Strings

1
Strings

• CS240
• Dick Steflik

2
What is a string
A null terminated array of characters
char thisIsAString10
0 1 2 3 4
5 6 7 8 9
\0
The \0 (null character) in the 0th position
indicates the string is empty Remember that the
name of an array is a pointer to it's first
element thisIsAString is the same as
s where char s thisIsAString
3
scanf
scanf is used for reading data from stdin
according to some format specifier. For strings
the format specifier is s, scanf will read
characters from stdin , up to the first blank and
assign the string to the specified variable.
scanf(s,s) printf(d s \n,strlen(s),s)
given the input from stdin abc def
ghi the output will be 3 abc
The format specifier can have a modifier such as
12s, this indicated that the maximum number of
characters to be read are 12, given the prev
example the output will be the same, the read
stops at the first blank
4
gets
To read whitespace
gets(b) printf("d s\n",strlen(b),b)
given the input a b c d the output will
be 7 a b c d
Problem with gets is there is nothing to tell it
when to stop, the same code above with the
input a b c d e f g h i j k l m will
give the output 25 a b c d e f g h i j k l m it
does this by overflowing the array reserved for b
and overwriting whatever happens to be defined
next in memory. This is a classic buffer
overflow Even though gets is useful it is
unsafe and should be avoided
5
fgets
fgets(targetstring,maxlength,filepointer)
char b5 fgets(b,5,stdin)
printf(d s\n,strlen(b),b) given
the input aaaaaaaaaaa the output is
4 aaaa Successive fgets calls will keep reading
from stdin as long as there is input char
a10 , b10 fgets(b,10,stdin) printf("d
s\n",strlen(b),b) fgets(a,10,stdin)
printf("d s\n",strlen(a),a) given the
input aaaaaaaaaabbbbbbbbbbcccccccccc the
output is 9 aaaaaaaaa
9 abbbbbbbb
6
strcpy
char strcpy(char s1 , char s2) copies
s2 into the char array s1, s1 is returned
If s2 is longer than s1 then s1 will be
overflowed, indicating that you must be careful
using strcpy as it is unsafe instead use strncpy.

• strncpy(dst, src, dst_size-1)
• dstdst_size -1 \0 / just
  to be safe /
• 
  -or-
• / allocate the destination
  buffer when you need it /
• dst (char )malloc(strlen(src)
  1)
• srtcpy(dst, src)

7
strcat (string concatenation)
char strcat(char s1 , char s2)
append string s2 onto the end of s1. Same
problem as strcpy if s2 is longer than the array
defined for s1, the resulting string will
overflow s1 and write over whatever happens to be
defined in memory after s1. Better to use strncat
as strncat(dst, src, dst_size -
strlen(dst) -1)
8
strchr strrchr(search for a character)
char strchr(char s , char c) search
s for c return a pointer to first occurance,
return null if not found.
char strrchr(char s , char c)
search s for c return a pointer to last
occurance, return null if not found.
9
strcmp strncmp(comparing strings)
int strcmp(char s1 , char s2) // compares
s1 and s2 returns less than 0 if s1 is
lexically less than s2 0 if
s1 and s2 are lexically equal
greater than 1 if s1 is lexically greater than
s2 int strncmp(char s1 , char s2, int n)
// compares first n characters of
s1 and s2
10
strlen
Int strlen(char s1) returns the
length of the specified character string
11
strcasecmp strncasecmp
Case insensitive versions of strcmp and strncmp
same return values as strcmp and strncmp
12
strstr
char strstr(char s1 , char s2) find
first occurrence of s2 in s1, return pointer to
the occurrence
strpbrk
char strpbrk(char s1 , char s2) find
the first occurrence of any character in s2 in
s1, return a pointer to the occurrence or a
null if no character in s2 is found in s1
13
strspn
int strspn(char s1 , char s2) returns
the number of characters at the beginning of s1
that match s2
strcspn
int strcspn(char s1 , char s2) returns
the number of characters at the beginning of s1
that do not match s2
14
strtok (string tokenizer)
char strtok(char s1 , char s2)
repeated calls to strtok breakes the string s1
into tokens that are seperated by the
characters in s2), returns null if no more
tokens. Each returned token will ne null (\0)
terminated