Segment permissions

A program to be loaded by the system must have at least one loadable segment (although this is not required by the file format). When the system creates loadable segments' memory images, it gives access permissions as specified in the p_flags member. All bits included in the PF_MASKPROC mask are reserved for processor-specific semantics.

Segment flag bits, p_flags

Name	Value	Meaning
PF_X	0x1	Execute
PF_W	0x2	Write
PF_R	0x4	Read
PF_MASKPROC	0xf0000000	Unspecified

If a permission bit is 0, that type of access is denied. Actual memory permissions depend on the memory management unit, which may vary from one system to another. Although all flag combinations are valid, the system may grant more access than requested. However, a segment will never have write permission unless it is specified explicitly. The following table shows both the exact flag interpretation and the allowable flag interpretation.

Segment permissions

Flags	Value	Exact	Allowable
none	0	All access denied	All access denied
PF_X	1	Execute only	Read, execute
PF_W	2	Write only	Read, write, execute
PF_W+PF_X	3	Write, execute	Read, write, execute
PF_R	4	Read only	Read, execute
PF_R+PF_X	5	Read, execute	Read, execute
PF_R+PF_W	6	Read, write	Read, write, execute
PF_R+PF_W+PF_X	7	Read, write, execute	Read, write, execute

For example, typical text segments have read and execute -- but not write -- permissions. Data segments normally have read, write, and execute permissions.