Arrays and pointers to allocated memory

This is a discussion on Arrays and pointers to allocated memory within the C Programming forums, part of the General Programming Boards category; If you declare int x[500][1000]; does x[i] give you the pointer to the first element in the i:th-array? Or is ...

  1. #1
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    Arrays and pointers to allocated memory

    If you declare

    int x[500][1000];

    does x[i] give you the pointer to the first element in the i:th-array? Or is x[i] the whole i:th array itself?



    Also, a declaration

    char string[100];

    will give the expression sizeof(string) the value 100. But a declaration

    char *string;
    string = malloc(100);

    will give the same expression the value 4 cause then it's a ponter? Is there any way to se how large the allocated memory is? I mean, when you use free(string), it must be known somewere how big the allocated area is, right? And then you could be able to check how large area you allocated?

  2. #2
    ATH0 quzah's Avatar
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    Quote Originally Posted by TriKri
    If you declare

    int x[500][1000];

    does x[i] give you the pointer to the first element in the i:th-array? Or is x[i] the whole i:th array itself?
    Test it yourself. Print the address of x[0]. Now print the address of x[1]. Do a bit of math, get your answer.

    Quote Originally Posted by TriKri
    Is there any way to se how large the allocated memory is?
    No.


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  3. #3
    Just Lurking Dave_Sinkula's Avatar
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    I see another recommendation to visit this page:
    http://www.torek.net/torek/c/pa.html

    because
    does x[i] give you the pointer to the first element in the i:th-array? Or is x[i] the whole i:th array itself?
    the first element is an array.

    Quote Originally Posted by TriKri
    Is there any way to se how large the allocated memory is? I mean, when you use free(string), it must be known somewere how big the allocated area is, right? And then you could be able to check how large area you allocated?
    Just remember how much you ask for.
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    Registered User Mortissus's Avatar
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    Quote Originally Posted by TriKri
    If you declare

    int x[500][1000];

    does x[i] give you the pointer to the first element in the i:th-array? Or is x[i] the whole i:th array itself?
    The "whole array" is acctually a pointer to a block of memory. The address of the first element is the same address of this whole array. The answer is yes if you think this way. If not, then make some tests and take your conclusions.


    Quote Originally Posted by TriKri
    Also, a declaration

    char string[100];

    will give the expression sizeof(string) the value 100. But a declaration

    char *string;
    string = malloc(100);

    will give the same expression the value 4 cause then it's a ponter? Is there any way to se how large the allocated memory is? I mean, when you use free(string), it must be known somewere how big the allocated area is, right? And then you could be able to check how large area you allocated?
    Yes, but it depends on the operational system.

  5. #5
    ATH0 quzah's Avatar
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    Quote Originally Posted by Mortissus
    The "whole array" is acctually a pointer to a block of memory.
    No it isn't. Not any more than a "whole int" is "acctually a pointer to a block of memory". Array names, though similar in most ways, are not pointers.


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    Thanx for all the answers. Now I feel I understand the difference between arrays and pointers a little bit better.

    But what about writing
    Code:
    int myarray[];
    myarray = malloc(100 * sizeof(int));
    Does myarray really become an array or does myarray become a pointer? Now I am lost.

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    Code:
    int myarray[];
    is not a valid declaration.
    If you understand what you're doing, you're not learning anything.

  8. #8
    Frequently Quite Prolix dwks's Avatar
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    Quote Originally Posted by TriKri
    But what about writing
    Code:
    int myarray[];
    myarray = malloc(100 * sizeof(int));
    Does myarray really become an array or does myarray become a pointer? Now I am lost.
    myarray starts as a pointer, and it ends as a pointer to some memory. The memory can be addressed as an array just like other pointers.
    dwk

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  9. #9
    and the hat of int overfl Salem's Avatar
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    > is not a valid declaration.
    It was, deep in the mists of time.
    http://www.cs.bell-labs.com/who/dmr/primevalC.html
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    System Novice siavoshkc's Avatar
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    Is there any way to se how large the allocated memory is? I mean, when you use free(string), it must be known somewere how big the allocated area is, right? And then you could be able to check how large area you allocated?
    Good point. When malloc() (or new in C++) allocates memory it writes a header for it that keeps some metadata like the size of it. There should be a way of reading this header for example with a function lile getmeminf(), why not?

    And about arrays and pointers: http://www.cplusplus.com/doc/general/Arrptr.html
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  11. #11
    and the hat of int overfl Salem's Avatar
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    > There should be a way of reading this header for example with a function lile getmeminf(), why not?
    There might be, but there is no PORTABLE way to find out, short of writing your own wrapper around malloc and free to keep track of the size.

    So have a look through your compiler specific documentation, and decide for yourself whether the loss of portability is worth it.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  12. #12
    System Novice siavoshkc's Avatar
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    If such thing exists it should be found in <new> header, but I didn't find any function to do such thing.
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    Frequently Quite Prolix dwks's Avatar
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    If such a thing exists (which it doesn't) in <new> (which it doesn't), it would be C++, not C.
    dwk

    Seek and ye shall find. quaere et invenies.

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  14. #14
    System Novice siavoshkc's Avatar
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    For C it should be found in stdlib.h among other related functions that are:
    Code:
    void *  calloc(__in size_t _NumOfElements, __in size_t _SizeOfElements);
    void   free(__inout_opt void * _Memory);
    void * malloc(__in size_t _Size);
    void * realloc(__in_opt void * _Memory, __in size_t _NewSize);
    It is strange to me that there is no such function. It should be standard in C/C++ and should take the pointer as argument and return its size at least. I don't know what information can be found in the header of allocated memory.
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  15. #15
    and the hat of int overfl Salem's Avatar
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    Well since my code usually looks like
    Code:
    size_t howmany = 123;
    int *myArr = malloc ( sizeof *myArr * howmany );
    Why would I need a library function to tell me how many bytes have been allocated when I already have a variable telling me how many bytes have been allocated?

    And if you do pass that to another function with say
    foo ( myArr );
    Why is it so hard not to change it to
    foo ( myArr, howmany );
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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