.Dd October 7, 2004
.Os Darwin
.Dt COMPAT 5
.Sh NAME
.Nm compat
.Nd manipulate compatibility settings
.Sh SYNOPSIS
.Ev COMMAND_MODE=legacy|unix2003
.Lp
.Fd #define _POSIX_C_SOURCE
.Fd #define _APPLE_C_SOURE
.Fd #define _NONSTD_SOURCE
.Fd #define __LP64__
.Sh DESCRIPTION
Setting the environment variable
.Ev COMMAND_MODE
to the value legacy causes utility programs to behave as closely to Mac OS X 10.3's utility programs as possible.  When in this mode all of 10.3's flags are accepted, and in some cases extra flags are accepted, but no flags that were used in 10.3 will have been removed or changed in meaning.  Any behavioral changes in this mode are documented in the LEGACY sections of the individual utilities.
.Pp
Setting the environment variable
.Ev COMMAND_MODE 
to the value unix03 causes utility programs to obey the
.St -susv3
standards even if doing so would alter the behavior of flags used in 10.3.
.Pp
.Ev COMMAND_MODE's
value is case insensitive and if it is unset or set to something other than legacy or unix03 it behaves as if it were set to unix03.
.Pp
Defining
.Dv _NONSTD_SOURCE
causes library and kernel calls to behave as closely to Mac OS X 10.3's library and kernel calls as possible.  Any behavioral changes in this mode are documented in the LEGACY sections of the individual function calls.
.Pp
Defining any of
.Dv _POSIX_C_SOURCE,
.Dv _APPLE_C_SOURE,
or
.Dv __LP64__
causes library and kernel calls to conform to
.St -susv3
standards even if doing so would alter the behavior of functions used in 10.3.  Defining
.Dv _POSIX_C_SOURCE
also removes functions, types, and other interfaces that are not part of
.St -susv3
from the normal C namespace, while
.Dv _APPLE_C_SOURE
does not.
.Pp
Failing to define any of those symbols currently acts as if you have defined 
.Dv _NONSTD_SOURCE
but it is expected that in a future OS X release the default behavior will change to be as if
.Dv _APPLE_C_SOURE
were defined.
.Sh STANDARDS
With COMMAND_MODE set to unix2003 utility functions conform to 
.St -susv3
.Pp
With
.Dv _POSIX_C_SOURCE,
.Dv _APPLE_C_SOURE,
or
.Dv __LP64__
system and library calls conform to
.St -susv3
.Sh BUGS
Different parts of a program can be compiled with different compatibility settings.  The resultant program will normally work as expected, for example a regex created by the
.St -susv3
.Fn regcomp 3
can be passed to the legacy
.Fn regfree 3
with no unexpected results.  Some cases are less clear cut, for example
what does the programmer intend when they use the
.Fn -susv3
.Fn regcomp 3
to compile a regex, but the legacy
.Fn regexec 3
to execute it?  Any interpretation will surprise someone.