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Memory Model CS 113: Introduction to C Instructor: Saikat Guha Cornell University Spring 2007, Lecture 4 Memory Model CS 113, Spring 2007, Lecture 4
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Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

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Page 1: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

Memory ModelCS 113: Introduction to C

Instructor: Saikat Guha

Cornell University

Spring 2007, Lecture 4

Memory Model CS 113, Spring 2007, Lecture 4

Page 2: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

Memory

◮ Program code◮ Function variables

◮ Arguments◮ Local variables◮ Return location

◮ Global Variables◮ Statically Allocated◮ Dynamically

Allocated

addr=0xFFFFFFFF

addr=0x00000000

Code

Global Static

GlobalDynamic(Heap)

fact(4)

fact(5)

main()

FunctionStack Frame

return addr

Local Var 1

Local Var 2

Local Var 3

Local Var 4

Arg 3 next fn

Arg 2 next fn

Arg 1 next fn

Memory Model CS 113, Spring 2007, Lecture 4

Page 3: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

Stores

◮ Function local variables

◮ Temporary variables

◮ Arguments for next function call

◮ Where to return when function ends

Memory Model CS 113, Spring 2007, Lecture 4

Page 4: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

Managed by compiler

◮ One stack frame each time function called

◮ Created when function called

◮ Stacked on top (under) one another

◮ Destroyed at function exit

Memory Model CS 113, Spring 2007, Lecture 4

Page 5: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()

Memory Model CS 113, Spring 2007, Lecture 4

Page 6: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

Memory Model CS 113, Spring 2007, Lecture 4

Page 7: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5)

Memory Model CS 113, Spring 2007, Lecture 4

Page 8: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1

Memory Model CS 113, Spring 2007, Lecture 4

Page 9: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

Memory Model CS 113, Spring 2007, Lecture 4

Page 10: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

fact(4)

Memory Model CS 113, Spring 2007, Lecture 4

Page 11: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1 arg

3

Memory Model CS 113, Spring 2007, Lecture 4

Page 12: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1 arg

3

fact(3) 0

n==1 arg

2

Memory Model CS 113, Spring 2007, Lecture 4

Page 13: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1 arg

3

fact(3) 0

n==1 arg

2

fact(2) 0

n==1 arg

1

Memory Model CS 113, Spring 2007, Lecture 4

Page 14: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1 arg

3

fact(3) 0

n==1 arg

2

fact(2) 0

n==1 arg

1

fact(1) 1

n==1

Memory Model CS 113, Spring 2007, Lecture 4

Page 15: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1 arg

3

fact(3) 0

n==1 arg

2

fact(2) 0

n==1

1

res

Memory Model CS 113, Spring 2007, Lecture 4

Page 16: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1 arg

3

fact(3) 0

n==1

2

res

Memory Model CS 113, Spring 2007, Lecture 4

Page 17: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1

6

res

Memory Model CS 113, Spring 2007, Lecture 4

Page 18: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()arg

5

fact(5) 0

n==1

24

res

Memory Model CS 113, Spring 2007, Lecture 4

Page 19: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Stack

int fact(int n) {int res;

if (n == 1)

return 1;

res = fact(n-1);

return n * res;

}

int main() {

int res = fact(5);

return 0;

}

main()120

res

Memory Model CS 113, Spring 2007, Lecture 4

Page 20: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

Stack games

◮ Locate the stack

◮ Find the direction ofstack growth

◮ Finding size of stackframe

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1 arg

3

fact(3) 0

n==1

2

res

Memory Model CS 113, Spring 2007, Lecture 4

Page 21: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

What can go wrong

◮ Run out of stackspace

◮ Unintentionallychange values on thestack

◮ In some otherfunction’s frame

◮ Even return addressfrom function

◮ Access memory evenafter frame isdeallocated

main()arg

5

fact(5) 0

n==1 arg

4

fact(4) 0

n==1 arg

3

fact(3) 0

n==1

2

res

Memory Model CS 113, Spring 2007, Lecture 4

Page 22: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

Memory Recap

◮ Program code◮ Function variables

◮ Arguments◮ Local variables◮ Return location

◮ Global Variables◮ Statically Allocated◮ Dynamically

Allocated

addr=0xFFFFFFFF

addr=0x00000000

Code

Global Static

GlobalDynamic(Heap)

fact(4)

fact(5)

main()

FunctionStack Frame

return addr

Local Var 1

Local Var 2

Local Var 3

Local Var 4

Arg 3 next fn

Arg 2 next fn

Arg 1 next fn

Memory Model CS 113, Spring 2007, Lecture 4

Page 23: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

Heap

Heap

Needed for long-term storage that needs to persistacross multiple function calls.

Managed by programmer

◮ Created by ptr = malloc(size)

◮ Destroyed by free(ptr)

MUST check the return value from malloc

MUST explicitly free memory when no longer in use

Memory Model CS 113, Spring 2007, Lecture 4

Page 24: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Heap

int main() {int *p, *q, *r;

p = (int *)malloc(sizeof(int));

q = (int *)malloc(

sizeof(int) * 10);

r = (int *)malloc(sizeof(int));

if (p == NULL || !q || !r) {... do cleanup ...

return 1;

}

free(p);

... do other stuff ...

Memory Model CS 113, Spring 2007, Lecture 4

Page 25: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Heap

int main() {int *p, *q, *r;

p = (int *)malloc(sizeof(int));

q = (int *)malloc(

sizeof(int) * 10);

r = (int *)malloc(sizeof(int));

if (p == NULL || !q || !r) {... do cleanup ...

return 1;

}

free(p);

... do other stuff ...

4 bytesMetadata

Memory Model CS 113, Spring 2007, Lecture 4

Page 26: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Heap

int main() {int *p, *q, *r;

p = (int *)malloc(sizeof(int));

q = (int *)malloc(

sizeof(int) * 10);

r = (int *)malloc(sizeof(int));

if (p == NULL || !q || !r) {... do cleanup ...

return 1;

}

free(p);

... do other stuff ...

4 bytesMetadata

40 bytes

Metadata

Memory Model CS 113, Spring 2007, Lecture 4

Page 27: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Heap

int main() {int *p, *q, *r;

p = (int *)malloc(sizeof(int));

q = (int *)malloc(

sizeof(int) * 10);

r = (int *)malloc(sizeof(int));

if (p == NULL || !q || !r) {... do cleanup ...

return 1;

}

free(p);

... do other stuff ...

4 bytesMetadata

40 bytes

Metadata

4 bytesMetadata

Memory Model CS 113, Spring 2007, Lecture 4

Page 28: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

The Heap

int main() {int *p, *q, *r;

p = (int *)malloc(sizeof(int));

q = (int *)malloc(

sizeof(int) * 10);

r = (int *)malloc(sizeof(int));

if (p == NULL || !q || !r) {... do cleanup ...

return 1;

}

free(p);

... do other stuff ...

40 bytes

Metadata

4 bytesMetadata

Memory Model CS 113, Spring 2007, Lecture 4

Page 29: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

Heap games

◮ Locate the heap

◮ How freespace ismanaged

◮ Find how memory isallocated

◮ How isfragmentationavoided

4 bytesMetadata

40 bytes

Metadata

Memory Model CS 113, Spring 2007, Lecture 4

Page 30: Memory Model - CS 113: Introduction to C · Memory Program code Function variables Arguments Local variables Return location Global Variables Statically Allocated Dynamically Allocated

What can go wrong

◮ Run out of heapspace malloc returns 0

◮ Unintentionallychange other heapdata

◮ Or clobber heapmetadata

◮ Access memory evenfree’d

◮ free memory twice

◮ Create a memoryleak

4 bytesMetadata

40 bytes

Metadata

Memory Model CS 113, Spring 2007, Lecture 4


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