"C" malloc problem! Help!

Discussion in 'Computer Science & Culture' started by superluminal, Feb 2, 2006.

Thread Status:
Not open for further replies.
  1. superluminal . Registered Senior Member

    Messages:
    10,717
    Ok, I have this problem.

    Here's part of the header for your information:
    Code:
    typedef unsigned char       UB1;
    typedef unsigned short int UB2;
    typedef unsigned int         UB4;
    
    UB1 *ld_data_base;
    UB4 *ld_data_ptr;
    UB1 *data_item;
    I malloc the two vars as shown. 'ld_data_base' is a block that will contain some variable length strings or other data. 'ld_data_ptr' is a list of pointers hat will point to the beginning of each variable length element in 'ld_data_base'.

    Code:
       ld_data_base = (UB1 *) malloc(21);
       ld_data_ptr = (UB4 *) malloc(3);
    I load the element pointers based on some table. Here I've used fixed offsets (6,14):

    Code:
       *(ld_data_ptr+0) = (UB4)(ld_data_base);
       *(ld_data_ptr+1) = (UB4)(ld_data_base+6);
       *(ld_data_ptr+2) = (UB4)(ld_data_base+14);
    So, I get for example,

    *(ld_data_ptr+0) = :00CA3430
    *(ld_data_ptr+1) = :00CA3436
    *(ld_data_ptr+2) = :00CA343E

    Which is fine. Now I malloc some temporary space for something else:

    Code:
       data_item = (UB1 *) malloc(1);
    and this happens:

    *(ld_data_ptr+0) = 0x00CA3430
    *(ld_data_ptr+1) = 0x00CA3436
    *(ld_data_ptr+2) = 0xE

    Why dose malloc-ing 'data_item' cause the value stored in the memory location 'ld_data_ptr+2' to get corrupted?!?! It's as if the last malloc is not respecting the previously allocated space.

    Running Borland C++ Builder 5 on Windows XP.

    Any help will be GREATLY appreciated!
     
  2. Google AdSense Guest Advertisement



    to hide all adverts.
  3. Zephyr Humans are ONE Registered Senior Member

    Messages:
    3,371
    The argument to malloc is size in bytes, while an int is 2 or 4 bytes depending on your system. I think you want:
    Code:
       ld_data_base = (UB1 *) malloc(21*sizeof(UB1));
       ld_data_ptr = (UB4 *) malloc(3*sizeof(UB4));
    etc

    Please Register or Log in to view the hidden image!



    (sizeof is a builtin operator that gives the size of a type in bytes)
     
  4. Google AdSense Guest Advertisement



    to hide all adverts.
  5. AntonK Technomage Registered Senior Member

    Messages:
    1,083
    You can also use calloc which takes in two parameters, namely the size of a single element, and how many elements you want. It basically does the same thing as above.

    -AntonK
     
  6. Google AdSense Guest Advertisement



    to hide all adverts.
  7. superluminal . Registered Senior Member

    Messages:
    10,717
    Zephyr,

    Yes indeedy it does! That fixed it. I haven't malloc'ed anything in a while and completely forgot that. Thanks a lot!
     
Thread Status:
Not open for further replies.

Share This Page