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CMSC 212 – S05 (lect 5)
Announcements
●Program #1A
– Due Today at 8pm
●Program #1B
– Due in two weeks
– Will be available by 8pm tonight
●Reading
– Chapter 10 (for today, skip 10.2)
– Chapter 5 (by next Tuesday)
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Guidelines and tips
Prepare for your exams
Study with the several resources on Docsity
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Structure, Combining Structure and Typedef - Low Level Programming | CMSC 212, Study notes of Computer Science
Material Type: Notes; Professor: Hollingsworth; Class: INTRO TO LOW-LEVEL PROG; Subject: Computer Science; University: University of Maryland; Term: Unknown 1989;
Typology: Study notes
Pre 2010
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Announcements
l^
Program #1A– Due Today at 8pm
l^
Program #1B– Due in two weeks– Will be available by 8pm tonight
l^
Reading– Chapter 10 (for today, skip 10.2)– Chapter 5 (by next Tuesday)
Structures
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Syntax: struct [tag] { member-list } [variable-list];– [ ] indicate optional portions depending on the version used– { } and ; are required
l^
Notes:– used to declare related items– similar to classes (without methods) in Java
l^
Example:struct {
int a;char b;float c; } x; •^
Declares a single variable x with 3 fields
Combining struct and typedef
l^
Often useful to use typedef and struct together:typedef struct {
int a;char b;float c; } Simple; l^
Declarations then look like:Simple x;Simple y[20], z; l^
Header Files–
If structure used by more than one file, put it in a header file.– If you want programmers who will be using your implementation to seethis struct, put it in the header file.– Use
#include
to include definition in each file where used.
Accessing Fields of Structures
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Consider this definition:struct {
int a;char b;float c; } x, y[10]; l^
Simple structure access:– x.a = 3;– printf("field b = %c\n", x.b);
l^
Accessing arrays of structures:– y[7].c = 3.14;– y[i].b = 'a';
Bit Fields
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Part of the Power of C is control over data layout– If you need a field with exactly 13 bits, you can define it– size must be less than or equal to the default size for that type– Useful for:
- defining fields that interact with hardware– managing pre-defined file formats– saving every last bit of space
- Syntax:
type
variablename:size;
l^
Examples:int foo:13;unsigned int foo:4;typedef struct {
/* from project #1 */
unsigned int opCode:4;unsigned int r1:4;unsigned int r2:4;unsigned int r3:4;unsigned int address:16;} instruction;
Passing Structures as Arguments
typedef struct { char productName[200];int quantity;float unitPrice; } Transaction;void printReceipt( Transaction trans){
printf("product = %s\n", trans.productName);printf("
quanity = %d\n", trans.quanity);
printf("
price = %f\n", trans.unitPrice);
} int main(void){Transaction foo;printReceipt(foo); l^
Uses call by value– Entire structure is copied to printReceipt
typedef struct { char productName[200];int quantity;float unitPrice; } Transaction;void initReceipt(Transaction *trans){
trans->productName[0] = ‘\0’;trans->quanity = 0;trans->unitPrice = 12.50; } int main(void){Transaction foo;initReceipt(&foo);
Passing a reference to a Structure
Indicates pointer type variable
Indicates pass the reference (the address)
Unions
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Syntax is similar to structs l^
Rather than storing each field, only stores
one
of the fields
-^
Handy when need to stores different things based on criteria
l^
Syntax:–
union [tag] { member-list } [variable-list];– again, the things in [ ] are optional– the { } and ; are required l^
Examples:union {
float f;int i; } objname;objname.f = 3.14159;
Memory Allocation
l^
Structures are placed in consecutive memory locations– memory is rounded up to alignment
- characters take 1 byte• integers normally take 4 bytes and start at aligned addresses
l^
Can query the size of a structure or variable– sizeof(structure name) or sizeof(variable name)
0x000x040x080x0c0x100x
struct { int a;char b;int c; } x; x = { 3, 'a', 10 };
3 0 0 0
0x (^0) (^0) (^0)
? ?^
?
0xA
Project #1B
l^
int getFile(char *fileName, char file[MEM_SIZE][80]);– reads the passed fileName into the array file– returns the number of lines read– returns a –1 on error
- can’t find the file• too many lines