hi.
i have a function like this
when i call it with my dynamic matrix everything is ok.Code:void print_matrix(int **t)
but if i call it with a static matrix i obtain a warning (and a segmentation foultCode:int **Dmatrix; Dmatrix = malloc(N * sizeof(int*)); //N righe for (i = 0; i < N; i++) { Dmatrix[i] = malloc(N * sizeof(int)); } print_matrix(Dmatrix);)
warning is:Code:int Smatrix[N][N]; print_matrix(Smatrix);
warning: passing argument 1 of ‘print_matrix’ from incompatible pointer type
int v[10][10] indicates 10 rows and 10 columns. But **v indicates multiple rows and columns. we can change that rows and columns any time.
If we pass this to function it allocates based on the index. So it needs that much memory to store that array or pointer in a function.
So if you pass with Smatrix[N][N] it needs N*N memory but in the pointer it is different. that is why it gives warning
The problem is that when it is passed to a function as an argument, Smatrix, like any other array, is converted to a pointer to its first element. Therefore, what is passed is an int (*)[N], i.e., a pointer to an array of N ints. This is certainly different from an int**, i.e., a pointer to a pointer to an int.
Look up a C++ Reference and learn How To Ask Questions The Smart WayOriginally Posted by Bjarne Stroustrup (2000-10-14)
int v[10][10]; is stored in memory as 100 consecutive integers, which the compiler allows you to access two dimensionally by internally performing a simple stride calculation: index = y*10+x.
int **v; is a pointer to 10 other pointers which each point to 10 ints (after the appropriate allocation). To get to any one of those ints requires following two pointers to where they point to.
As you can see, they both require completely different work "under the hood". There is no way any compiler can treat these the same way because they're fundamentally very different.
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Advice: Take only as directed - If symptoms persist, please see your debugger
Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"
I think this illustrates the difference.
Mainframe assembler programmer by trade. C coder when I can.
Not bad, though even those images don't do everything required to show how different they really are.
For example, the arrow from the declaration to the first box in the second image indicates the following of a pointer, but the arrow in the first image is not, because in that case the array is the data and no pointers are involved at all. So that arrow should be removed.
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Advice: Take only as directed - If symptoms persist, please see your debugger
Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"
This better?
Mainframe assembler programmer by trade. C coder when I can.
Great! I wasn't expecting you to spend any more effort to produce such a nice diagram, but hopefully the OP really gets it now
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Advice: Take only as directed - If symptoms persist, please see your debugger
Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"
Perhaps it can go into the FAQ somehow.
Mainframe assembler programmer by trade. C coder when I can.
There is the SourceForge Wiki. Feel free to create an article on the matter.
7. It is easier to write an incorrect program than understand a correct one.
40. There are two ways to write error-free programs; only the third one works.*
