How do you cast to a pointer of an array?
for example I would like to do the following:
I get an error for the cast "(int *[2])". And I also tried "(int[2]*)" and "(int**)"Code:int *data[2]; data = (int *[2]) malloc(sizeof(int[2])*SIZE);
I know I could just create a an array of ints and make it twice the size but it makes more visual sense to pair values like this.
I also require the cast because the gcc compiler throws an error otherwise.
It's a precedence issue with your definition of data. The brackets have a higher precedence than the asterisk so you're defining an array of 2 int pointers. To define a pointer to int[2] arrays you need parentheses like this:
And you of course don't need the cast on malloc.Code:int (*data)[2];
Firstly, wouldn't this give me an array of 2 int pointers? I want a pointer to int arrays of size 2Originally Posted by algorism
Second my gcc compiler throws an error without the cast. NOT a warning; "error: incompatible types when assigning to type ‘int *[2]’ from type ‘void *’"
EDIT:
to be clear, the error for my cast is not about it being a wrong cast, the syntax itself is wrong. The code I posted may be a little misleading with the cast I used so let me post that again with a more telling cast:
and the error for the second line is: "error: expected ‘)’ before ‘*’ token"Code:int *data[2]; data = (int[2]*) malloc(sizeof(int[2])*SIZE);
Last edited by jdodle; 05-31-2016 at 03:44 PM.
Firstly, you're an idiot.
Secondly, screw you.
whoa... whats the hostility for? I was just trying to clarify what the problem is... I'm sorry if it came off as arrogant or something.
How are you compiling this program? Are you by chance using a C++ compiler instead of a C compiler? If you're using malloc() correctly with a C compiler you shouldn't need the cast.Second my gcc compiler throws an error without the cast. NOT a warning; "error: incompatible types when assigning to type ‘int *[2]’ from type ‘void *’"
It did.I'm sorry if it came off as arrogant or something.
Jim
I am using gcc with flags "-std=c99 -g -O3". However it could have been adjusted, are there ways to change how the compiler recognizes errors and warnings when installing gcc?
In any case, the question is not really about how to fix the error or get around the error (I know how I can avoid it), it is about how to cast to a pointer for array's of size N. fixing the error is more of a side effect.
Last edited by jdodle; 05-31-2016 at 04:48 PM.
Did you include the <stdlib.h> include file?
Post a small complete program that illustrates your problem. And no fixing the error is not a side effect, it is a requirement.In any case, the question is not really about how to fix the error or get around the error (I know how I can avoid it), it is about how to cast to a pointer for array's of size N. fixing the error is more of a side effect.
You should not be casting the return value of malloc() this cast can mask serious problems, if the program doesn't compile without the cast then you're doing something seriously wrong.
Jim
yes I have it included.
and here is the compilation log:Code:#include <stdlib.h> int main (int argc, char *argv[]) { int *test[5]; test = malloc(sizeof(test[5])*10); free(test); }
Code:$ gcc -o test test.c test.c: In function ‘main’: test.c:7: error: incompatible types when assigning to type ‘int *[5]’ from type ‘void *’I think you misunderstand what I mean. The question "what pointer cast do I use?" is purely an educational question, I could theoretically ask this question without writing a single line of code. Of course in implementation, obviously my code will not work if it doesn't even compile. In this way it is certainly a requirement. I can, and know, how to rewrite my code to avoid this problem with the type cast but I am curious as to what the cast should be. In this way, my question is leaning towards purely educational, with a real world application.
This is new to me, even with other compilers I have used, not casting malloc gives a compiler warning, granted I usually ignore it but this seems to suggest that I should be casting the return.
Would you mind providing some situations where casting is a bad idea here?
Firstly, wouldn't this give me an array of 2 int pointers? I want a pointer to int arrays of size 2
cdecl.org says:
int (*data)[2]
declare data as pointer to array 2 of int
Code:int (*data)[2] = malloc(sizeof *data);All of them?Would you mind providing some situations where casting is a bad idea here?
Optimize memory management
FAQ > Casting malloc - Cprogramming.com
Would data[2] be out of bounds? if so, this is not what I want.
> Would data[2] be out of bounds? if so, this is not what I want.
Yes it would, and I've no idea now what you want.
> Would you mind providing some situations where casting is a bad idea here?Code:#include <stdio.h> #include <stdlib.h> #define ASIZE 10 void foo ( ) { // cdecl> explain int (*data)[2]; // declare data as pointer to array 2 of int int (*data)[2]; data = malloc( sizeof(*data) * ASIZE ); // the minor dimension is fixed at 2, but you // can have as many of the major dimension as // you malloc. for ( int i = 0 ; i < ASIZE ; i++ ) { data[i][0] = 0; data[i][1] = 1; } free(data); } void bar ( ) { // cdecl> explain int *data[2]; // declare data as array 2 of pointer to int int *data[2]; data[0] = malloc( sizeof(*data[0]) * ASIZE ); data[1] = malloc( sizeof(*data[1]) * ASIZE ); // the major dimension is fixed at 2, but you // can have as many of the minor dimension as // you malloc. for ( int i = 0 ; i < ASIZE ; i++ ) { data[0][i] = 0; data[1][i] = 1; } free(data[0]); free(data[1]); } int main ( ) { foo(); bar(); return 0; } $ gcc -std=c99 -Wall -Wextra -g foo.c $ valgrind ./a.out ==3976== Memcheck, a memory error detector ==3976== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al. ==3976== Using Valgrind-3.6.1-Debian and LibVEX; rerun with -h for copyright info ==3976== Command: ./a.out ==3976== ==3976== ==3976== HEAP SUMMARY: ==3976== in use at exit: 0 bytes in 0 blocks ==3976== total heap usage: 3 allocs, 3 frees, 160 bytes allocated ==3976== ==3976== All heap blocks were freed -- no leaks are possible ==3976== ==3976== For counts of detected and suppressed errors, rerun with: -v ==3976== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)
Yes, in older versions of C, omitting the inclusion of stdlib.h would leave malloc without a prototype - at which point the compiler would automatically assume malloc returned an int. On machines where pointers have different size to int (or machines which use different registers for different types), the code is invisibly broken because the cast hides the error.
The error is because you can't assign an array in C.int *test[5];
test = malloc(sizeof(test[5])*10);
free(test);
and here is the compilation log:
$ gcc -o test test.c
test.c: In function ‘main’:
test.c:7: error: incompatible types when assigning to type ‘int *[5]’ from type ‘void *’
test[0] = malloc.... is valid
test = malloc is NOT valid
You can't synthesise int test[5][10] from int *test[5] with a single call to malloc.
int (*test)[10] = malloc(5 * sizeof(*test) ); synthesises int test[5][10], but it only works if the minor dimension is a compile time constant.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper.
