Lecture 8 - Memory Layout of Data, Padding, Binary File I/O

Question 1

Everything that contains a value uses..

Answer 1

• memory
• memory space looks like a long continuous stream of bytes
• everything that contains a value occupies 1+ bytes of memory

Question 2

variables near each other and location

Answer 2

variables that are defined near each otters are usually near to each other in memory

Question 3

structures in memory

Answer 3

• structures are placed in memory just like arrays
• guaranteed to be packed next to each other

Question 4

determining the size of variables and types?

Answer 4

• variable of certain size may have a different allocated size on different machines/compilers
• we don’t want code to misbehave when compiled on different system but don’t have to remember what the size is

Question 5

strncpy()

Answer 5

• copies one string into another string
  strcpy(dest, src, size);
• size is max num of chars to copy from src to dest
• allows us to not have to remember size across compilers by putting sizeof(dest)

Question 6

padding

Answer 6

• structures are often padded so that data elements occur at the correct offset
• ex. ints are 4-byte aligned, longs are 8-byte, etc.
• if unaligned, some can’t handle it/it’s very slow

Question 7

how to align structures to use space efficiently

Answer 7

• orders the fields top to bottom by relative size
• doubles, long, pointers, long, int/float, short, char
• makes padding just be at end of struct

Question 8

fread()

Answer 8

• given open FILE, read “raw” memory items to or from a file
• allows you to dump a data struct directly into a binary format file
  read(void *ptr, int size, int num, FILE *fp);

Question 9

fwrite()

Answer 9

• given an open FILE, write “raw” memory to/from a file
• allows you to “dump” a data struct directly into a binary format file
• fwrite(void *ptr, int size, int num, FILE *fp);

Question 10

return values of fread() and fwrite()

Answer 10

• return the number of items that were read or written
  -on error, they return a shot item count or zero

Question 11

uses of fread()

Answer 11

• reading binary files
• reading structs from files
• efficiently reading large data blocks

Question 12

fwrite() curiosities

Answer 12

• void* argument in fwrite() means we can pass a pointer to anything

Question 13

more on fread()/fwrite()

Answer 13

• moves a “memory image” to or from a file
• essentially byte file I/O
• file is NOT PORTABLE
• no data type checking
• different systems have different formats for ints, floats, etc.