unions

For example, say we have a union typedef union { unsigned long U32; float f; }U_U32_F; When a variable of this union type is declared, is there a way to set an initial value? U_U32_F u = 0xffffffff; // Does not work...is there a correct syntax for this? Use an initializer list: U_U32_F u = { 0xffffffff }; You can set other members than the first one via U_U32_F u = { .f = 42.0 }; Try U_U32_F u = {0xffffffff}; Note that per-member union initialization doesn't work on pre-C99 compilers, of which there is a depressing number out there. The current Microsoft C compiler doesn't support it, for

Do you have any horror stories to tell? The GCC Manual recently added a warning regarding -fstrict-aliasing and casting a pointer through a union: [...] Taking the address, casting the resulting pointer and dereferencing the result has undefined behavior [emphasis added], even if the cast uses a union type, e.g.: union a_union { int i; double d; }; int f() { double d = 3.0; return ((union a_union *)&d)->i; } Does anyone have an example to illustrate this undefined behavior? Note this question is not about what the C99 standard says, or does not say. It is about the actual functioning of gcc ,

I have learned but don't really get unions. Every C or C++ text I go through introduces them (sometimes in passing), but they tend to give very few practical examples of why or where to use them. When would unions be useful in a modern (or even legacy) case? My only two guess would be programming microprocessors when you have very limited space to work with, or when you're developing an API (or something similar) and you want to force the end user to have only one instance of several objects/types at one time. Are these two guesses even close to right? vz0 Unions are usually used with the

I can do this in c++/g++: struct vec3 { union { struct { float x, y, z; }; float xyz[3]; }; }; Then, vec3 v; assert(&v.xyz[0] == &v.x); assert(&v.xyz[1] == &v.y); assert(&v.xyz[2] == &v.z); will work. How does one do this in c with gcc? I have typedef struct { union { struct { float x, y, z; }; float xyz[3]; }; } Vector3; But I get errors all around, specifically line 5: warning: declaration does not declare anything line 7: warning: declaration does not declare anything user287561 according to http://gcc.gnu.org/onlinedocs/gcc/Unnamed-Fields.html#Unnamed-Fields -fms-extensions will enable the