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DosAliasMem creates a private Read/Write alias or an LDT code segment alias to part of an existing memory object. The alias object is accessible only to the process that created it. The original object must be accessible to the caller of DosAliasMem.


DosAliasMem (pMem, cbSize, ppAlias, flags)


pMem (PMEM) input
Contains the address of the memory to be aliased. It must be on a page boundary (that is, 4K aligned), but may specify an address within a memory object.
cbSize (CBSIZE) input
Specifies the size in bytes for the memory to alias. The entire range must lie within a single memory object and must be committed if OBJ_SELMAPALL is specified.
ppAlias (PPALIAS) output
Address of a location in which the address of the aliased memory is returned. The corresponding LDT selector is not explicitly returned but may be calculated by using the Compatibility Mapping Algorithm
sel = (SEL) ((ULONG) (*ppAlias) >> 13 | 7)
flags (FLAGS) input 
Flags are defined as follows:
OBJ_SELMAPALL (0x00000800)
OBJ_SELMAPALL creates a Read/Write 32 bit alias to the address specified. The entire range must be committed, start on page boundary and be within the extent of a single memory object. An LDT selector is created to map the entire range specified.
If OBJ_SELMAPALL is not specified, then size is rounded up to a 4K multiple and the alias created inherits the permissions from the pages of the original object.
OBJ_TILE may be specified, but currently this is enforced whether or not specified. This forces LDT selectors to be based on 64K boundaries.
SEL_CODE (0x00000001)
Marks the LDT alias selector(s) Read-Executable code selectors.
SEL_USE32 (0x00000002)
Used with OBJ_SELMAPALL, otherwise ignored. Marks the first alias LDT selector as a 32 bit selector by setting the BIG/C32 bit.

Return Code

ulrc (APIRET) returns
DosAliasMem returns one of the following values:
  • 0 NO_ERROR


An export for DosAliasMem does not appear in versions of OS2386.LIB distributed prior to Warp Server for e-business. When using older versions, the following statements should be added to the link edit .DEF file

importsDosAliasMem = DOSCALLS.298

An alias is removed by calling DosFreeMem with the alias address.

Though it is possible to create a Read/Write alias to a code segment to allow code modification this is not recommended. On Pentium® processors, and later, pipe-lining techniques used by the processor might allow the processor not to be aware of the modified code, if appropriate pipe-line serialization is not performed by the programmer. For further information see the processor documentation.

Example Code

#include <os2.h>

int main(int argc, char *argv[], char *envp[])
   PVOID pAlias;
   PVOID pMem;
   APIRET rc;

   /* alias a read-only shared memory object as a private read/write */
   /* object. This will allow clients of this object to read only    */
   /* while allowing the owner to update it.                         */

   rc=DosAllocSharedMem( pMem,NULL,128*1024,

   rc = DosAliasMem(pMem, 128*1024,  pAlias, OBJ_TILE);
   return 0;

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