Getting bool from C to C++ and back

C's and C++'s bool type are different, but, as long as you stick to the same compiler (in your case, gcc), it should be safe, as this is a reasonable common scenario.

In C++, bool has always been a keyword. C didn't have one until C99, where they introduced the keyword _Bool (because people used to typedef or #define bool as int or char in C89 code, so directly adding bool as a keyword would break existing code); there is the header stdbool.h which should, in C, have a typedef or #define from _Bool to bool. Take a look at yours; GCC's implementation looks like this:

/*
 * ISO C Standard:  7.16  Boolean type and values  <stdbool.h>
 */

#ifndef _STDBOOL_H
#define _STDBOOL_H

#ifndef __cplusplus

#define bool        _Bool
#define true        1
#define false        0

#else /* __cplusplus */

/* Supporting <stdbool.h> in C++ is a GCC extension.  */
#define _Bool        bool
#define bool        bool
#define false        false
#define true        true

#endif /* __cplusplus */

/* Signal that all the definitions are present.  */
#define __bool_true_false_are_defined        1

#endif        /* stdbool.h */

Which leads us to believe that, at least in GCC, the two types are compatible (in both size and alignment, so that the struct layout will remain the same).

Also worth noting, the Itanium ABI, which is used by GCC and most other compilers (except Visual Studio; as noted by Matthieu M. in the comments below) on many platforms, specifies that _Bool and bool follow the same rules. This is a strong garantee. A third hint we can get is from Objective-C's reference manual, which says that for Objective-C and Objective-C++, which respect C's and C++'s conventions respectively, bool and _Bool are equivalent; so I'd pretty much say that, though the standards do not guarantee this, you can assume that yes, they are equivalent.

Edit:

If the standard does not guarantee that _Bool and bool will be compatible (in size, alignment, and padding), what does?

When we say those things are "architecture dependent", we actually mean that they are ABI dependent. Every compiler implements one or more ABIs, and two compilers (or versions of the same compiler) are said to be compatible if they implement the same ABI. Since it is expected to call C code from C++, as this is ubiquitously common, all C++ ABIs I've ever heard of extend the local C ABI.

Since OP asked about Beaglebone, we must check the ARM ABI, most specifically the GNU ARM EABI used by Debian. As noted by Justin Time in the comments, the ARM ABI indeed declares C++'s ABI to extend C's, and that _Bool and bool are compatible, both being of size 1, alignment 1, representing a machine's unsigned byte. So the answer to the question, on the Beaglebone, yes, _Bool and bool are compatible.

The language standards say nothing about this (I'm happy to be proven wrong about this, I couldn't find anything), so it can't be safe if we just limit ourselves to language standards. But if you're picky about which architectures you support you can find their ABI documentation to see if it will be safe.

For example, the amd64 ABI document has a footnote for the _Bool type that says:

This type is called bool in C++.

Which I can't interpret in any other way than that it will be compatible.

Also, just musing about this. Of course it will work. Compilers generate code that both follow an ABI and the behavior of the largest compiler for the platform (if that behavior is outside the ABI). A big thing about C++ is that it can link to libraries written in C and a thing about libraries is that they can be compiled by any compiler on the same platform (this is why we have ABI documents in the first place). Can there be some minor incompatibility at some point? Sure, but that's something you'd better solve by a bug report to the compiler maker rather than workaround in your code. I doubt bool would be something compiler makers would screw up.