Surviving C and PostgreSQL
CS186 Supplemental Session
9/13/04
Matt Denny, Paul Huang, Murali Rangan
(Originally prepared by Shariq Rizvi, Wei Xu,
Shawn Jeffery)
Outline
•
•
•
•
A Review of C
PostgreSQL Basics
Tour of Assignment 1
2Q
A Review of C
• Review Pitfalls of C Programming for
Java Programmers
• Pointers and Arrays
• Strings
• Segmentation Faults
• For more information, consult “ The C
Programming Language” by Kernighan
and Ritchie or tutorial on web site
• Pointer = variable containing address of another variable
float f;
float *f_addr;
f
/* data variable */
/* pointer variable */
f_addr
any float
?
?
?
4300
4304
any address
f_addr = &f; /* & = address operator */
f
f_addr
?
4300
4300
4304
*f_addr = 3.2;
f
/* indirection operator */
f_addr
3.2
4300
4300
4304
float g=*f_addr; /* indirection:g is now 3.2 */
f = 1.3;
f
f_addr
1.3
4300
4300
4304
Arrays and Pointers
int month[12]; /* month is a pointer to base address
430: system allocates 12*sizeof(int)
bytes at 430 */
month[3] = 7;
/* integer at address (430+3*sizeof(int))
is now 7 */
ptr = month + 2; /* ptr points to month[2],
i.e. ptr =(430+2 * sizeof(int)) =
438 */
ptr[5] = 12;
/* int at address (434+5*sizeof(int)) is
now 12; same as month[7] */
• Now , month[6], *(month+6), (month+4)[2],
ptr[4], *(ptr+4) are all the same integer variable.
Strings
#include <stdio.h>
main()
char
char
char
{
msg[10]; /* array of 10 chars */
*p;
/* pointer to a char */
msg2[]=“Hello”; /* msg2 = ‘H’’e’’l’’l’’o’’\0’ */
msg = “Bonjour”; /* ERROR. msg has a const address.
Think of space allocation*/
p
= “Bonjour”; /* address of “Bonjour” goes into p */
p = msg; /* OK */
p[0] = ‘H’, p[1] = ‘i’,p[2]=‘\0’;
/* *p and msg are now “Hi”
Warning: be careful if you have space
allocated for p!!*/
}
Segmentation Fault?
• You reference memory that the OS doesn’t
want you to
• What to check?
• Dereferencing a null pointer 99%
– Output or trace with a debugger
– Check array bounds