DosAllocSeg

This call allocates a segment of memory to a requesting process.

DosAllocSeg

(Size, Selector, AllocFlags)

Size (USHORT) - input

Number of bytes requested. The value specified must be less than or equal to 65535. A value of zero indicates 65536 bytes.
Selector (PSEL) - output
Address where the selector of the segment allocated is returned.
AllocFlags (USHORT) - input
Bit indicators describing the characteristics of the segment being allocated. The bits that can be set and their meanings are:

Bit

Description
15-4
Reserved and must be set to zero.
3
If segment is shared, it can be decreased in size by DosReallocSeg.
2
Segment may be discarded by the system in low memory situations.
1
Segment is shareable through DosGetSeg.
0
Segment is shareable through DosGiveSeg.
rc (USHORT) - return
Return code descriptions are:
• NO_ERROR 8
  ERROR_NOT_ENOUGH_MEMORY
  87
  ERROR_INVALID_PARAMETER
  Remarks

  DosAllocSeg allows a process to allocate a data segment up to 64KB in size, which is movable and swappable by the system. If your application needs to accommodate a large data structure that exceeds the 64KB limit, DosAllocHuge may be issued to allocate multiple segments as one huge block of memory.

  A segment allocated by DosAllocSeg with AllocFlags bit 2 set can be discarded by the system to remedy a low memory situation when the segment is not in use. Upon allocation, a discardable segment is locked and ready for access. The caller issues DosUnlockSeg when it is finished using the segment. The next time the caller needs to access the segment, it must issue DosLockSeg. During the time a segment is locked, it cannot be discarded, but it can still be swapped.

  Allocate memory as discardable when it is needed to hold data for only short periods of time; for example, saved bit map images for obscured windows. Once the system discards a segment, the caller must reallocate the segment with DosReallocSeg and regenerate the data. Reallocating the segment automatically locks it for the first access.

  A segment may also be designated as shared with another process. If a process issues DosAllocSeg with AllocFlags bit 0 set, then the segment allocated is shareable through DosGiveSeg. To share the segment in this manner, the owning process can then issue DosGiveSeg to obtain a selector for the sharer to use. The owning process then passes the selector to the sharer using some means of interprocess communication. The sharing process can use the selector to access the shared segment. If the shared segment has been designated discardable (AllocFlags bit 2 is also set), the sharer must issue DosLockSeg to lock the segment.

  Memory allocated with DosAllocSeg is freed by a call to DosFreeSeg.

  Note: This request may be issued from privilege level 2. However, the segment is allocated as a privilege level 3 segment.

  
  Family API Considerations

  Some options operate differently in the DOS mode than in the OS/2 mode. Therefore, the following restrictions apply to DosAllocSeg when coding for the DOS mode:

  o

  Requested Size value is rounded up to the next paragraph (16-byte).
  o
  Selector is the actual segment address allocated.
  o
  AllocFlags must be set to zero.

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