Microsoft KB Archive/42756

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Problems Not Including MALLOC.H in Compact, Large Model

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Q42756

6.00 6.00a 6.00ax 7.00 | 6.00 6.00a | 1.00 1.50 MS-DOS | OS/2 | WINDOWS kbprg -- The information in this article applies to: - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax - Microsoft C for OS/2, versions 6.0 and 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0 and 1.5 -- SUMMARY ======= In the compact and large memory models, it is vital that the malloc function be prototyped as returning a void far pointer. The recommended method is to include the header file MALLOC.H. This file contains the prototype for malloc. The default data-pointer size in the compact and large models is 32 bits. The default return type of any function, including malloc, when it is not prototyped is a 16-bit short integer. Thus, the segment portion of a far address will be destroyed unless the compiler knows it is dealing with a 32-bit type. The compiler versions 5.1, 6.0, 6.0a, and 6.0ax will generate the following warnings if the warning level is set high enough: C4016: 'malloc': no function return type, using int as default C4017: 'malloc': no function prototype given Microsoft C/C++ version 7.0 and Visual C++ version 1.0 and 1.5 will generate the following warning: C4013: 'malloc' undefined: assuming extern returning int It is a good idea to compile with the highest warning level so that problems like this can be caught at compile time. MORE INFORMATION ================ If you attempt to use the UNIX coding style of including MEMORY.H rather than MALLOC.H, you will encounter problems at run time. MEMORY.H does not prototype any of the memory allocation functions; it prototypes only memory copy and move functions. In a segmented architecture that can have data and code pointers of different sizes, this may have serious ramifications. The compiler will generate warnings for the following sample code: Sample Code: /* Compile options needed: /AL /W4 */ #include void main { char *addr; size_t nbytes = 100; addr = (char *) malloc( nbytes ); } Additional reference words: kbinf 6.00 6.00a 6.00ax 7.00 1.00 1.50 KBCategory: kbprg KBSubcategory: CLngIss

Keywords : kb16bitonly

Issue type :

Technology : kbVCsearch kbAudDeveloper kbPTProdChange kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbCComp600DOS kbCComp600aDOS kbCComp600axDOS kbCComp600OS2 kbCComp600aOS2 kbCVC700DOS