CS3101-1 Programming Languages

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CS3101-1Programming Languages CLecture 5

• Matthew P. Johnson
• Columbia University
• Fall 2003

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Important Reminder

• Do not let me fail to repeat Qs
• Remind me if I forget
• ? Win valuable prizes

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Agenda

• Hw3 due last night
• Hw4 assigned tonight
• Last time arrays ptrs
• This time
• More on ptrs
• Structures
• Not objects!
• Struct ptrs, struct params
• Self-ref structs
• More mallac, casting
• Typedef
• Unions Bitfields
• More I/O
• Var-length args
• Final exam

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Multi-D arrays as ftn args

• Any dims after the first (0th) must be specified
• int ftn(int days212) ?
• int ftn(int days12) ?
• int ftn(int (days)12)
• Remember pass int days212 ? receive
  int(days)12
• Q Why do others need to be specified?
• Q Why neednt the first?
• A 2 in days2 ignored, so days is equiv to
  days
• passing array same as passing ptr
• So passing a ptrto what?
• Passing a ptr to int12s
• Remember referent type tells ptr the number of
  bytes that compose its referent

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Array-of-ptrs v. 2-dim arrays

• Almost the same but not quite
• int a1020
• 10x20 2-dim array ? 1020 200 ints
• each row same length
• a16 ? (a1)6 ? (a16) ? ((a1) 6)
• ? (a 120 6) ? (a 26)
• In each addition, offset has implicit coeff of
  sizeof(type)
• sizeof(int) 4, sizeof(int20) 420
• why we need the length of the rows!
• See twodim.c
• int b10
• No ints created, but 10 int s created
• Each can be set to point to an int20 ( a row)
• Or could be set to diff length arrays

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Array-of-ptrs v. 2-dim arrays

• Also, we wrote
• int (days)12 as the same as int days212
• A ptr to length-12 int arrays
• int b10 the ptr analog of int a1020
• 10 int ptrs
• Q Why not b20?
• A These look analogous, but are not
• int (days)12 is one ptr, a ptr that points to
  a length-12 int array
• int b10 is 10 ptrs, each of which points to
  int(s)
• Interped as a table, this gives a 10x? table
• we had a 10x20 table 10 rows
• ? 10 ptrs to ints

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Array-of-ptrs v. 2-dim arrays

• Diff between int (d)10 and
• int d10?
• have higher precedence than
• In second, 10 attaches to d, then to result ?
  array of int-ptrs
• In first, attaches to d, then 10 to result ?
  ptr to int-array(s)
• Just remember , () bind tighter than

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v. ()

• You should know diff bet int f() and int (pf)()
  (without helpful names)
• Be careful with s and s/()s
• Q What does the following mean?
• char ((x()))()
• A left as an exercise for the listener

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Array-of-strs v. 2-D char arrs

• char names Abc, d, efgh
• names array of char, each pts to
  just-large-enough string (A,b,c,\0)
• char names5 Abc, d, efgh
• len-15 array, with empty space between entries
• Abc\0 d\0 efgh\0

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Command-line args

• ? main ftn params
• Canonical main ftn
• int main(int argc, char argv)
• Looks like Javas main method OO
• public static void main(String args)
• argv array of strings
• argv0 exec name!
• Other elms cmd-line args, in order type
• Args sep-ed by spaces
• To include space in string, surround by quotes

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Command-line args

• Q What is argv?
• A ? (argv)
• ? (argv1)
• ? argv1
• ? argv10
• ? char 0 of arg 1
• Q What is argv0?
• A ? something of arg 0
• ? something of char 1 of arg 0
• ? val of char 1 of arg 0
• Q What is (argv)0?
• A ? 0th of (argv1)
• ? 0th of arg 1
• ? char 0 of arg 1
• ? argv

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Ftn pointers

• Ftns arent values vars cant be set to them
• But ptrs can point to them!
• ? Can pass one ftn to another, so second can call
  first!
• Ftn name simpliciter (no parens) evals to ptr to
  the ftn
• To call, dereference ftn, in parens, followed by
  args
• res (cmp)(pa, pb)
• Dereferencing ftn ptr is actually not required,
  but reminds us that cmp is a ptr

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Ftn pointers

• Example qsort
• Sorts according to some ordering of items
• Ordering specified by implementation of cmp ftn
• How does qsort find out which cmp to call?
• Cmp (or a ptr to it) is passed to qsort as a
  param
• NB type depends on param-list types and return
  type
• Show ftnptrs.c

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Proper main ftns

• int main(int argc, char argv)
• return 0
• NB Ftn signature is similar to Java equiv, minus
  OO
• public static void main(String args)
• Since arrays dont know their length, this info
  is sent separately
• argc argument count
• argv argument vector
• Also, argv0 is always the exec name (e.g.,
  a.out)

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Meta-pointers

• Given any type, exists ptr to type
• int ? int
• char ? char
• int ? int
• ptr to a ptr to an int
• Ever used?
• Yes! -qsort

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Pointer op review

• Q suppose say
• int x 10
• int ip x
• int ipp ip
• ? ipp ?
• A 10
• Q suppose say
• int ip
• int ipp ip / ! /
• int x 10
• ip x
• Any problem, since ip is not initiated?
• A No, ip not set, but it has mem reserved for it
  - picture

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Pointer op review

• Q suppose say
• int ip
• ip 10
• Any problem?
• A Yes! ip not set ? ip who knows?
• Q suppose say
• int ftn()
• int x 10
• return x
• Any problem?
• A Yes! X is popped off stack on return
• dangling pointer

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Structures

• Primary sort of user-defined complex data type
• type inst composed of arb set of other vars
• Perhaps contains other structs (usually w/ptrs)
• No order
• Multiple data types
• Intuitions
• struct a record containing fields/DB row
• struct Java object - methods

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struct example Point

• Declare a struct to represent a point in the
  plane
• struct Point
• int x
• int y
• Now can create Point vars
• struct Point pt
• NB type of pt is struct Point, not simply Point
• As with enum types
• Point is called the struct tag

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struct example Point

• Can access struct members with dot op
• struct-name.mem
• pt.x 10
• Can also do static init
• struct Point pt 10, 10

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structs

• Notice no pointers here (yet)
• ? declaration of struct obtains mem for all elms
• Like def/declar of prims, arrays

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Nested structs

• Structs can be nested (structs within structs)
• struct Rect
• struct Point lowerleft
• struct Point upperright
• struct Rect screen
• screen.lowerleft.x 5
• picture

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Struct ops

• Field access .
• Address-of
• Assignment
• Values of all struct members (recursively)
  constitute val of struct
• s1 s2 ? all fields of s1 set to vals in s2
• Unlike arrays!
• Of course, can only assign between same struct
  types

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struct op

• struct Point pt1 5,10,
• p2 30,40
• pt2 pt1
• pt2.x 13
• ? pt1.x ?
• ? 5
• We have 4 distinct ints in mem
• Changing one does not affect the other

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C structs v. Java objs

• In Java, all objs are implicitly pass-by-ref
• Not the case for C structs!
• Also, no concept of public/private struct mems
• Everything is public-access
• Concept of inside not even applicable
• No methods in structs, just data
• NB passing structures would be much more work
  (slower) w/o pointers

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structs as params, return vals

• Suppose want to add two points
• Intuition add xs, add ys
• Would be nice to say
• pt3 pt1 pt2
• Programmatic op overloading
• Cant in C see 3101-2
• Next best use a function
• Take two pt params
• Return a pt

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Add pts

• struct Point addpt(struct Point p1,
• struct Point p2)
• stuct Point sum
• sum.x p1.x p2.x
• sum.y p1.y p2.y
• return sum
• Can we do better?

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Add pts

• Yes struct params are passed-by-val
• ? we can modify them
• struct Point addpt(struct Point p1,
• struct Point p2)
• p1.x p2.x
• p1.y p2.y
• return p1
• No extra var

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Struct ptrs

• Ptrs can point to any var type
• Declare struct ptrs thus
• struct Point pp
• pp is a struct Point can apply dot op
• Dot op has (naturally) high prec.
• pt.x 5 ? (pt.x) 5
• But not quite high enough
• pp.x ? (pp.x) ? ??
• Dot higher than ? need parens

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Shortcut op ?

• pp.x ? (pp.x) ??
• (pp).x ? pp?x ? works
• ?, dot have same precedence
• Assocs L-to-R
• r.lowerleft.x ? (r.lowerleft).x
• rp?lowerleft.x ? (rp?lowerleft).x
• Q pp?x ? what?
• A ? has highest prec
• ? (pp?x)
• (), , ., ? have highest precedence
• See table on page 52

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Static inits

• Can combine array static inits and struct static
  inits
• struct Point pts
• 10, 20,
• 30, 40
• Shorter
• struct Point pts 10, 20, 30, 40
• Braces around rows optional

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sizeof structs

• sizeof applies to arrays
• For any locally defined array,
• sizeof(ar) sizeof(ar0)len
• sizeof(elm type)len
• ? len sizeof(ar) / sizeof(ar0)
• For pointers or array params (same thing), length
  is unkown

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sizeof structs

• sizeof applies to structs
• sizeof pt
• sizeof (struct Point)
• Gives actual bytes needed to store the struct
• May be more than just sum of field sizes
• Implementation reasons

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sizeof structs

• Consider
• struct S
• int i
• char c
• May turn out
• sizeof (struct S) 8 44
• ? sizeof sAr 8len

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Self-ref structs

• Claimed when struct var defined, all mems
  defined
• What about self-referential data structures
  linked lists, trees, etc.
• struct Listnode
• int val
• struct Listnode next
• Define var
• struct Listnode first
• In mem, we autoly create 1) an int and 2) a
  Listnode
• ? 1) an int and 2) a) an int and b) a Listnode
• ? off to the races
• Any instance, fully expanded, is infinite

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Self-ref structs

• Soln next is not a struct, but a ptr
• struct Listnode
• int val
• struct Listnode next
• Now define var
• struct Listnode head
• In mem, we autoly create 1) an int and 2) a ptr
• Thats all!

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Self-ref structs

• To expand
• Create another node with malloc
• Set prev nodes next value to address of new
• Always set new next value to null
• Traversal is straight-forward
• struct Listnode curr
• for (curr head curr ! NULL curr
  curr?next)

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Self-ref structs

• struct Listnode addnewnode(struct Listnode
  curr, int i)
• struct Listnode node
• (struct Listnode)
• malloc(sizeof(struct Listnode))
• node?i i
• curr?next node
• return node

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Casting ptrs

• NB we cast result from malloc
• Its type is void, the generic ptr
• What happens when we cast a ptr?
• Is its referent affected?
• No
• Remember int is a var that is a ptr (to an
  int)
• void is a var that is a ptr (to something)
• So casting a ptr changes how much mem is
  interped as its referent
• But not the referent itself

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Casting ptr example

• int x 1025
• int ip x
• int cp (char)ip
• Q What do we get if we read four bytes as four
  chars?
• A 0, 0, 4, 1
• 2563 2562 42561 12560
• byte-string representation

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malloc example

• Goal duplicate string
• char strdup(char s)
• char p (char)malloc(strlen(s)1)
• if (p ! NULL)
• strcpy(p,s)
• return p

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malloc example NB

• strlen doesnt include \0
• Add 1 to get in-mem size
• Used strcpy
• One of ftns in string.h
• Others
• strncpy(s,t,n)
• At most n chars
• Assumes s ref has enough room
• strcmp(s,t)
• -1 s lt t
• 0 s t
• 1 s gt t

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string.h ftns

• 3. strcat(s,t)
• Concantation to to end of s
• 4. strncat(s,t,n)
• ltn chars of t
• 5. strchr(s,c)
• Ptr to first appearance of c in s
• 6. strrchr(s,c)
• Ptr to last appearance of c in s

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Dynamic memory

• malloc allocates memory dynamically
• As opposed to regular var definition
• Local vars are automatic
• pushed, popped to stack
• malloc-ed mem stays on heap until freed
• Some langs have garbage collection not C
• Main dynamic memory ftns (stdlib.h)
• void malloc(int size)
• If none available returns NULL
• Memory is not initilialized!
• void free (void p)

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Dynamic memory example

• Suppose want 53 Ball objects
• Can do
• Ball53 theballs
• Suppose want n Ball objects, with n
  user-specified
• Q Will this work
• Balln theballs
• A No! array-size must be constant/Known at
  compile time
• Soln obtain memory dynamically
• Ball theballs (Ball)malloc(nsizeof(Ball))
• if (theballs ! NULL)
• free(theballs)

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Dynamic memory dangers

• dangling pointers
• free(p)
• p 10
• Mem at address p may be reclaimed
• Dont try to read/write there
• Soln
• free(p)
• p NULL
• Memory leaks
• p NULL
• Were done with the mem at address p
• But we forgot where it is! that much less mem
  now
• Soln
• free(p)
• p NULL

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Dynamic memory dangers

• Generalization
• When finished with dynamic mem, always
• Free it
• Reset ptr to null
• Q Why doesnt free automatically reset p to
  null?
• A p is passed by value!
• Q Any other possible soln?
• A Yes, pass meta-pointer (but C doesnt do this)

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Dynamic memory

• Freeing is done in reverse order
• Suppose have two listnodes
• node1?next node2
• To free both
• free(node1?next)
• free(node1)
• Q What if freed node1 first?
• A Not allowed to access node1?next afterward
• Of course, can still free node2 if we have a
  separate ptr

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typedef

• Used to define new types
• Really, merely creates synonym for existing
  type
• Examples
• 1. typedef int Length
• ? can declare ints with Length
• Length x 10
• 2. typedef char String
• String s hi

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typedef examples

• 3. typedef struct Listnode Node
• struct Listnode n ? Node n
• 4. typedef double (DDFtn) (double)
• DDFtn ftn sin

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typedef benefits

• Replace long type constructions with simple names
• Part of C, not CPP
• (simple) typedefs could be done with CPP
• Give semantic meaning to types
• Suppose want all Length vars to change from short
  to long
• But other shorts should stay shorts
• ? just change def of Length!

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Unions

• Fairly rarely used should be aware of
• Idea declare var can take on one val of mult
  types
• Switch between
• Syntax similar to that of structs
• union utag
• int i
• double d
• char s
• un
• un.i 3
• un.f 3.5
• Retains only most recently set val

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Accessing unions

• How does it remember which field you most
  recently set?
• It doesnt access the right one
• un.i, un.f, up?i
• Size of union is size of largest elm
• Other construals
• un struct that has only 1 elm at a time
• un struct s.t. all mems have offset 0

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Bitfields

• Also infrequently used
• Here we use unsigneds
• struct
• unsigned int a 1
• unsigned int b 2
• unsigned int c 1
• bits
• Bitfields get machine-dependent and messy
• Usually bit masks are better choice
• if (att (HIDDEN SYSTEM))

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I/O

• Weve used I/O streams
• stdin from keyboard
• stdout to screen
• stderr to screen, for errors
• Can redirect to, from
• a.out lt in.txt gt out.txt
• Can also pipe from one exec to another
• progA progB
• As output sent to B as input
• Done this before?
• Yes! ls more

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I/O ftns

• Simplest I/O ftns
• getchar()
• putchar(c)
• More sophisticated
• scanf
• printf
• Saw most important conv wildcards
• Others
• printf(.s, max, s) at most max chars of s
• scanf(1s, s) gives ptr to first
  non-whitepace char

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File access

• Know how to access files by redirection lt, gt
• Can also do so programmatically
• FILE fopen(char name, char mode)
• Nodes r, w, a
• Returns file ptr or NULL if failed
• int fclose(fp)

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File access

• Most I/O routines have file-spec analogs
• int getc(fp)
• int putc(c, fp)
• NB diff names because no function overloading
• int fputs(s, fp)
• char fgets(s, fp)
• int fscanf(fp, format, )
• int fprintf(fp, format, )
• Passing stdin/stdout reduces to calling the
  regular routines

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Low-level I/O

• All I/O routines discussed so far are high-level
  (!)
• Rely on lower-level routines
• On Unix, these are system calls like
• read(fd, buf, n)
• write(fd, buf, n)
• n chosen from
• 0 stdin
• 1 stdout
• 2 stderr
• How to open file?
• Fd creat(name, parms) / no e in creat /
• Unlink(name)
• Lseek(fd, offset, origin)
• See course in OSs

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Error-handling

• Built-in error stream strerr
• Also goes to screen
• Redirected indly machine/shell-dependent?
• Should return success val from main
• 0 success
• !0 error
• Can also exit with success val from elsewhere
• exit(0)
• Can also assert what should be true
• assert(a gt 0)
• If expr is false, we quit
• Feature recently added to Java

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Var-length param lists

• Seen var-length param lists to scanf, printf
• Claimed no ftn over-loading in C
• Very strange! How does this work?
• Declarations
• int scanf(char format, )
• int printf(char format, )
• As one says in original, ellipses in original.
• is part of C

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Var-length param lists

• varargs.h , included by stdargs.h, contains
  macros
• va_list
• type used for var that pts to arg in list
• va_start(ap, fmt)
• Sets ap to first arg
• va_arg(ap, type-name)
• Returns current arg interped as type-name
• va_end(ap)
• Cleans up
• Q How do we know when to quit?
• A Pass in info
• Q But we didnt pass it to printf/scanf
• A Yes we did
• in the form of appearances of wildcards in the
  pattern string
• Content of macros left as an exercise
• sum.c

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Grading

• Final will be hard/challenging
• Ave score probably near 70
• Final grades are curved
• Rule of thumb mean stdev
• Mean B/B-
• -stdev one letter grade
• See mean/stdevs on web to estimate your grade

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Next time final exam

• Closed books/notes/everything
• 2 hours/class time
• Questions
• Vocab extern, static, etc.
• Find errors/read code/predict output
• Write code
• See web for a semi-definitive list of topics
• Jake will be proctoring the exam
• But well both have OH this week(end)
• Come in if you have questions!

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The future

• Q What happens when OO is added to C?
• A See CS3101-2 C, beginning the week after
  the final
• Same time, same channel
• hw4 TBA
• Please fill out mid-term course evals!
• Link will be on classpage tonight
• Available until day of final
• Sign in and good luck!