Using __BYTE_ORDER without endian.h

Using __BYTE_ORDER without endian.h

[Rafał Miłecki]

Hi,

So far we were using uClibc in OpenWrt and my following code was working fine:

#include <byteswap.h>
#include <errno.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#if __BYTE_ORDER == __BIG_ENDIAN
#define cpu_to_le32(x) bswap_32(x)
#define le32_to_cpu(x) bswap_32(x)
#elif __BYTE_ORDER == __LITTLE_ENDIAN
#define cpu_to_le32(x) (x)
#define le32_to_cpu(x) (x)
#else
#error "Unsupported endianness"
#endif

Unfortunately is is *silently* broken with musl. When using musl and
not including endian.h directly the condition:
#if __BYTE_ORDER == __BIG_ENDIAN
is always true.

Could you fix this?

I expect either:
1) __BYTE_ORDER to be always available & set correctly
2) Compile error because of __BYTE_ORDER being undefined

I think such silent miscompilation is sth that has to be avoided.

-- 
Rafał

Re: Using __BYTE_ORDER without endian.h

[Jens Gustedt]

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Am Montag, den 22.06.2015, 08:57 +0200 schrieb Rafał Miłecki:
> Hi,
> 
> So far we were using uClibc in OpenWrt and my following code was working fine:
> 
> #include <byteswap.h>
> #include <errno.h>
> #include <stdint.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <unistd.h>
> 
> #if __BYTE_ORDER == __BIG_ENDIAN
> #define cpu_to_le32(x) bswap_32(x)
> #define le32_to_cpu(x) bswap_32(x)
> #elif __BYTE_ORDER == __LITTLE_ENDIAN
> #define cpu_to_le32(x) (x)
> #define le32_to_cpu(x) (x)
> #else
> #error "Unsupported endianness"
> #endif
> 
> Unfortunately is is *silently* broken with musl. When using musl and
> not including endian.h directly the condition:
> #if __BYTE_ORDER == __BIG_ENDIAN
> is always true.

This is correct behavior. Identifiers that are unknown to the
preprocessor always evaluate to 0 and 0 == 0 is true.

> Could you fix this?
> 
> I expect either:
> 1) __BYTE_ORDER to be always available & set correctly
> 2) Compile error because of __BYTE_ORDER being undefined
> 
> I think such silent miscompilation is sth that has to be avoided.

You should fix your erroneous code by prefixing with something like

#if !defined(__BYTE_ORDER) && !defined(__BIG_ENDIAN)

Jens



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