values. This conflicts with the glibc extension where %as, %a[ and
%aS mean to allocate the string for the data read. A strictly
conforming C99 program using %as, %a[ or %aS in a scanf format string
-will misbehave under glibc.
+will misbehave under glibc if it does not include <stdio.h> and
+instead declares scanf itself; if it gets the declaration of scanf
+from <stdio.h>, it will use a C99-conforming version.
Compiler limitations
====================
The macros __STDC_IEC_559__, __STDC_IEC_559_COMPLEX__ and
-__STDC_ISO_10646__ are properly supposed to be defined by the
-compiler, and to be constant throughout the translation unit (before
-and after any library headers are included). However, they mainly
-relate to library features, and the necessary magic has yet to be
-implemented for GCC to predefine them to the correct values for the
-library in use, so glibc defines them in <features.h>. Programs that
-test them before including any standard headers may misbehave.
+__STDC_ISO_10646__ are properly supposed to be constant throughout the
+translation unit (before and after any library headers are included).
+However, they mainly relate to library features, and GCC only knows to
+preinclude <stdc-predef.h> to get their definitions in version 4.8 and
+later. Programs that test them before including any standard headers
+may misbehave with older compilers.
GCC doesn't support the optional imaginary types. Nor does it
understand the keyword _Complex before GCC 3.0. This has the
corresponding impact on the relevant headers.
-glibc's use of extern inline conflicts with C99: in C99, extern inline
-means that an external definition is generated as well as possibly an
-inline definition, but in GCC it means that no external definition is
-generated. When GCC's C99 mode implements C99 inline semantics, this
-will break the uses of extern inline in glibc's headers. (Actually,
-glibc uses `extern __inline', which is beyond the scope of the
-standard, but it would clearly be very confusing for `__inline' and
-plain `inline' to have different meanings in C99 mode.)
-
glibc's <tgmath.h> implementation is arcane but thought to work
correctly; a clean and comprehensible version requires compiler
builtins.
For most of the headers required of freestanding implementations,
glibc relies on GCC to provide correct versions. (At present, glibc
-provides <stdint.h>, and GCC doesn't.)
-
-Implementing MATH_ERRNO, MATH_ERREXCEPT and math_errhandling in
-<math.h> needs compiler support: see
+provides <stdint.h>, and GCC doesn't before version 4.5.)
-http://sources.redhat.com/ml/libc-hacker/2000-06/msg00008.html
-http://sources.redhat.com/ml/libc-hacker/2000-06/msg00014.html
-http://sources.redhat.com/ml/libc-hacker/2000-06/msg00015.html
+The definition of math_errhandling conforms so long as no translation
+unit using math_errhandling is compiled with -fno-math-errno,
+-fno-trapping-math or options such as -ffast-math that imply these
+options. math_errhandling is only conditionally defined depending on
+__FAST_MATH__; the compiler does not provide the information needed
+for more exact definitions based on settings of -fno-math-errno and
+-fno-trapping-math, possibly for only some source files in a program.
Issues with headers
===================
-There are various technical issues with the definitions contained in
-glibc's headers, listed below. The list below assumes GCC 3.3.2, and
-relates to i686-linux; older GCC may lead to more problems in the
-headers.
-
-Note that the _t suffix is reserved by POSIX, but not by pure ISO C.
-Also, the Single Unix Specification generally requires more types to
-be included in headers (if _XOPEN_SOURCE is defined appropriately)
-than ISO C permits.
+None known.
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