Cirrus Logic CS485 Manuel d'utilisateur télécharger pdf (Page 32)

CS485G Spring 2015 32

34 Unions

1. Snow day: 3/6/2015

2. Lecture 18, 3/9/2015

3. A union type is like a struct, but the ﬁelds all start at offset 0, so

they overlap.

4. This method allows us to view the same data as different types.

5. It also lets us look at individual bytes of numeric types to see the

byte ordering.

6. Example:

1 union {

2 char c[8]; // 8

1 = 8 bytes

3 short s[4]; // 4

2 = 8 bytes

4 int i[2]; // 2

4 = 8 bytes

5 long l[1]; // 1

8 = 8 bytes (on i86_64)

6 float f[2]; // 2

4 = 8 bytes

7 } dw;

8 dw.f[0] = 3.1415;

9 printf("as float: %f; as integer: %d;\n"

10 "as two shorts: %d, %d\n",

11 dw.f[0], dw.i[0], dw.s[0], dw.s[1]);

Result:

as float: 3.141500; as integer: 1078529622;

as two shorts: 3670, 16457

35 Linux x86 memory layout

1. Simpliﬁed version of allocation of 4GB virtual space

start name purpose properties

0 unused prevent errors no access

0x08000000 text program read-only

data initialized data read, write; static size

bss uninitialized data read, write; static size

heap allocatable data read, write; grows up

stack activation records read, write; grows down

0xC0000000 kernel kernel code, shared no access

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Cirrus Logic CS485 Manuel d'utilisateur Page 32