Struct Address
Objective
- Learn basics of data structures
- Learn how memory may be padded within data structures
Review Basics
Here is the basic use of data structures in C:
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// Declare data structure in C using typedef2
typedef struct {3
int i;4
char c;5
float f;6
} my_struct_t;7
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// Pass data structure as a copy9
void struct_as_param(my_struct_t s) {10
s.i = 0;11
s.c = 'c';12
}13
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// Pass data structure as a pointer15
void struct_as_pointer(my_struct_t *p) {16
p->i = 0;17
p->c = 'c';18
}19
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// Zero out the struct21
void struct_as_pointer(my_struct_t *p) {22
memset(p, 0, sizeof(*p));23
}Padding
- Use the struct below, and try this sample code
- Note that there may be a compiler error in the snippet below that you are expected to resolve on your own
- Struct should ideally be placed before the main() and the
printf()should be placed inside of themain() - You should use your SJ embedded board because the behavior may be different on a different compiler or the board
- Now un-comment the
packedattribute such that the compiler packs the fields together, and print them again.
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typedef struct {2
float f1; // 4 bytes3
char c1; // 1 byte4
float f2;5
char c2;6
} /*__attribute__((packed))*/ my_s;7
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// TODO: Instantiate a struct of type my_s with the name of "s"9
printf("Size : %d bytes\n"10
"floats 0x%p 0x%p\n"11
"chars 0x%p 0x%p\n",12
sizeof(s), &s.f1, &s.f2, &s.c1, &s.c2);Note:
- Important: In your submission (could be comments in your submitted code), provide your summary of the two print-outs. Explain why they are different, and try to draw conclusions based on the behavior.