More refinements

man7/feature_test_macros.7

@@ -52,7 +52,7 @@ When a function requires that a feature test macro is defined,
 the manual page SYNOPSIS includes a note of the following form
 (this example from the
 .BR chmod (2)
-man page):
+manual page):
 .RS
 .sp
 .B #include <sys/stat.h>
@@ -96,8 +96,25 @@ cc -D_XOPEN_SOURCE=500        # Or any value > 500
 .PP
 Note that, as described below, some feature test macros are
 defined be default, so that it may not always be necessary to
-explicitly specify the feature test macro(s) shown
-in the SYNOPSIS.
+explicitly specify the feature test macro(s) shown in the
+SYNOPSIS.
+
+In a few cases, manual pages use a shorthand for expressing the
+feature test macro requirements (this example from
+.BR readahead (2)):
+.RS
+.nf
+
+.B #define _GNU_SOURCE
+.B #include <fcntl.h>
+.sp
+.BI "ssize_t readahead(int " fd ", off64_t *" offset ", size_t " count );
+.fi
+.RE
+.PP
+This syntax is used 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
@@ -273,10 +290,9 @@ 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
 .PP
-When
-.BR gcc (1)
-is invoked, the following macros are defined by default:
+The following macros are defined by default:
 .BR _BSD_SOURCE ,
 .BR _SVID_SOURCE ,
 .BR _POSIX_SOURCE ,