

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score 1 · Answer 3 · answered Mar 30 '12 at 12:44

It is only necessary if you used static allocation for your array thought. Because the generate code create a continuous memory block for the array, like pointed out ruakh.

However if you use dynamic allocation it is not necessary, you only need to pass pointers.

Regards

Passing multi-dimensional arrays to functions in C

3 Answers3