Re: byteswap.h behavior

[Bruno Haible <bruno@clisp.org>]

Collin Funk wrote:
> I worry though that in some cases programs will be accustomed to
> glibc's behavior. Since all of the functions are just macros to static
> inline functions in <bits/byteswap.h> arguments will only be evaluated
> once. It seems like it could be the cause of some unexpected bugs...

The simple fix to your worry is: Let the user use '-Wall' routinely.
gcc has a warning option '-Wsequence-point', included in '-Wall'.
clang has a warning option '-Wunsequenced', included in '-Wall'.
The do the job, and I have not seen them produce false warnings ever.

$ gcc -Wall foo.c
foo.c: In function ‘main’:
foo.c:15:59: warning: operation on ‘value_macro’ may be undefined [-Wsequence-point]
   15 |   printf ("2. %#" PRIX32 "\n", bswap_32_macro (value_macro++));
      |                                                           ^
foo.c:7:31: note: in definition of macro ‘bswap_32_macro’
    7 |                            (((x) & 0x00FF0000) >> 8) |  \
      |                               ^
foo.c:15:59: warning: operation on ‘value_macro’ may be undefined [-Wsequence-point]
   15 |   printf ("2. %#" PRIX32 "\n", bswap_32_macro (value_macro++));
      |                                                           ^
foo.c:7:31: note: in definition of macro ‘bswap_32_macro’
    7 |                            (((x) & 0x00FF0000) >> 8) |  \
      |                               ^
foo.c:15:59: warning: operation on ‘value_macro’ may be undefined [-Wsequence-point]
   15 |   printf ("2. %#" PRIX32 "\n", bswap_32_macro (value_macro++));
      |                                                           ^
foo.c:7:31: note: in definition of macro ‘bswap_32_macro’
    7 |                            (((x) & 0x00FF0000) >> 8) |  \
      |                               ^

$ clang -Wall foo.c
foo.c:15:59: warning: multiple unsequenced modifications to 'value_macro' [-Wunsequenced]
   15 |   printf ("2. %#" PRIX32 "\n", bswap_32_macro (value_macro++));
      |                                                           ^~
foo.c:5:31: note: expanded from macro 'bswap_32_macro'
    5 | #define bswap_32_macro(x) ((((x) & 0x000000FF) << 24) | \
      |                               ^
    6 |                            (((x) & 0x0000FF00) << 8) |  \
      |                               ~
1 warning generated.

Assume this warning option is used. Then you can leave byteswap.h as it is.

Bruno