CMSC 104 Spring 2014, Section 02, Lecture 22 Jason Tang

Pointers
CMSC 104 Spring 2014, Section 02, Lecture 22
Jason Tang
Topics
• Pointers
• Pointer Arithmetic
• String Pointers
Indirect Variables (Review)
• What if the value of a variable X happens to be the
address of another variable Y?
• If one does not change value of X, but instead changes
the memory that X is indirectly referencing, then Y
will change!
• Recall that every variable refers to a unique memory
address
Pointer
Where have I
heard this before?
• Special type of variable whose value is the address of
some other piece of memory
• If pointing to nothing at all, it has the value NULL
• NULL is not the nul character (‘\0’)
• Use & operator to get the address of a variable
• Use * operator to “dereference” a pointer
• Dereferencing NULL will cause program to die
Example of a Pointer
#include <stdio.h>
int main(void) {
char s[] = "Hello, there!";
char *t = &s[0];
printf("%s\n", t);
return 0;
}
Example of a Pointer
#include <stdio.h>
int main(void) {
char s[] = "Hello, there!";
char *t = &s[0];
t “points” to the
printf("%s\n", t);
first element of s
return 0;
}
This works because %s format specifier (and
every other built-in string routine) accepts
either a char array or a char pointer
Why Use Pointers?
• Create “aliases” of other variables
• Effectively what an indirect variable really is in C
• Manipulate array contents
• Use dynamic memory (next week’s lecture)
Pointer Example 2
#include <stdio.h>
void f(int *jim, int *bob);
int main(void) {
int x = 5;
printf("x was %d\n", x);
f(&x, &x);
printf("x is now %d\n", x);
return 0;
}
void f(int *jim, int *bob) {
*jim = *jim + 10;
*bob = *bob * 3;
}
Pointer Example 2
void f(int *jim, int *bob) {
*jim = *jim + 10;
*bob = *bob * 3;
}
x
jim
bob
Indirectly Modifying array
Contents
#include <stdio.h>
int main(void) {
char s[] = "Hello, there!";
char *t = &s[0];
t = t + 4;
*t = ' ';
t = t + 3;
*t = ' ';
printf("%s\n", s);
return 0;
}
Pointer Arithmetic
...
t = t + 4;
*t = ' ';
t = t + 3;
*t = ' ';
...
s[]
H
*t
e
l
l
o
...
*(t + 4) = ' ';
*(t + 7) = ' ';
...
,
*(t + 4)
t
*(t + 7)
h
e
r
e
! \0
Crashing Programs
#include <stdio.h>
int main(void) {
char s[] = "Hello, there!";
char *t = NULL;
/* *t = 'A'; <-- line A */
/* printf("%s\n", t); <-- line B */
return 0;
}
1. Compile and run this program as is.
2. Then uncomment line A, recompile, and run.
3. Then comment line A, uncomment line B, and run.
Crashing Programs
#include <stdio.h>
int main(void) {
char s[] = "Hello, there!";
char *t = NULL;
/* *t = 'A'; <-- line A */
/* printf("%s\n", t); <-- line B */
return 0;
}
• Dereferencing NULL or otherwise manipulating a NULL
pointer cause a “segmentation fault”
• Program will almost always crash and burn
Strings and Pointers
• Many built-in string functions return pointers to some
subset of an existing string:
• strchr()
• strstr(), strfry(), etc.
strchr()
• Returns pointer to first occurrence of target character
within a string, or NULL if that character not found
• NULL is not the \0 character
H
e
l
l
o
strchr(s, ‘l’)
,
t
h
e
r
strchr(s, ‘h’)
e
! \0
NULL
strchr(s, ‘x’)
strchr() example
#include <stdio.h>
#include <string.h>
int main(void) {
char buf[80], *s = buf;
fgets(buf, 80, stdin);
int count = 0;
while (s != NULL) {
s = strchr(s, ' ');
if (s != NULL) {
count = count + 1; s = s + 1;
}
}
printf("Number of spaces in input = %d\n", count);
return 0;
}