This is part of Family API which allow to create dual-os version of program runs under OS/2 and DOS

Note: This is legacy API call. It is recommended to use 32-bit equivalent

2021/09/17 04:47 · prokushev · 0 Comments
2021/08/20 03:18 · prokushev · 0 Comments


This call deallocates a memory segment.


DosFreeSeg (Selector)


  • Selector (SEL) - input : Selector of the segment to be freed.

Return Code

rc (USHORT) - return

Return code descriptions are:

  • 0 NO_ERROR


DosFreeSeg frees selectors to segments returned by allocation calls to DosAllocSeg, DosAllocShrSeg, and DosAllocHuge. In addition, DosFreeSeg frees a selector returned by a call to DosCreateCSAlias. If a CS alias selector has been created for a data segment by a call to DosCreateCSAlias, the CS alias selector is still valid after the segment's data selector has been freed.

When allocated memory is shared, all selectors to the shared memory must be freed before the memory is deallocated. For example, if memory allocated by DosAllocSeg or DosAllocHuge has been given to another process with DosGiveSeg, the giver usually frees its selector by a call to DosFreeSeg. The recipient, in turn, frees the selector passed to it, after it has accessed the shared memory with DosGetSeg.

DosFreeSeg decrements the reference count for named shared segments allocated by DosAllocShrSeg. Access to the segment with DosGetShrSeg increments this count. When the count is 0, the memory is deallocated.

Family API Considerations

Some options operate differently in the DOS mode than in OS/2 mode. Therefore, the following restriction applies to DosFreeSeg when coding for the DOS mode:

If DosFreeSeg is issued on a CSAliased segment it deallocates the associated memory. This is inconsistent with the OS/2 mode, because DosFreeSeg must be performed on both the original and CSAliased selectors.

Example Code

C Binding

  USHORT  rc = DosFreeSeg(Selector);
  SEL              Selector;      /* Selector */
  USHORT           rc;            /* return code */

This example allocates a segment of 30,250 bytes and then discards the segment.

  #define NUMBER_OF_BYTES 30250
  SEL    Selector;
  USHORT rc;
     rc = DosAllocSeg(NUMBER_OF_BYTES,     /* # of bytes requested */
                      &Selector,           /* Selector allocated */
                     ALLOC_FLAG);         /* Allocation flags */
   rc = DosFreeSeg(Selector);            /* Segment selector */

MASM Binding

  EXTRN  DosFreeSeg:FAR
  PUSH   WORD    Selector      ;Selector
  CALL   DosFreeSeg

Returns WORD