gcc/doc/implement-c.texi

@c Copyright (C) 2001-2021 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node C Implementation
@chapter C Implementation-Defined Behavior
@cindex implementation-defined behavior, C language

A conforming implementation of ISO C is required to document its
choice of behavior in each of the areas that are designated
``implementation defined''.  The following lists all such areas,
along with the section numbers from the ISO/IEC 9899:1990, ISO/IEC
9899:1999 and ISO/IEC 9899:2011 standards.  Some areas are only
implementation-defined in one version of the standard.

Some choices depend on the externally determined ABI for the platform
(including standard character encodings) which GCC follows; these are
listed as ``determined by ABI'' below.  @xref{Compatibility, , Binary
Compatibility}, and @uref{http://gcc.gnu.org/readings.html}.  Some
choices are documented in the preprocessor manual.
@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.  Some choices are made by the
library and operating system (or other environment when compiling for
a freestanding environment); refer to their documentation for details.

@menu
* Translation implementation::
* Environment implementation::
* Identifiers implementation::
* Characters implementation::
* Integers implementation::
* Floating point implementation::
* Arrays and pointers implementation::
* Hints implementation::
* Structures unions enumerations and bit-fields implementation::
* Qualifiers implementation::
* Declarators implementation::
* Statements implementation::
* Preprocessing directives implementation::
* Library functions implementation::
* Architecture implementation::
* Locale-specific behavior implementation::
@end menu

@node Translation implementation
@section Translation

@itemize @bullet
@item
@cite{How a diagnostic is identified (C90 3.7, C99 and C11 3.10, C90,
C99 and C11 5.1.1.3).}

Diagnostics consist of all the output sent to stderr by GCC@.

@item
@cite{Whether each nonempty sequence of white-space characters other than
new-line is retained or replaced by one space character in translation
phase 3 (C90, C99 and C11 5.1.1.2).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@end itemize

@node Environment implementation
@section Environment

The behavior of most of these points are dependent on the implementation
of the C library, and are not defined by GCC itself.

@itemize @bullet
@item
@cite{The mapping between physical source file multibyte characters
and the source character set in translation phase 1 (C90, C99 and C11
5.1.1.2).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@end itemize

@node Identifiers implementation
@section Identifiers

@itemize @bullet
@item
@cite{Which additional multibyte characters may appear in identifiers
and their correspondence to universal character names (C99 and C11 6.4.2).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The number of significant initial characters in an identifier
(C90 6.1.2, C90, C99 and C11 5.2.4.1, C99 and C11 6.4.2).}

For internal names, all characters are significant.  For external names,
the number of significant characters are defined by the linker; for
almost all targets, all characters are significant.

@item
@cite{Whether case distinctions are significant in an identifier with
external linkage (C90 6.1.2).}

This is a property of the linker.  C99 and C11 require that case distinctions
are always significant in identifiers with external linkage and
systems without this property are not supported by GCC@.

@end itemize

@node Characters implementation
@section Characters

@itemize @bullet
@item
@cite{The number of bits in a byte (C90 3.4, C99 and C11 3.6).}

Determined by ABI@.

@item
@cite{The values of the members of the execution character set (C90,
C99 and C11 5.2.1).}

Determined by ABI@.

@item
@cite{The unique value of the member of the execution character set produced
for each of the standard alphabetic escape sequences (C90, C99 and C11
5.2.2).}

Determined by ABI@.

@item
@cite{The value of a @code{char} object into which has been stored any
character other than a member of the basic execution character set
(C90 6.1.2.5, C99 and C11 6.2.5).}

Determined by ABI@.

@item
@cite{Which of @code{signed char} or @code{unsigned char} has the same
range, representation, and behavior as ``plain'' @code{char} (C90
6.1.2.5, C90 6.2.1.1, C99 and C11 6.2.5, C99 and C11 6.3.1.1).}

@opindex fsigned-char
@opindex funsigned-char
Determined by ABI@.  The options @option{-funsigned-char} and
@option{-fsigned-char} change the default.  @xref{C Dialect Options, ,
Options Controlling C Dialect}.

@item
@cite{The mapping of members of the source character set (in character
constants and string literals) to members of the execution character
set (C90 6.1.3.4, C99 and C11 6.4.4.4, C90, C99 and C11 5.1.1.2).}

Determined by ABI@.

@item
@cite{The value of an integer character constant containing more than one
character or containing a character or escape sequence that does not map
to a single-byte execution character (C90 6.1.3.4, C99 and C11 6.4.4.4).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The value of a wide character constant containing more than one
multibyte character or a single multibyte character that maps to
multiple members of the extended execution character set, or
containing a multibyte character or escape sequence not represented in
the extended execution character set (C90 6.1.3.4, C99 and C11
6.4.4.4).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The current locale used to convert a wide character constant consisting
of a single multibyte character that maps to a member of the extended
execution character set into a corresponding wide character code (C90
6.1.3.4, C99 and C11 6.4.4.4).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{Whether differently-prefixed wide string literal tokens can be
concatenated and, if so, the treatment of the resulting multibyte
character sequence (C11 6.4.5).}

Such tokens may not be concatenated.

@item
@cite{The current locale used to convert a wide string literal into
corresponding wide character codes (C90 6.1.4, C99 and C11 6.4.5).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The value of a string literal containing a multibyte character or escape
sequence not represented in the execution character set (C90 6.1.4,
C99 and C11 6.4.5).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.

@item
@cite{The encoding of any of @code{wchar_t}, @code{char16_t}, and
@code{char32_t} where the corresponding standard encoding macro
(@code{__STDC_ISO_10646__}, @code{__STDC_UTF_16__}, or
@code{__STDC_UTF_32__}) is not defined (C11 6.10.8.2).}

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}.  @code{char16_t} and
@code{char32_t} literals are always encoded in UTF-16 and UTF-32
respectively.

@end itemize

@node Integers implementation
@section Integers

@itemize @bullet
@item
@cite{Any extended integer types that exist in the implementation (C99
and C11 6.2.5).}

GCC does not support any extended integer types.
@c The __mode__ attribute might create types of precisions not
@c otherwise supported, but the syntax isn't right for use everywhere
@c the standard type names might be used.  Predefined typedefs should
@c be used if any extended integer types are to be defined.  The
@c __int128_t and __uint128_t typedefs are not extended integer types
@c as they are generally longer than the ABI-specified intmax_t.

@item
@cite{Whether signed integer types are represented using sign and magnitude,
two's complement, or one's complement, and whether the extraordinary value
is a trap representation or an ordinary value (C99 and C11 6.2.6.2).}

GCC supports only two's complement integer types, and all bit patterns
are ordinary values.

@item
@cite{The rank of any extended integer type relative to another extended
integer type with the same precision (C99 and C11 6.3.1.1).}

GCC does not support any extended integer types.
@c If it did, there would only be one of each precision and signedness.

@item
@cite{The result of, or the signal raised by, converting an integer to a
signed integer type when the value cannot be represented in an object of
that type (C90 6.2.1.2, C99 and C11 6.3.1.3).}

For conversion to a type of width @math{N}, the value is reduced
modulo @math{2^N} to be within range of the type; no signal is raised.

@item
@cite{The results of some bitwise operations on signed integers (C90
6.3, C99 and C11 6.5).}

Bitwise operators act on the representation of the value including
both the sign and value bits, where the sign bit is considered
immediately above the highest-value value bit.  Signed @samp{>>} acts
on negative numbers by sign extension.

As an extension to the C language, GCC does not use the latitude given in
C99 and C11 only to treat certain aspects of signed @samp{<<} as undefined.
However, @option{-fsanitize=shift} (and @option{-fsanitize=undefined}) will
diagnose such cases.  They are also diagnosed where constant
expressions are required.

@item
@cite{The sign of the remainder on integer division (C90 6.3.5).}

GCC always follows the C99 and C11 requirement that the result of division is
truncated towards zero.

@end itemize

@node Floating point implementation
@section Floating Point

@itemize @bullet
@item
@cite{The accuracy of the floating-point operations and of the library
functions in @code{<math.h>} and @code{<complex.h>} that return floating-point
results (C90, C99 and C11 5.2.4.2.2).}

The accuracy is unknown.

@item
@cite{The rounding behaviors characterized by non-standard values
of @code{FLT_ROUNDS} @gol
(C90, C99 and C11 5.2.4.2.2).}

GCC does not use such values.

@item
@cite{The evaluation methods characterized by non-standard negative
values of @code{FLT_EVAL_METHOD} (C99 and C11 5.2.4.2.2).}

GCC does not use such values.

@item
@cite{The direction of rounding when an integer is converted to a
floating-point number that cannot exactly represent the original
value (C90 6.2.1.3, C99 and C11 6.3.1.4).}

C99 Annex F is followed.

@item
@cite{The direction of rounding when a floating-point number is
converted to a narrower floating-point number (C90 6.2.1.4, C99 and C11
6.3.1.5).}

C99 Annex F is followed.

@item
@cite{How the nearest representable value or the larger or smaller
representable value immediately adjacent to the nearest representable
value is chosen for certain floating constants (C90 6.1.3.1, C99 and C11
6.4.4.2).}

C99 Annex F is followed.

@item
@cite{Whether and how floating expressions are contracted when not
disallowed by the @code{FP_CONTRACT} pragma (C99 and C11 6.5).}

Expressions are currently only contracted if @option{-ffp-contract=fast},
@option{-funsafe-math-optimizations} or @option{-ffast-math} are used.
This is subject to change.

@item
@cite{The default state for the @code{FENV_ACCESS} pragma (C99 and C11
7.6.1).}

This pragma is not implemented, but the default is to ``off'' unless
@option{-frounding-math} is used in which case it is ``on''.

@item
@cite{Additional floating-point exceptions, rounding modes, environments,
and classifications, and their macro names (C99 and C11 7.6, C99 and
C11 7.12).}

This is dependent on the implementation of the C library, and is not
defined by GCC itself.

@item
@cite{The default state for the @code{FP_CONTRACT} pragma (C99 and C11
7.12.2).}

This pragma is not implemented.  Expressions are currently only
contracted if @option{-ffp-contract=fast},
@option{-funsafe-math-optimizations} or @option{-ffast-math} are used.
This is subject to change.

@item
@cite{Whether the ``inexact'' floating-point exception can be raised
when the rounded result actually does equal the mathematical result
in an IEC 60559 conformant implementation (C99 F.9).}

This is dependent on the implementation of the C library, and is not
defined by GCC itself.

@item
@cite{Whether the ``underflow'' (and ``inexact'') floating-point
exception can be raised when a result is tiny but not inexact in an
IEC 60559 conformant implementation (C99 F.9).}

This is dependent on the implementation of the C library, and is not
defined by GCC itself.

@end itemize

@node Arrays and pointers implementation
@section Arrays and Pointers

@itemize @bullet
@item
@cite{The result of converting a pointer to an integer or
vice versa (C90 6.3.4, C99 and C11 6.3.2.3).}

A cast from pointer to integer discards most-significant bits if the
pointer representation is larger than the integer type,
sign-extends@footnote{Future versions of GCC may zero-extend, or use
a target-defined @code{ptr_extend} pattern.  Do not rely on sign extension.}
if the pointer representation is smaller than the integer type, otherwise
the bits are unchanged.
@c ??? We've always claimed that pointers were unsigned entities.
@c Shouldn't we therefore be doing zero-extension?  If so, the bug
@c is in convert_to_integer, where we call type_for_size and request
@c a signed integral type.  On the other hand, it might be most useful
@c for the target if we extend according to POINTERS_EXTEND_UNSIGNED.

A cast from integer to pointer discards most-significant bits if the
pointer representation is smaller than the integer type, extends according
to the signedness of the integer type if the pointer representation
is larger than the integer type, otherwise the bits are unchanged.

When casting from pointer to integer and back again, the resulting
pointer must reference the same object as the original pointer, otherwise
the behavior is undefined.  That is, one may not use integer arithmetic to
avoid the undefined behavior of pointer arithmetic as proscribed in
C99 and C11 6.5.6/8.

@item
@cite{The size of the result of subtracting two pointers to elements
of the same array (C90 6.3.6, C99 and C11 6.5.6).}

The value is as specified in the standard and the type is determined
by the ABI@.

@end itemize

@node Hints implementation
@section Hints

@itemize @bullet
@item
@cite{The extent to which suggestions made by using the @code{register}
storage-class specifier are effective (C90 6.5.1, C99 and C11 6.7.1).}

The @code{register} specifier affects code generation only in these ways:

@itemize @bullet
@item
When used as part of the register variable extension, see
@ref{Explicit Register Variables}.

@item
When @option{-O0} is in use, the compiler allocates distinct stack
memory for all variables that do not have the @code{register}
storage-class specifier; if @code{register} is specified, the variable
may have a shorter lifespan than the code would indicate and may never
be placed in memory.

@item
On some rare x86 targets, @code{setjmp} doesn't save the registers in
all circumstances.  In those cases, GCC doesn't allocate any variables
in registers unless they are marked @code{register}.

@end itemize

@item
@cite{The extent to which suggestions made by using the inline function
specifier are effective (C99 and C11 6.7.4).}

GCC will not inline any functions if the @option{-fno-inline} option is
used or if @option{-O0} is used.  Otherwise, GCC may still be unable to
inline a function for many reasons; the @option{-Winline} option may be
used to determine if a function has not been inlined and why not.

@end itemize

@node Structures unions enumerations and bit-fields implementation
@section Structures, Unions, Enumerations, and Bit-Fields

@itemize @bullet
@item
@cite{A member of a union object is accessed using a member of a
different type (C90 6.3.2.3).}

The relevant bytes of the representation of the object are treated as
an object of the type used for the access.  @xref{Type-punning}.  This
may be a trap representation.

@item
@cite{Whether a ``plain'' @code{int} bit-field is treated as a
@code{signed int} bit-field or as an @code{unsigned int} bit-field
(C90 6.5.2, C90 6.5.2.1, C99 and C11 6.7.2, C99 and C11 6.7.2.1).}

@opindex funsigned-bitfields
By default it is treated as @code{signed int} but this may be changed
by the @option{-funsigned-bitfields} option.

@item
@cite{Allowable bit-field types other than @code{_Bool}, @code{signed int},
and @code{unsigned int} (C99 and C11 6.7.2.1).}

Other integer types, such as @code{long int}, and enumerated types are
permitted even in strictly conforming mode.

@item
@cite{Whether atomic types are permitted for bit-fields (C11 6.7.2.1).}

Atomic types are not permitted for bit-fields.

@item
@cite{Whether a bit-field can straddle a storage-unit boundary (C90
6.5.2.1, C99 and C11 6.7.2.1).}

Determined by ABI@.

@item
@cite{The order of allocation of bit-fields within a unit (C90
6.5.2.1, C99 and C11 6.7.2.1).}

Determined by ABI@.

@item
@cite{The alignment of non-bit-field members of structures (C90
6.5.2.1, C99 and C11 6.7.2.1).}

Determined by ABI@.

@item
@cite{The integer type compatible with each enumerated type (C90
6.5.2.2, C99 and C11 6.7.2.2).}

@opindex fshort-enums
Normally, the type is @code{unsigned int} if there are no negative
values in the enumeration, otherwise @code{int}.  If
@option{-fshort-enums} is specified, then if there are negative values
it is the first of @code{signed char}, @code{short} and @code{int}
that can represent all the values, otherwise it is the first of
@code{unsigned char}, @code{unsigned short} and @code{unsigned int}
that can represent all the values.
@c On a few unusual targets with 64-bit int, this doesn't agree with
@c the code and one of the types accessed via mode attributes (which
@c are not currently considered extended integer types) may be used.
@c If these types are made extended integer types, it would still be
@c the case that -fshort-enums stops the implementation from
@c conforming to C90 on those targets.

On some targets, @option{-fshort-enums} is the default; this is
determined by the ABI@.

@end itemize

@node Qualifiers implementation
@section Qualifiers

@itemize @bullet
@item
@cite{What constitutes an access to an object that has volatile-qualified
type (C90 6.5.3, C99 and C11 6.7.3).}

Such an object is normally accessed by pointers and used for accessing
hardware.  In most expressions, it is intuitively obvious what is a read
and what is a write.  For example

@smallexample
volatile int *dst = @var{somevalue};
volatile int *src = @var{someothervalue};
*dst = *src;
@end smallexample

@noindent
will cause a read of the volatile object pointed to by @var{src} and store the
value into the volatile object pointed to by @var{dst}.  There is no
guarantee that these reads and writes are atomic, especially for objects
larger than @code{int}.

However, if the volatile storage is not being modified, and the value of
the volatile storage is not used, then the situation is less obvious.
For example

@smallexample
volatile int *src = @var{somevalue};
*src;
@end smallexample

According to the C standard, such an expression is an rvalue whose type
is the unqualified version of its original type, i.e.@: @code{int}.  Whether
GCC interprets this as a read of the volatile object being pointed to or
only as a request to evaluate the expression for its side effects depends
on this type.

If it is a scalar type, or on most targets an aggregate type whose only
member object is of a scalar type, or a union type whose member objects
are of scalar types, the expression is interpreted by GCC as a read of
the volatile object; in the other cases, the expression is only evaluated
for its side effects.

When an object of an aggregate type, with the same size and alignment as a
scalar type @code{S}, is the subject of a volatile access by an assignment
expression or an atomic function, the access to it is performed as if the
object's declared type were @code{volatile S}.

@end itemize

@node Declarators implementation
@section Declarators

@itemize @bullet
@item
@cite{The maximum number of declarators that may modify an arithmetic,
structure or union type (C90 6.5.4).}

GCC is only limited by available memory.

@end itemize

@node Statements implementation
@section Statements

@itemize @bullet
@item
@cite{The maximum number of @code{case} values in a @code{switch}
statement (C90 6.6.4.2).}

GCC is only limited by available memory.

@end itemize

@node Preprocessing directives implementation
@section Preprocessing Directives

@xref{Implementation-defined behavior, , Implementation-defined
behavior, cpp, The C Preprocessor}, for details of these aspects of
implementation-defined behavior.

@itemize @bullet
@item
@cite{The locations within @code{#pragma} directives where header name
preprocessing tokens are recognized (C11 6.4, C11 6.4.7).}

@item
@cite{How sequences in both forms of header names are mapped to headers
or external source file names (C90 6.1.7, C99 and C11 6.4.7).}

@item
@cite{Whether the value of a character constant in a constant expression
that controls conditional inclusion matches the value of the same character
constant in the execution character set (C90 6.8.1, C99 and C11 6.10.1).}

@item
@cite{Whether the value of a single-character character constant in a
constant expression that controls conditional inclusion may have a
negative value (C90 6.8.1, C99 and C11 6.10.1).}

@item
@cite{The places that are searched for an included @samp{<>} delimited
header, and how the places are specified or the header is
identified (C90 6.8.2, C99 and C11 6.10.2).}

@item
@cite{How the named source file is searched for in an included @samp{""}
delimited header (C90 6.8.2, C99 and C11 6.10.2).}

@item
@cite{The method by which preprocessing tokens (possibly resulting from
macro expansion) in a @code{#include} directive are combined into a header
name (C90 6.8.2, C99 and C11 6.10.2).}

@item
@cite{The nesting limit for @code{#include} processing (C90 6.8.2, C99
and C11 6.10.2).}

@item
@cite{Whether the @samp{#} operator inserts a @samp{\} character before
the @samp{\} character that begins a universal character name in a
character constant or string literal (C99 and C11 6.10.3.2).}

@item
@cite{The behavior on each recognized non-@code{STDC #pragma}
directive (C90 6.8.6, C99 and C11 6.10.6).}

@xref{Pragmas, , Pragmas, cpp, The C Preprocessor}, for details of
pragmas accepted by GCC on all targets.  @xref{Pragmas, , Pragmas
Accepted by GCC}, for details of target-specific pragmas.

@item
@cite{The definitions for @code{__DATE__} and @code{__TIME__} when
respectively, the date and time of translation are not available (C90
6.8.8, C99 6.10.8, C11 6.10.8.1).}

@end itemize

@node Library functions implementation
@section Library Functions

The behavior of most of these points are dependent on the implementation
of the C library, and are not defined by GCC itself.

@itemize @bullet
@item
@cite{The null pointer constant to which the macro @code{NULL} expands
(C90 7.1.6, C99 7.17, C11 7.19).}

In @code{<stddef.h>}, @code{NULL} expands to @code{((void *)0)}.  GCC
does not provide the other headers which define @code{NULL} and some
library implementations may use other definitions in those headers.

@end itemize

@node Architecture implementation
@section Architecture

@itemize @bullet
@item
@cite{The values or expressions assigned to the macros specified in the
headers @code{<float.h>}, @code{<limits.h>}, and @code{<stdint.h>}
(C90, C99 and C11 5.2.4.2, C99 7.18.2, C99 7.18.3, C11 7.20.2, C11 7.20.3).}

Determined by ABI@.

@item
@cite{The result of attempting to indirectly access an object with
automatic or thread storage duration from a thread other than the one
with which it is associated (C11 6.2.4).}

Such accesses are supported, subject to the same requirements for
synchronization for concurrent accesses as for concurrent accesses to
any object.

@item
@cite{The number, order, and encoding of bytes in any object
(when not explicitly specified in this International Standard) (C99
and C11 6.2.6.1).}

Determined by ABI@.

@item
@cite{Whether any extended alignments are supported and the contexts
in which they are supported (C11 6.2.8).}

Extended alignments up to @math{2^{28}} (bytes) are supported for
objects of automatic storage duration.  Alignments supported for
objects of static and thread storage duration are determined by the
ABI.

@item
@cite{Valid alignment values other than those returned by an _Alignof
expression for fundamental types, if any (C11 6.2.8).}

Valid alignments are powers of 2 up to and including @math{2^{28}}.

@item
@cite{The value of the result of the @code{sizeof} and @code{_Alignof}
operators (C90 6.3.3.4, C99 and C11 6.5.3.4).}

Determined by ABI@.

@end itemize

@node Locale-specific behavior implementation
@section Locale-Specific Behavior

The behavior of these points are dependent on the implementation
of the C library, and are not defined by GCC itself.