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Microsoft KB Archive/10758

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Knowledge Base


Managing Memory with calloc() and malloc()

Article ID: 10758

Article Last Modified on 12/1/2003



APPLIES TO

  • The C Run-Time (CRT), when used with:
    • Microsoft C Professional Development System 5.1
    • Microsoft C Professional Development System 6.0
    • Microsoft C Professional Development System 6.0a
    • Microsoft C Professional Development System 6.0a
    • Microsoft C Professional Development System 5.1
    • Microsoft C Professional Development System 6.0
    • Microsoft C Professional Development System 6.0a
    • Microsoft C/C++ Professional Development System 7.0
    • Microsoft Visual C++ 1.0 Professional Edition
    • Microsoft Visual C++ 1.5 Professional Edition



This article was previously published under Q10758


In an application developed using the large memory model, the calloc() and malloc() functions appear to lose large blocks of memory when the segment value of an allocation changes. For example, if an application requests two 16K memory blocks, and the segment value in the second pointer is different from the segment value in the first pointer, the difference between the pointers is approximately 24,000 bytes.

This behavior is caused by the method the malloc() function uses to perform memory allocations. The malloc() function is tuned to process a large number of (relatively) small memory allocations very quickly. The malloc() function allocates a large block of memory from MS-DOS (8K is the default size), and allocates memory from the block until it is full. When an application is greater than 8K, malloc() may allocate an additional 8K block of memory and use part of that block to satisfy the allocation. If the next request does not fit in the space remaining in a given segment, malloc() must request an another segment. In the example given above, the new allocation is almost 8K beyond the last byte of the previous allocation.

An application can reduce the amount of wasted space by reducing the size of the memory block malloc() allocates by changing the value of the _amblksiz value. Include the following declaration in your application:

   extern unsigned int _amblksiz;
        

Set _amblksiz to the desired value. Smaller values of _amblksiz waste less space at the expense of slower performance for malloc(). However, when an application allocates large memory blocks, it tends to make fewer calls to malloc() and the performance degradation is minimal. Setting _amblksiz to 2 in a test program reduced the space loss to 42 bytes without noticeably degrading its performance.


Additional query words: kbinf 1.00 1.50 5.10 6.00 6.00a 6.00ax 7.00

Keywords: kb16bitonly KB10758