Choosing a Segment Address

Choosing a Segment Address

Normally there is no need to map a segment to any particular virtual address. You specify addr as 0 and IRIX picks an unused virtual address. This is the usual method and the recommended one.

You can specify a nonzero value in addr to request a particular base address for the new segment. You specify MAP_FIXED in flags to say that addr is an absolute requirement, and that the segment must begin at addr or not be created. If you omit MAP_FIXED, mmap() takes a nonzero addr as a suggestion only.

Segments at Fixed Offsets

In rare cases you may need to create two or more mapped segments with a fixed relationship between their base addresses. This would be the case when there are offset values in one segment that refer to the other segment, as diagrammed in Figure 1-1.

Figure 1-1 : Segments With a Fixed Offset Relationship In Figure 1-1, a word in one segment contains an offset value A giving the distance in bytes to an object in a different mapped segment. Offset A is accurate only when the two segments are separated by a known distance, offset S.

You can create segments in such a relationship using the following procedure.

Map a single segment large enough to encompass the lengths of all segments that need fixed offsets. Use 0 for addr, allowing IRIX to pick the base address. Let this base address be B.
Map the smaller segments over the larger one. For the first (the one at the lowest relative position), specify B for addr and MAP_FIXED in flags.
For the remaining segments, specify B+S for addr and MAP_FIXED in flags.

The initial, large segment establishes a known base address and reserves enough address space to hold the other segments. The later mappings replace the first one, which cannot be used for its own sake.

Segments at a Fixed Address

You can specify any value for addr. IRIX creates the mapping if there is no conflict with an existing segment, or returns an error if the mapping is impossible. However, you cannot normally tell what virtual addresses will be available for mapping in any particular installation or version of the operating system.

There are three exceptions. First, after IRIX has chosen an address for you, you can always map a new segment of the same or shorter length at the same address. This allows you to map different parts of a file into the same segment at different times (see "Mapping Portions of a File").

Second, the low 4 MB of the address space are unused (see "Address Space Boundaries"). It is a very bad idea to map anything into the 0 page since that makes it hard to trap the use of uninitialized pointers. But you can use other parts of the initial 4 MB for mapping.

Third, the MIPS Application Binary Interface (ABI) specification (an extension of the System V ABI published by AT&T) states that addresses from 0x3000 0000 through 0x3ffc 0000 are reserved for user-defined segment base addresses.

You may specify values in this range as addr with MAP_FIXED in flags. When you map two or more segments into this region, no two segments can occupy the same 256 KB unit. This rule ensures that segments always start in different pages, even when the maximum possible page size is in use. For example, if you want to create two segments each of 4096 bytes, you can place one at 0x30000000 through 0x3000 0fff and the other at 0x3004 0000 through 0x3004 0fff. (256 KB is 0x0004 0000.)

Note: If two programs in the same system attempt to map different objects to the same absolute address, the second attempt fails.