As mentioned in Chapter 1, a neat feature of Linux is its ability to execute files compiled for other operating systems. Of course, this is possible only if the files include machine code for the same computer architecture on which the kernel is running. Two kinds of support are offered for these “foreign” programs:
Emulated execution: necessary to execute programs that include system calls that are not POSIX-compliant
Native execution: valid for programs whose system calls are totally POSIX-compliant
Microsoft MS-DOS and Windows programs are emulated: they cannot be natively executed, since they include APIs that are not recognized by Linux. An emulator like DOSemu or Wine (which appeared in the example at the end of the previous section) is invoked to translate each API call into an emulating wrapper function call, which in turn uses the existing Linux system calls. Since emulators are mostly implemented as User Mode applications, we don’t discuss them further.
On the other hand, POSIX-compliant programs compiled on operating
systems other than Linux can be executed without too much trouble,
since POSIX operating systems offer similar APIs. (Actually, the APIs
should be identical, although this is not always the case.) Minor
differences that the kernel must iron out usually refer to how system
calls are invoked or how the various signals are numbered. This
information is stored in execution domain descriptors
of type
exec_domain
.
A
process specifies its
execution domain by setting the
personality
field of its descriptor and storing the address of the corresponding
exec_domain
data structure in the
exec_domain
field. A process can change its
personality by issuing a suitable system call named
personality( )
; typical values assumed by the
system call’s parameter are listed in Table 20-6. The C library does not include a
corresponding wrapper routine because programmers are not expected to
directly change the personality of their programs. Instead, the
personality( )
system call should be issued by the
glue code that sets up the execution context of the process (see the
next section).
Table 20-6. Main personalities supported by the Linux kernel
Personality |
Operating system |
---|---|
|
Standard execution domain |
|
System V Release 4 |
|
System V Release 3 |
|
SCO Unix Version 3.2 |
|
SCO OpenServer Release 5 |
|
Unix System V/386 Release 3.2.1 |
|
Interactive Unix |
|
BSD Unix |
|
SunOS |
|
Xenix |
|
SGI Irix-5 32 bit |
|
SGI Irix-6 32 bit |
|
SGI Irix-6 64 bit |
|
RISC OS |
|
Sun’s Solaris |
|
Caldera’s UnixWare 7 |