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C Review Pointers, Arrays, and I/O

C Review Pointers, Arrays, and I/O. CS61c Summer 2006 Michael Le. C Advice. Draw stuff out Variables are boxes, pointers are arrows Give a type your variables! & returns a value whose type has one more star than the type of the variable int quux; int* baz = &quux;

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C Review Pointers, Arrays, and I/O

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  1. C ReviewPointers, Arrays, and I/O CS61c Summer 2006 Michael Le

  2. C Advice • Draw stuff out • Variables are boxes, pointers are arrows • Give a type your variables! • & returns a value whose type has one more star than the type of the variable • int quux; int* baz = &quux; • Execute the fundamental operations one at a time • variable lookup, pointer deference, etc

  3. Tracing Pointers – Warm Up What will y contain? int main(int argc, char** argv) { int y, *z; y = 4; z = &y; y = *z + 9; return 0; }

  4. y z int int* Tracing Pointers – Warm Up What will y contain? int main(int argc, char** argv) { int y, *z; y = 4; z = &y; y = *z + 9; return 0; }

  5. y z int int* 0x4 Tracing Pointers – Warm Up What will y contain? int main(int argc, char** argv) { int y, *z; y = 4; z = &y; y = *z + 9; return 0; }

  6. y z int int* 0x4 Tracing Pointers – Warm Up What will y contain? int main(int argc, char** argv) { int y, *z; y = 4; z = &y; y = *z + 9; return 0; }

  7. y z int int* 0xD Tracing Pointers – Warm Up What will y contain? int main(int argc, char** argv) { int y, *z; y = 4; z = &y; y = *z + 9; return 0; }

  8. y z int int* 0xD Tracing Pointers – Warm Up What will y contain? int main(int argc, char** argv) { int y, *z; y = 4; z = &y; y = *z + 9; return 0; } It contains 0xD. What is that in binary? In decimal?

  9. Tracing Pointers – More Levels What is in foo and bar at the end of this program? int main(int argc, char** argv) { int foo, *bar, **baz, quux; bar = &quux; foo = 4; baz = &bar; **baz = 13; bar = &foo; **baz = 9; return 0; }

  10. foo quux bar baz int int int* int** 410 Tracing Pointers – More Levels What is in foo and quux at the end of this program? int main(int argc, char** argv) { int foo, *bar, **baz, quux; bar = &quux; foo = 4; baz = &bar; **baz = 13; bar = &foo; **baz = 9; return 0; }

  11. foo quux bar baz int int int* int** 410 1310 Tracing Pointers – More Levels What is in foo and quux at the end of this program? int main(int argc, char** argv) { int foo, *bar, **baz, quux; bar = &quux; foo = 4; baz = &bar; **baz = 13; bar = &foo; **baz = 9; return 0; }

  12. foo quux bar baz int int int* int** 410 1310 Tracing Pointers – More Levels What is in foo and quux at the end of this program? int main(int argc, char** argv) { int foo, *bar, **baz, quux; bar = &quux; foo = 4; baz = &bar; **baz = 13; bar = &foo; **baz = 9; return 0; }

  13. foo quux bar baz int int int* int** 910 1310 Tracing Pointers – More Levels What is in foo and quux at the end of this program? int main(int argc, char** argv) { int foo, *bar, **baz, quux; bar = &quux; foo = 4; baz = &bar; **baz = 13; bar = &foo; **baz = 9; return 0; }

  14. What’s wrong with this program? int modifyCount(int x) { x = x – 1; } int main(int argc, char** argv) { int x = 4; /* want to change x */ modifyCount(x); return 0; }

  15. x int 410 What’s wrong with this program? int modifyCount(int x) { x = x – 1; } int main(int argc, char** argv) { int x = 4; /* want to change x */ modifyCount(x); return 0; }

  16. x x int int 410 410 What’s wrong with this program? int modifyCount(int x) { x = x – 1; } int main(int argc, char** argv) { int x = 4; /* want to change x */ modifyCount(x); return 0; }

  17. x x int int 410 310 What’s wrong with this program? int modifyCount(int x) { x = x – 1; } int main(int argc, char** argv) { int x = 4; /* want to change x */ modifyCount(x); return 0; }

  18. x int 410 What’s wrong with this program? int modifyCount(int x) { x = x – 1; } int main(int argc, char** argv) { int x = 4; /* want to change x */ modifyCount(x); return 0; } We never changed x! How do we fix this?

  19. Use Pointers! int modifyCount(int* x) { *x = *x – 1; } int main(int argc, char** argv) { int x = 4; /* want to change x */ modifyCount(&x); return 0; }

  20. x x int int* 410 What’s wrong with this program? int modifyCount(int* x) { *x = *x – 1; } int main(int argc, char** argv) { int x = 4; /* want to change x */ modifyCount(&x); return 0; }

  21. Pointers and ++/-- Suppose we have the following program: int main(int argc, char** argv) { int i, j; int* p = &argc; /* argc = 1 */ i = (*p)++; argc = 1; j = ++(*p); return 0; } What is in i and j?

  22. Pointers and ++/-- Assuming x and y have type int… • y = x++; is equivalent to y=x; x=x+1; • y = ++x; is equivalent to x=x+1; y=x;

  23. Pointers and ++/-- Suppose we have the following program: int main(int argc, char** argv) { int i, j; int* p = &argc; /* argc = 1 */ i = (*p)++; argc = 1; j = ++(*p); return 0; } What is in i and j? i = 1 and j = 2 i = *p; *p = *p + 1; *p = *p + 1; j = *p;

  24. Pointers and [] • x[i] can always be rewritten as *(x+i) and vice versa • Array types can often be converted into their corresponding pointer counterparts • int foo[] is equivalent to int* foo • int* bar[] is equivalent to int** bar • You can at most change one set of [] safely • Changing more requires knowing how the array looks in memory

  25. Pointers and ++/-- Suppose we have the following program: int main(int argc, char** argv) { int i, j; int* p = &argc; /* argc = 1 */ i = (*p)++; argc = 0; j = ++(*p); return 0; } What is in i and j? Both contain 1 i = *p; *p = *p + 1; *p = *p + 1; j = *p;

  26. printf, scanf, and their cousins • printf (and its cousins) are special functions that do not have a fixed argument list • for each format specifier (i.e. %d), an additional argument needs to be supplied • Examples: • printf(“%d”, 4); • printf(“%s%d%c”, “CS”, 0x3D, ‘c’);

  27. printf, scanf, and their cousins • Unlike printf, with scanf for each format specifier (i.e. %d), an additional argument needs to be supplied that has type pointer • Examples: • int z; scanf(“%d”, &z); • char foo[5]; int d; scanf(“%s %d”, foo, &d);

  28. C Program Walkthrough What happens with this program? void quux(int foo) { char a[4]; char* baz = (char*)(&foo); printf("%c%c%c%c", baz[0], *(baz + 1), baz[1+1], baz[sprintf(a, "123")]); } int main(...) { quux(0x4d495053); }

  29. foo int 0x4d495053 C Program Walkthrough What happens with this program? void quux(int foo) { char a[4]; char* baz = (char*)(&foo); printf("%c%c%c%c", baz[0], *(baz + 1), baz[1+1], baz[sprintf(a, "123")]); } int main(...) { quux(0x4d495053); }

  30. C Program Walkthrough What happens with this program? void quux(int foo) { char a[4]; char* baz = (char*)(&foo); printf("%c%c%c%c", baz[0], *(baz + 1), baz[1+1], baz[sprintf(a, "123")]); } int main(...) { quux(0x4d495053); } foo int 0x4d495053 baz char*

  31. C Program Walkthrough What happens with this program? void quux(int foo) { char a[4]; char* baz = (char*)(&foo); printf("%c%c%c%c", baz[0], *(baz + 1), baz[1+1], baz[sprintf(a, "123")]); } int main(...) { quux(0x4d495053); } foo int 0x4d495053 baz char*

  32. C Program Walkthrough What happens with this program? void quux(int foo) { char a[4]; char* baz = (char*)(&foo); printf("%c%c%c%c", baz[0], *(baz + 1), baz[1+1], baz[sprintf(a, "123")]); } int main(...) { quux(0x4d495053); } foo int 0x4d495053 4d 49 50 53 baz char*

  33. C Program Walkthrough What happens with this program? void quux(int foo) { char a[4]; char* baz = (char*)(&foo); printf("%c%c%c%c", baz[0], *(baz + 1), baz[1+1], baz[sprintf(a, "123")]); } int main(...) { quux(0x4d495053); } foo int 0x4d495053 4d 49 50 53 baz char*

  34. C Program Walkthrough What happens with this program? void quux(int foo) { char a[4]; char* baz = (char*)(&foo); printf("%c%c%c%c", baz[0], *(baz + 1), baz[1+1], baz[sprintf(a, "123")]); } int main(...) { quux(0x4d495053); } foo int 0x4d495053 4d 49 50 53 baz char* It will print out “MIPS”

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