Computer Systems: A Programmer\'s Perspective, 3rd Edition, 3.70: https://www.ch

Question

Computer Systems: A Programmer's Perspective, 3rd Edition, 3.70:
https://www.chegg.com/homework-help/Computer-Systems-3rd-edition-chapter-3-problem-70HWP-solution-9780134092669

Explanation / Answer

/* Consider the following union declaration: */ /* union ele { */ /* struct { */ /* int *p; */ /* int y; */ /* } e1; */ /* struct { */ /* int x; */ /* union ele *next; */ /* } e2; */ /* }; */ /* This declaration illustrates that structures can be embedded within unions. */ /* The following procedure (with some expressions omitted) operates on a */ /* linked list having these unions as list elements: */ /* void proc (union ele *up) */ /* { */ /* up-> */ /* = *(up-> */ /* } */ /* A. What would be the offsets (in bytes) of the following fields: */ /* e1.p: */ /* e1.y: */ /* e2.x: */ /* e2.next: */ /* B. How many total bytes would the structure require? */ /* C. The compiler generates the following assembly code for the body of proc: */ /* up at %ebp+8 */ /* movl 8(%ebp), %edx */ /* movl 4(%edx), %ecx */ /* movl (%ecx), %eax */ /* movl (%eax), %eax */ /* subl (%edx), %eax */ /* movl %eax, 4(%ecx) */ /* On the basis of this information, fill in the missing expressions in the code */ /* for proc. Hint: Some union references can have ambiguous interpretations. */ /* These ambiguities get resolved as you see where the references lead. There */ /* is only one answer that does not perform any casting and does not violate */ /* any type constraints. */ ///////////////////////////////////////////////////////////////////////////////////////////////////// /* suppose both int and pointer are 4 bytes long */ /* A. */ /* e1.p: 0 */ /* e1.y: 4 */ /* e2.x: 0 */ /* e2.next: 4 */ /* B. */ /* Total: 8 bytes */ /* C. */ /* movl 8(%ebp), %edx */ // get up /* movl 4(%edx), %ecx */ // next = (up->e2).next /* movl (%ecx), %eax */ // p = (next->e1).p /* movl (%eax), %eax */ // x = *p /* subl (%edx), %eax */ // x2 = x - (up->e2).x /* movl %eax, 4(%ecx) */ // (next->e1).y = x2 union ele { struct { int *p; int y; } e1; struct { int x; union ele *next; } e2; }; void proc (union ele *up) { union ele *next = (up->e2).next; (next->e1).y = *((next->e1).p) - (up->e2).x; }

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