Various improvements.

man7/feature_test_macros.7

@@ -112,12 +112,23 @@ feature test macro requirements (this example from
 .fi
 .RE
 .PP
-This syntax is used in cases where only a single 
+This format is employed in cases where only a single 
 feature test macro can be used to expose the function
 declaration, and that macro is not defined by default.
 .SS Feature test macros understood by glibc
 Linux/glibc understands the following feature test macros:
 .\" FIXME ? add _ATFILE_SOURCE to this list
+.TP
+.B __STRICT_ANSI__
+ISO Standard C.
+This macro is implicitly defined by
+.BR gcc (1)
+when invoked with, for example, the
+.I -std=c99
+or
+.I -ansi
+flag.
+
 .TP
 .B _POSIX_C_SOURCE
 Defining this macro with the value 1 causes header files to expose
@@ -139,24 +150,12 @@ with the value 1.
 .B _XOPEN_SOURCE
 Defining this macro with any value causes header files to expose
 definitions conforming to POSIX.1, POSIX.2, and XPG4.
-Defining with the value 500 or greater additionally exposes
+Defining this macro with the value 500 or greater additionally exposes
 definitions for SUSv2 (UNIX 98).
-Defining with the value 600 or greater additionally exposes
+Since glibc 2.2, defining this macro with the value 600
+or greater additionally exposes
 definitions for SUSv3 (UNIX 03; i.e., the POSIX.1-2001 base specification
 plus the XSI extension) and C 99 definitions.
-.IP
-Defining
-.B _XOPEN_SOURCE
-also implicitly defines
-.BR _POSIX_C_SOURCE :
-with the value 2, if
-.B _XOPEN_SOURCE
-is less than 500;
-with the value 199506L, if 500 <=
-.B _XOPEN_SOURCE
-< 600; or with the value 200112L if
-.B _XOPEN_SOURCE
->= 600.
 .TP
 .B _XOPEN_SOURCE_EXTENDED
 If this macro is defined with the value 1, and the
@@ -221,16 +220,25 @@ System V-derived definitions.
 (SVID == System V Interface Definition; see
 .BR standards (7).)
 .TP
+.B _ATFILE_SOURCE
+Defining this macro with any value cause header files to expose
+declarations of a range of functions with the suffix "at";
+see
+.BR openat (2).
+.TP
 .B _GNU_SOURCE
 Defining this macro (with any value) is equivalent to defining
 .BR _BSD_SOURCE ,
 .BR _SVID_SOURCE ,
+.BR _ATFILE_SOURCE ,
 .BR _LARGEFILE64_SOURCE ,
-.BR _ISOC99_SOURCE
+.BR _ISOC99_SOURCE ,
+.BR _XOPEN_SOURCE_EXTENDED ,
+.BR _POSIX_SOURCE ,
 .BR _POSIX_C_SOURCE
-with the value 200112L, and
+with the value 200112L (199506L in glibc versions before 2.5), and
 .BR _XOPEN_SOURCE
-with the value 600.
+with the value 600 (500 in glibc versions before 2.2).
 In addition, various GNU-specific extensions are also exposed.
 Where standards conflict, BSD definitions are disfavored.
 .TP
@@ -290,24 +298,70 @@ and result in a run-time error if the check fails.
 Use of this macro requires compiler support, available with
 .BR gcc (1)
 since version 4.0.
-.SS Feature test macros defined by default
+.SS Default definitions, implicit definitions, and combining definitions
 .PP
-The following macros are defined by default:
+The following feature test macros are defined by default:
 .BR _BSD_SOURCE ,
 .BR _SVID_SOURCE ,
 .BR _POSIX_SOURCE ,
 and
-.BR _POSIX_C_SOURCE =199506L.
+.BR _POSIX_C_SOURCE =200112L
-If individual macros are defined, then other macros are disabled
+(199506L in glibc versions before 2.5).
-unless they are also explicitly defined.
+.PP
-(Exception: if
+If any of
+.BR __STRICT_ANSI__ ,
+.BR _ISOC99_SOURCE ,
+.BR _POSIX_SOURCE ,
+.BR _POSIX_C_SOURCE  ,
+.BR _XOPEN_SOURCE ,
+.BR _XOPEN_SOURCE_EXTENDED ,
+.BR _BSD_SOURCE ,
+or
+.BR _SVID_SOURCE
+is explicitly defined, then
+.BR _BSD_SOURCE ,
+and
+.BR _SVID_SOURCE
+are not defined by default.
+
+If
+.BR _POSIX_SOURCE
+and
 .BR _POSIX_C_SOURCE
-is not otherwise defined,
+are not explicitly defined,
-then it is always defined with the value 200112L
+and either
-(199506L in glibc versions before 2.4),
+.BR __STRICT_ANSI__
-unless the compiler is invoked in one of its standard modes, for example, the
+is not defined or
-.I -std=c99
+.BR _XOPEN_SOURCE
-flag.)
+is defined with a value of 500 or more, then
+.RS 6
+.IP * 3
+.B _POSIX_SOURCE
+is defined with the value 1; and
+.IP *
+.B _POSIX_C_SOURCE
+is defined with one of the following values:
+.RS 6
+.IP \(bu 3
+2,
+if 
+.BR XOPEN_SOURCE
+is defined with a value less than 500;
+.IP \(bu
+199506L,
+if 
+.BR XOPEN_SOURCE
+is defined with a value greater than or equal to 500 and less than 600;
+or
+.IP \(bu
+200112L (199506L in glibc versions before 2.5),
+if 
+.BR XOPEN_SOURCE
+is undefined, or
+is defined with a value greater than or equal to 600.
+.RE
+.RE
+.PP
 Multiple macros can be defined; the results are additive.
 .SH CONFORMING TO
 POSIX.1 specifies
@@ -317,9 +371,11 @@ and
 .BR _XOPEN_SOURCE .
 .BR _XOPEN_SOURCE_EXTENDED
 was specified by XPG4v2 (aka SUSv1).
+
 .BR _FILE_OFFSET_BITS
 is not specified by any standard,
 but is employed on some other implementations.
+
 .BR _BSD_SOURCE ,
 .BR _SVID_SOURCE ,
 .BR _GNU_SOURCE ,