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In my embedded development, I have trained myself to use unions instead of type-punned-pointer casts to access one data type as another. This document says thi

In my embedded development, I have trained myself to use unions instead of type-punned-pointer casts to access one data type as another. This document says this is a GCC-specific extension. Is this true? Or is it one of those things that's not standardized, but the compiler vendors all do it anyway?

cremno 12y ago
It's legal C99. See DR258 / C99+TC3. But be careful, reading trap representations is UB. http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_283.htm http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_283.htm http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
effdee 12y ago
Type-punning via unions is allowed in both C99 [0] and C11. [0] see "ISO/IEC 9899:1999 Technical Corrigendum 3", http://www.open-std.org/Jtc1/sc22/wg14/www/docs/n1235.pdf http://www.open-std.org/Jtc1/sc22/wg14/www/docs/n1235.pdf
oshepherd 12y ago
Type punning by pointer is explicitly UB per the standard. Type punning by union is implementation defined behavior. Everyone fortunately defines it to Do The Right Thing (TM). The truly standard supported way to type pun is by memcpy float f = ...; int x; memcpy(&x, &f, sizeof x);
- jakobegger 12y ago
  Yes. Use memcpy if you need to do this! This is especially important when you're on a platform that requires aligned pointers. For example, the following code will crash on ARM char bytes[5] = {0}; float flt = *(float*)&(bytes[1]); Using memcpy works on any platform with 4 byte floats: char bytes[5] = {0}; float flt; memcpy(&flt, &(bytes[1]), 4);
  - markrages 12y ago
    But the union won't crash either (the compiler will keep it aligned), the conversion compiles to no-op, and doesn't waste memory. Embedded targets are always memory-constrained. If they aren't, you are wasting money on hardware.
    - jakobegger 12y ago
      I was thinking about the case when you get unaligned data (eg you are reading from a file or from the network). Then you need to copy the data anyway.

cremno 12y ago

It's legal C99. See DR258 / C99+TC3. But be careful, reading trap representations is UB. http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_283.htm http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_283.htm http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

effdee 12y ago

Type-punning via unions is allowed in both C99 [0] and C11. [0] see "ISO/IEC 9899:1999 Technical Corrigendum 3", http://www.open-std.org/Jtc1/sc22/wg14/www/docs/n1235.pdf http://www.open-std.org/Jtc1/sc22/wg14/www/docs/n1235.pdf

oshepherd 12y ago

Type punning by pointer is explicitly UB per the standard. Type punning by union is implementation defined behavior. Everyone fortunately defines it to Do The Right Thing (TM). The truly standard supported way to type pun is by memcpy float f = ...; int x; memcpy(&x, &f, sizeof x);

jakobegger 12y ago
Yes. Use memcpy if you need to do this! This is especially important when you're on a platform that requires aligned pointers. For example, the following code will crash on ARM char bytes[5] = {0}; float flt = *(float*)&(bytes[1]); Using memcpy works on any platform with 4 byte floats: char bytes[5] = {0}; float flt; memcpy(&flt, &(bytes[1]), 4);
- markrages 12y ago
  But the union won't crash either (the compiler will keep it aligned), the conversion compiles to no-op, and doesn't waste memory. Embedded targets are always memory-constrained. If they aren't, you are wasting money on hardware.
  - jakobegger 12y ago
    I was thinking about the case when you get unaligned data (eg you are reading from a file or from the network). Then you need to copy the data anyway.