lib/: Use the comma operator to perform lvalue conversion

[alejandro-colomar]

Compound literals are lvalues, and thus somewhat dangerous. Their address can be taken, and they can be assigned to.

We were using statement expressions to perform lvalue conversion on compound literals, transforming them to rvalues, and thus removing their dangers. However, statement expressions are non-standard, and quite complex within the compiler, so it would be interesting to use simpler compiler features to achieve the same.

The comma operator also performs lvalue conversion, and we can use a dummy (void)0 expression to introduce it. This is significantly simpler, and is more portable than the statement expression (it is valid all the way back to ANSI C89).

By using a simpler feature, we have a smaller risk of running into a compiler bug.

Suggested-by: @uecker
Cc: @chrisbazley
Cc: @kees
Cc: @flatcap

---

Revisions:

v1b

• Rebase

$ git rd 
1:  9558736a5b42 = 1:  5732c14f453e lib/: Use the comma operator to perform lvalue conversion

v1c

• Rebase

$ git rd 
1:  5732c14f = 1:  8956e27a lib/: Use the comma operator to perform lvalue conversion

v1d

• Rebase

$ git rd 
1:  8956e27a = 1:  5a03c618 lib/: Use the comma operator to perform lvalue conversion

v1e

• Clarify portability.

$ git rd
1:  5a03c618 ! 1:  b7ab3f97 lib/: Use the comma operator to perform lvalue conversion
    @@ Commit message
     
         The comma operator also performs lvalue conversion, and we can use
         a dummy (void)0 expression to introduce it.  This is significantly
    -    simpler, and is more portable than the statement expression (it is valid
    -    all the way back to ANSI C89).
    +    simpler, and is more portable than the statement expression: it is valid
    +    all the way back to C99 (the comma operator and the (void)0 expression
    +    are portable to C89, but the compound literal is from C99).
     
         By using a simpler feature, we have a smaller risk of running into
         a compiler bug.

v1f

• Rebase

$ git rd 
1:  b7ab3f97a0c8 = 1:  617ded6a961c lib/: Use the comma operator to perform lvalue conversion

v2

• Add rvalue() and ptr_cast(), to encapsulate these operations.

$ git rd 
1:  617ded6a961c = 1:  617ded6a961c lib/: Use the comma operator to perform lvalue conversion
-:  ------------ > 2:  b2f581e3a2de lib/cast.h: rvalue(): Add macro for performing lvalue conversion
-:  ------------ > 3:  ad5d5b419079 lib/: Use rvalue() instead of its pattern
-:  ------------ > 4:  f2878fbaf173 lib/cast.h: ptr_cast(): Add macro
-:  ------------ > 5:  cbd1a16c7541 lib/: Use ptr_cast() instead of its pattern

v2b

• Rebase

$ git rd -U0
1:  617ded6a961c ! 1:  512aba26e5a2 lib/: Use the comma operator to perform lvalue conversion
    @@ lib/sizeof.h
    -
    - ## lib/string/strerrno.h ##
    -@@
    - 
    - 
    - // strerrno - string errno
    --#define strerrno()  ({(const char *){strerror(errno)};})
    -+#define strerrno()  ((void)0, (const char *){strerror(errno)})
    - 
    - 
    - #endif  // include guard
2:  b2f581e3a2de = 2:  5bc74aa0449d lib/cast.h: rvalue(): Add macro for performing lvalue conversion
3:  ad5d5b419079 ! 3:  ed9069c3cde6 lib/: Use rvalue() instead of its pattern
    @@ lib/sizeof.h
    -
    - ## lib/string/strerrno.h ##
    -@@
    - 
    - 
    - // strerrno - string errno
    --#define strerrno()  ((void)0, (const char *){strerror(errno)})
    -+#define strerrno()  rvalue((const char *){strerror(errno)})
    - 
    - 
    - #endif  // include guard
4:  f2878fbaf173 = 4:  f787e34499fa lib/cast.h: ptr_cast(): Add macro
5:  cbd1a16c7541 ! 5:  04855d90a948 lib/: Use ptr_cast() instead of its pattern
    @@ lib/search/l/lfind.h
    -
    - ## lib/string/strerrno.h ##
    -@@
    - 
    - 
    - // strerrno - string errno
    --#define strerrno()  rvalue((const char *){strerror(errno)})
    -+#define strerrno()  ptr_cast(const char, strerror(errno))
    - 
    - 

v2c

• Rebase

$ git rd 
1:  512aba26e5a2 = 1:  473f4bfe7f23 lib/: Use the comma operator to perform lvalue conversion
2:  5bc74aa0449d = 2:  1d1201430ee1 lib/cast.h: rvalue(): Add macro for performing lvalue conversion
3:  ed9069c3cde6 = 3:  ffd6f93a1b1d lib/: Use rvalue() instead of its pattern
4:  f787e34499fa = 4:  7a5c6c94ff63 lib/cast.h: ptr_cast(): Add macro
5:  04855d90a948 = 5:  0fec8a9bd8c2 lib/: Use ptr_cast() instead of its pattern

v2d

• Update includes.

$ git rd -U0
1:  473f4bfe7f23 = 1:  473f4bfe7f23 lib/: Use the comma operator to perform lvalue conversion
2:  1d1201430ee1 = 2:  1d1201430ee1 lib/cast.h: rvalue(): Add macro for performing lvalue conversion
3:  ffd6f93a1b1d ! 3:  f9c915931851 lib/: Use rvalue() instead of its pattern
    @@ lib/alloc/calloc.h
    + #include <stdlib.h>
    + 
    ++#include "cast.h"
    + #include "exit_if_null.h"
    + #include "sizeof.h"
    + 
    + 
    @@ lib/alloc/malloc.h
    + #include <stdlib.h>
    + 
    + #include "attr.h"
    ++#include "cast.h"
    + #include "exit_if_null.h"
    + #include "sizeof.h"
    + 
    @@ lib/sizeof.h
    + #endif
    + #include <sys/types.h>
    + 
    ++#include "cast.h"
    ++
4:  7a5c6c94ff63 = 4:  611f249602be lib/cast.h: ptr_cast(): Add macro
5:  0fec8a9bd8c2 ! 5:  81f34a70a7f9 lib/: Use ptr_cast() instead of its pattern
    @@ lib/alloc/calloc.h
    + #include "cast.h"
    + #include "exit_if_null.h"
    +-#include "sizeof.h"
    + 
    @@ lib/alloc/malloc.h
    + #include <stdlib.h>
    + 
    + #include "attr.h"
    +-#include "cast.h"
    + #include "exit_if_null.h"
    + #include "sizeof.h"