Summary
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 struct |
– structure containing one or multiple fields, each with its own type (or compound type)
– size is combined size of all the fields, padded for byte alignment
– anonymous or named |
| union |
– structure containing one of several fields, each with its own type (or compound type)
– size is size of largest field
– anonymous or named |
| Bit fields |
– structure fields with width in bits
– aligned and ordered in architecture-dependent manner can result in inefficient code |
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| Assuming a 32-bit x86 processor, evaluate sizeof(struct foo). |
struct foo {
short s;
union {
int i;
char c;
} u;
unsigned int flag_s : 1;
unsigned int flag_u : 2;
unsigned int bar;
}; |
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| Q: How can we rearrange the fields to minimize the size of struct foo? |
| A: order from largest to smallest. What is the sizeof(struct foo)? |
struct foo {
union {
int i;
char c;
} u;
unsigned int bar;
short s;
unsigned int flag_s : 1;
unsigned int flag_u : 2;
}; |
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