posix_memalign.3: Improve discussion of feature test macros and header files for valloc(3)

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/posix_memalign.3

@@ -23,7 +23,7 @@
 .\" License.
 .\"
 .\" 2001-10-11, 2003-08-22, aeb, added some details
-.TH POSIX_MEMALIGN 3  2010-09-20 "GNU" "Linux Programmer's Manual"
+.TH POSIX_MEMALIGN 3  2012-03-23 "GNU" "Linux Programmer's Manual"
 .SH NAME
 posix_memalign, memalign, valloc \- Allocate aligned memory
 .SH SYNOPSIS
@@ -31,10 +31,10 @@ posix_memalign, memalign, valloc \- Allocate aligned memory
 .B #include <stdlib.h>
 .sp
 .BI "int posix_memalign(void **" memptr ", size_t " alignment ", size_t " size );
+.BI "void *valloc(size_t " size );
 .sp
 .B #include <malloc.h>
 .sp
-.BI "void *valloc(size_t " size );
 .BI "void *memalign(size_t " boundary ", size_t " size );
 .fi
 .sp
@@ -64,9 +64,15 @@ _BSD_SOURCE ||
 Before glibc 2.12:
 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
+.ad b
+.br
+(The (nonstandard) header file
+.I <malloc.h>
+also exposes the declaration of
+.BR valloc ();
+no feature test macros are required.)
 .RE
 .PD
-.ad b
 .SH DESCRIPTION
 The function
 .BR posix_memalign ()
@@ -164,18 +170,9 @@ is declared in \fI<stdlib.h>\fP instead of \fI<malloc.h>\fP.
 According to SUSv2,
 .BR valloc ()
 is declared in \fI<stdlib.h>\fP.
-Libc4,5 and glibc declare it in \fI<malloc.h>\fP and perhaps also in
+Libc4,5 and glibc declare it in \fI<malloc.h>\fP, and also in
 \fI<stdlib.h>\fP
-(namely, if
-.B _GNU_SOURCE
-is defined, or
-.B _BSD_SOURCE
-is defined, or,
-for glibc, if
-.B _XOPEN_SOURCE_EXTENDED
-is defined, or, equivalently,
-.B _XOPEN_SOURCE
-is defined to a value not less than 500).
+if suitable feature test macros are defined (see above).
 .SH NOTES
 On many systems there are alignment restrictions, for example, on buffers
 used for direct block device I/O.