c++ - Are POD types always aligned? -

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for example, if declare long variable, can assume aligned on "sizeof(long)" boundary? microsoft visual c++ online says so, standard behavior?

some more info:

a. possible explicitely create misaligned integer (*bar):

char foo[5]

int * bar = (int *)(&foo[1]);

b. apparently, #pragma pack() affects structures, classes, , unions.

c. msvc documentation states pod types aligned respective sizes (but or default, , standard behavior, don't know)

as others have mentioned, isn't part of standard , left compiler implement sees fit processor in question. example, vc implement different alignment requirements arm processor x86 processors.

microsoft vc implements called natural alignment size specified #pragma pack directive or /zp command line option. means that, example, pod type size smaller or equal 8 bytes aligned based on size. larger aligned on 8 byte boundary.

if important control alignment different processors , different compilers, can use packing size of 1 , pad structures.

#pragma pack(push) #pragma pack(1)     struct example {    short data1;     // offset 0    short padding1;  // offset 2    long data2;      // offset 4 }; #pragma pack(pop)

in code, padding1 variable exists make sure data2 naturally aligned.

answer a:

yes, can cause misaligned data. on x86 processor, doesn't hurt @ all. on other processors, can result in crash or slow execution. example, alpha processor throw processor exception caught os. os inspect instruction , work needed handle misaligned data. execution continues. __unaligned keyword can used in vc mark unaligned access non-x86 programs (i.e. ce).


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