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@c GNU noreturn, stdnoreturn modules documentation
@c Copyright (C) 2019--2023 Free Software Foundation, Inc.
@c Permission is granted to copy, distribute and/or modify this document
@c under the terms of the GNU Free Documentation License, Version 1.3 or
@c any later version published by the Free Software Foundation; with no
@c Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
@c copy of the license is at <https://www.gnu.org/licenses/fdl-1.3.en.html>.
@node Non-returning Functions
@section Non-returning Functions
@cindex @code{_Noreturn}
@cindex @code{noreturn}
@cindex @code{stdnoreturn}
A "non-returning" function is a function which cannot return normally.
Instead of returning, it can loop forever, or it can transfer control via
@code{abort}, @code{execvp}, @code{exit}, @code{longjmp}, @code{throw}
(in C++), or similar mechanisms. Non-returning functions are
declared with a @code{void} return type.
It helps the compiler's ability to emit sensible warnings, following
data-flow analysis, to declare which functions are non-returning.
It can also help generate more-efficient code, as there is no need
to save a return address when calling a non-returning function.
Gnulib has multiple ways to support such a declaration:
@itemize @bullet
@item
The @code{_Noreturn} keyword. No modules are needed, as Gnulib
arranges for @code{<config.h>} to define @code{_Noreturn} to an
appropriate replacement on platforms lacking it.
Unfortunately, although this approach works for all current C versions,
the @code{_Noreturn} keyword is obsolescent in C23.
@item
The @samp{noreturn} module. It provides a way to put this declaration
at function declarations, at function definitions, and in function
pointer types. The identifiers to use are:
@itemize -
@item
@code{_GL_NORETURN_FUNC} for use in function declarations and function
definitions.
@item
@code{_GL_NORETURN_FUNCPTR} for use on function pointers.
@end itemize
@noindent
The include file is @code{<noreturn.h>}.
@end itemize
Which of the approaches to use? If the non-returning functions you
have to declare are unlikely to be accessed through function pointers,
you should use @code{_Noreturn}; otherwise the module
@code{noreturn} provides for better data-flow analysis and thus for
better warnings.
There is also an obsolete @code{stdnoreturn} module, but its use is no
longer recommended.
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