CS 2130

1
CS 2130

• Presentation 10
• Strings
• Library Fun Facts

2
Review

• Declared Scope Storage
  Class Initialized
• Outside all blocks 1 Global 1 static
  1 Once
• 2 File 2 auto
  2 Each time
• 3 Local
  
• static outside all blocks 1 Global 1 static
  1 Once
• 2 File 2 auto
  2 Each time
• 3 Local
  
• Inside a block 1 Global 1 static
  1 Once
• 2 File 2 auto
  2 Each time
• 3 Local
  
• static inside a block 1 Global 1 static
  1 Once
• 2 File 2 auto
  2 Each time
• 3 Local
  

3
Strings

• Important There are no strings in C

4
Strings

• Many languages include built-in support for
  Strings
• Typical examples include
• Basic
• Pascal
• Scheme/Lisp
• Java
• "Built-in" here implies behind the scenes details
  are managed by the compiler and/or the run-time
  environment

5
Strings in C

• There are no Strings in C.
• There is an agreed upon convention to store
  string like data structures in arrays of
  characters.
• Part of the convention includes terminating
  strings with a null character.
• In many cases "string" operations in C look just
  like those found in other languages.
• Do not be deceived. This is C...memory must be
  managed properly (by you)!

6
Compare

• Basic
• 10 a "Hello "
• 20 b "world"
• 30 a a b
• 40 print a
• run
• Hello World

• C
• char a "Hello "
• char b "world"
• strcat(a, b)
• printf("s\n", a)

7
Better?

• char a12 "Hello "
• char b "world"
• strcat(a, b)
• printf("s\n", a)

8
What's happening behind the scenes in Basic?

• Who cares!?!?!?

9
What's happening in C?

• char a12 "Hello "

a
H
e
l
l
o

\0
?
10
What's happening in C?

• char a12 "Hello "
• char b "world"

a
H
e
l
l
o

\0
b
w
o
r
l
d
\0
11
What's happening in C?

• char a12 "Hello "
• char b "world"
• strcat(a, b)

a
H
e
l
l
o

w
o
r
l
d
\0
b
w
o
r
l
d
\0
12
What's happening in C?

• char a12 "Hello "
• char b "world"
• strcat(a, b)
• printf("s\n", a)

a
H
e
l
l
o

w
o
r
l
d
\0
b
w
o
r
l
d
\0
Hello world
13
Strings

• We implement string-like data structures using
  character arrays
• char s110 / Not a string! /
• char s2 / Not a string! /
• char s310 "foo" / Not a string /
• char s4 "bar"
• What acts like a string?
• A pointer to a character followed by a sequence
  of characters terminating in a null byte
• char s510 "foo"
• f o o \0 ?? ?? ?? ?? ?? ??

14
Strings

• char s110 "foo"
• printf("s\n", s1)
• Works to print
• foo
• Note If s1 is not null terminated the printf
  will print a character string until it encounters
  a null character or causes an error (segment
  fault etc.)

15
Strings

• char s110
• s10 'f'
• s11 'o'
• s12 'o'
• printf("s\n", s1)
• Sample output
• fooâ-VGot 2
• ,_VGot 2
• Got 20
• _VGot 2
• _Got 16
• Got 1

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Strings

• char s110 "foo"
• char s2 "superduper"
• foo\0 is stored in space allocated for s1 (an
  array)
• on stack
• or in changeable data area
• plus?
• superduper\0 is stored in the constant data area
  with s2 pointing to it.

17
Strings

• char s110 "foo"
• char s2 "superduper"
• sizeof(s1) ?
• 1) 4
• 2) 10
• 3) Depends
• 4) 11

18
Strings

• char s110 "foo"
• char s2 "superduper"
• sizeof(s2) ?
• 4

19
Strings

• char s110 "foo"
• char s2 "superduper"
• sizeof(s2) ?
• 1) 1
• 2) 4
• 3) 10
• 4) 11

20
Contrast

• sizeof("Hello") ?
• 6
• strlen("Hello") ?
• ???

21
Strings

• char s110 "foo"
• char s2 "superduper"
• Legal?
• s2 s1

Yes!
22
Strings

• char s110 "foo"
• char s2 "superduper"
• Legal?
• s1 s2

No!
23
Strings

• char s110 "foo"
• char s2 "superduper"
• Does s2 s1 copy the strings?

No!
24
But it looks like it copies strings!

• char s110 "foo"
• char s2 "superduper"
• s2 s1
• printf("s\n", s1)
• printf("s\n", s2)
• What prints?

foo foo
25
Strings
\0
f
o
o
s1
\0
b
a
r
s2
s2 s1 What happens?
26
Strings
\0
f
o
o
s1
\0
b
a
r
s2
Memory Leak?
27
Strings
\0
f
o
o
s1
\0
b
a
r
s2
Depends!
28
Memory Leak?

• char s1 "foo"
• char s2 "bar"
• s2 s1
• No memory leak!

• char s1 "foo"
• char s2 malloc(4)
• s2 s1
• Memory Leak!

Paranoid delusional error checking omitted from
slide. Don't try this at home!
29
String Functions

• include
• strcpy(p, s)
• strcmp(p, s)
• strcat(p, s)
• strlen(s)
• Note strcpy and strcat dont malloc space
• p must be allocated to be large enough to hold
  the results

30
String Functions

• strcpy(s2, s1) returns what?
• s2
• Why?

• Can be used like this
• printf("s\n", strcpy(a, b))

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String Functions

• char s110 "foo"
• char s2 "foobar"
• strcpy(s2, s1) / Okay? /
• No!
• strcpy(s1, s2) / Okay? /
• Yes!

32
String Functions

• strdup - duplicate a string
• include
• char strdup(const char s)
• What does it do? Different than strcpy? Why?
• Cautions?

• All of the same concerns that we had with malloc
  and free!

33
Danger!

• gets
• char gets(char s)
• gets() reads a line from stdin into the buffer
  pointed to by s until either a terminating
  newline or EOF, which it replaces with '\0'. No
  check for buffer overrun is performed!
• Because it is impossible to tell without knowing
  the data in advance how many characters gets()
  will read, and because gets() will continue to
  store characters past the end of the buffer, it
  is extremely dangerous to use. It has been used
  to break computer security. Use fgets() instead.

34
Example

• include
• include
• include
• int main(int argc, char argv)
• char buffer10
• gets(buffer)
• printf("buffer s\n", buffer)
• return EXIT_SUCCESS

35
Well behaved?

• C\demo
• hello
• buffer hello

36
Well behaved?

• C\demo
• Now is the time for all good men to come to the
  aid of their
• buffer Now is the time for all good men to come
  to the aid of their
• In cygwin_except_handler
• Exception trapped!
• exception C0000005 at 20726F66
• exception ax 0 bx 256F52C cx 46 dx BFFC9490
• exception si 5 di 100113BF bp 20656D69 sp
  256F520
• exception is STATUS_ACCESS_VIOLATION
• Stack trace
• yada, yada, yada

37
strtok

• strtok - extract token from string
• include
• char strtok
• (char s, const char delim)
• A token' is a nonempty string of characters not
  occurring in the string delim, followed by \0 or
  by a character occurring in delim.
• The strtok() function can be used to parse the
  string s into tokens. The first call to strtok()
  should have s as its first argument. Subsequent
  calls should have the first argument set to NULL.
  Each call returns a pointer to the next token, or
  NULL when no more tokens are found.

38
strtok

• strtok - extract token from string
• include
• char strtok
• (char s, const char delim)
• If a token ends with a delimiter, this delimiting
  character is overwritten with a \0 and a pointer
  to the next character is saved for the next call
  to strtok. The delimiter string delim may be
  different for each call.
• Never use this function. This function modifies
  its first argument. The identity of the
  delimiting character is lost. This function
  cannot be used on constant strings.
• The strtok() function returns a pointer to the
  next token, or NULL if there are no more tokens.

39
Question

• It's highly probable that strtok contains a
  variable of what storage class?
• static
• register
• extern
• manual
• volatile

40
Questions?
41
Library Fun Facts
42
What are .a files?

• They're libraries (collections of .o files,
  basically). Most libraries live in /usr/lib.
  When you specify "-lm" to cc/gcc at link time, it
  reads that as "use a library named 'm'" and then
  expands 'm' by wrapping 'lib' and '.a' around it
  to make 'libm.a' (which it finds in /usr/lib).
• The library stuff is pretty natural for the
  installed libraries in /usr/lib. For
  special-purpose libraries, it's a little more
  cumbersome because you have to add two flags to
  the linking command
• -L/the/directory/with/the/lib/files
• and, for example
• -lpgm
• if you want to use libpgm.a in that directory.

43
Questions?
44
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