Many more improvements.

man7/feature_test_macros.7

@@ -116,8 +116,12 @@ 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
+The following paragraphs explain how feature test macros are handled
+in Linux glibc 2.\fIx\fP, \fIx\fP > 0.
+.\" The details in glic 2.0 are simpler, but combining a
+.\" a description of them with the details in later glibc versions
+.\" would make for a complicated description.
+Linux glibc understands the following feature test macros:
 .TP
 .B __STRICT_ANSI__
 ISO Standard C.
@@ -133,13 +137,16 @@ flag.
 .B _POSIX_C_SOURCE
 Defining this macro with the value 1 causes header files to expose
 definitions conforming to POSIX.1-1990 and ISO C (1990).
-Defining with the value L or greater additionally exposes
+Defining this macro with the value L or greater additionally exposes
 definitions for POSIX.2-1992.
-Defining with the value 199309L or greater additionally exposes
+Defining this macro with the value 199309L or greater additionally exposes
 definitions for POSIX.1b (real-time extensions).
-Defining with the value 199506L or greater additionally exposes
+.\" 199506L functionality is only available since glibc 2.1
+Defining this macro with the value 199506L or greater additionally exposes
 definitions for POSIX.1c (threads).
-Defining with the value 200112L exposes definitions corresponding
+Since glibc 2.3.3,
+defining this macro with the value 200112L or greater
+exposes definitions corresponding
 to the POSIX.1-2001 base specification (excluding the XSI extension).
 .TP
 .B _POSIX_SOURCE
@@ -201,7 +208,7 @@ large files with only a recompilation being required.)
 and on those systems this macro has no effect.
 .TP
 .B _BSD_SOURCE
-Defining this macro with any value cause header files to expose
+Defining this macro with any value causes header files to expose
 BSD-derived definitions.
 Defining this macro also causes BSD definitions to be preferred in
 some situations where standards conflict, unless one or more of
@@ -215,13 +222,13 @@ or
 is defined, in which case BSD definitions are disfavored.
 .TP
 .B _SVID_SOURCE
-Defining this macro with any value cause header files to expose
+Defining this macro with any value causes header files to expose
 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
+Defining this macro with any value causes header files to expose
 declarations of a range of functions with the suffix "at";
 see
 .BR openat (2).
@@ -236,7 +243,8 @@ Defining this macro (with any value) is equivalent to defining
 .BR _XOPEN_SOURCE_EXTENDED ,
 .BR _POSIX_SOURCE ,
 .BR _POSIX_C_SOURCE
-with the value 200112L (199506L in glibc versions before 2.5), and
+with the value 200112L (199506L in glibc versions before 2.5),
+.\" 199309L in glibc versions before 2.1
 .BR _XOPEN_SOURCE
 with the value 600 (500 in glibc versions before 2.2).
 In addition, various GNU-specific extensions are also exposed.
@@ -300,13 +308,15 @@ Use of this macro requires compiler support, available with
 since version 4.0.
 .SS Default definitions, implicit definitions, and combining definitions
 .PP
-The following feature test macros are defined by default:
+If no feature test macros are explicitly defined,
+then the following feature test macros are defined by default:
 .BR _BSD_SOURCE ,
 .BR _SVID_SOURCE ,
 .BR _POSIX_SOURCE ,
 and
 .BR _POSIX_C_SOURCE =200112L
-(199506L in glibc versions before 2.5).
+(199506L in glibc versions before 2.4).
+.\" 199309L in glibc versions before 2.1.
 .PP
 If any of
 .BR __STRICT_ANSI__ ,
@@ -354,7 +364,7 @@ if
 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),
+200112L (199506L in glibc versions before 2.4),
 if 
 .BR XOPEN_SOURCE
 is undefined, or
@@ -378,13 +388,13 @@ but is employed on some other implementations.
 
 .BR _BSD_SOURCE ,
 .BR _SVID_SOURCE ,
+.BR _ATFILE_SOURCE ,
 .BR _GNU_SOURCE ,
 .BR _FORTIFY_SOURCE ,
 .BR _REENTRANT ,
 and
 .BR _THREAD_SAFE
 are Linux (glibc) specific.
-.\" FIXME ? add _ATFILE_SOURCE to this list
 .SH NOTES
 .I <features.h>
 is a Linux/glibc specific header file.