Bug 47605

Summary: Document that C_Bool might be the wrong constant for C Booleans
Product: gcc Reporter: Tobias Burnus <burnus>
Component: fortranAssignee: Not yet assigned to anyone <unassigned>
Status: RESOLVED FIXED    
Severity: normal Keywords: documentation
Priority: P3    
Version: 4.6.0   
Target Milestone: ---   
Host: Target:
Build: Known to work:
Known to fail: Last reconfirmed: 2014-03-22 00:00:00

Description Tobias Burnus 2011-02-04 07:17:13 UTC 
James pointed out the following at
http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/3c2784c55d402ced

c_bool is typically one byte wide, and c_int 4. That means: Most C programs - except of those few which explicitly use _Bool, use a 4 byte and not a 1 byte Boolean variables.

I think one should put a warning into the manual - either in the C Binding part [1] or in the section about the ISO C Binding constants [2], or in both.

[1] http://gcc.gnu.org/onlinedocs/gfortran/Intrinsic-Types.html
[2]http://gcc.gnu.org/onlinedocs/gfortran/ISO_005fC_005fBINDING.html
Comment 1 Dominique d'Humieres 2014-03-22 19:34:16 UTC 
In [1] I see

> For logical types, please note that the Fortran standard only guarantees
> interoperability between C99's _Bool and Fortran's C_Bool-kind logicals and C99
> defines that true has the value 1 and false the value 0. Using any other integer
> value with GNU Fortran's LOGICAL (with any kind parameter) gives an undefined
> result. (Passing other integer values than 0 and 1 to GCC's _Bool is also
> undefined, unless the integer is explicitly or implicitly casted to _Bool.)

What should be added to that?
Comment 2 Dominique d'Humieres 2015-08-31 05:31:12 UTC 
> > For logical types, please note that the Fortran standard only guarantees
> > interoperability between C99's _Bool and Fortran's C_Bool-kind logicals and C99
> > defines that true has the value 1 and false the value 0. Using any other integer
> > value with GNU Fortran's LOGICAL (with any kind parameter) gives an undefined
> > result. (Passing other integer values than 0 and 1 to GCC's _Bool is also
> > undefined, unless the integer is explicitly or implicitly casted to _Bool.)
>
> What should be added to that?

No answer for over a year. Closing as FIXED.