Segmentation Fault

[Strat79]

This program is a double-linked list in C for my System's Software class.
Everything worked fine, until I coded the delete() and insert() functions.

The program compiles and executes on a window machine using Dev C++.
It also compiles without any errors or warnings on a linux machine using
gcc -g scan.c P3.c

However, when executing the a.out file, the program crashes with a segmentation fault.
My TA and I tried to find the bug for hours today, but to no avail. I would appreciate any help. Thanks.

[Ancient Dragon]

struct Object *delete(struct Object *start, int num)
{
      int i = 1;

counting in C and C++ always starts with 0, not 1.

I didn't get seg fault

The 7-th number in the list NUMBERS is '2'.
Deleted 10th element in list IDS.
Inserted 7-th element of NUMBERS into 10th index in list IDS.

IDS linked list:
I, 7, include
I, 5, stdio
I, 1, h
I, 7, include
I, 6, stdlib
I, 1, h
I, 6, define
I, 3, XYZ
I, 3, int
N, 1, 2
I, 4, main
I, 4, argc
I, 4, char
I, 4, argv
I, 3, int
I, 1, i
I, 1, j
I, 1, k
I, 3, int
I, 6, array1
I, 6, array2
I, 4, FILE
I, 5, Input
I, 5, fopen
I, 4, argv
I, 2, rb
I, 4, FILE
I, 6, Output
I, 5, fopen
I, 4, argv
I, 1, w
I, 3, int
I, 4, temp
I, 3, Int
I, 5, count
I, 3, int
I, 4, temp
I, 5, Array
I, 4, MaxL
I, 4, char
I, 3, str
I, 2, if
I, 5, Input
I, 4, NULL
I, 5, while
I, 4, feof
I, 5, Input
I, 6, fscanf
I, 5, Input
I, 1, d
I, 4, temp
I, 3, Int
I, 4, temp
I, 5, Array
I, 1, i
I, 4, temp
I, 3, Int

The 3-rd separator in the list SEPS is '<'.
Deleted 5th element in list NUMBERS.
Inserted 3-rd element of SEPS into 5th index in list NUMBERS.

NUMBERS linked list:
N, 2, 16
N, 1, 0
N, 1, 0
N, 7, 123.123
S, 1, <
N, 1, 0
N, 1, 2
N, 1, 0
N, 1, 0
N, 2, 10

Inserted '&&' into 1st index in list SEPS.

SEPS linked list:
S, 2, &&
O, 1, #
S, 1, white space
O, 1, <
O, 1, .
O, 1, >
S, 1, white space
O, 1, #
S, 1, white space
O, 1, <
O, 1, .
O, 1, >
S, 1, white space
O, 1, #
S, 1, white space
S, 1, white space
S, 1, white space
S, 1, white space
O, 1, (
S, 1, white space
O, 1, ,
S, 1, white space
S, 1, white space
O, 1, *
O, 1, [
O, 1, ]
O, 1, )
S, 1, white space
O, 1, {
T, 1, new line character
S, 1, white space
S, 1, white space
O, 1, =
S, 1, white space
O, 1, ,
S, 1, white space
S, 1, white space
O, 1, =
S, 1, white space
O, 1, ,
S, 1, white space
S, 1, white space
S, 1, white space
O, 1, =
S, 1, white space
O, 1, ;
S, 1, white space
S, 1, white space
O, 1, *
O, 1, ,
S, 1, white space
O, 1, *
O, 1, ;
S, 1, white space
S, 1, white space
O, 1, *
S, 1, white space
O, 1, =
S, 1, white space
O, 1, (
O, 1, [
O, 1, ]
O, 1, ,
S, 1, white space
O, 1, "
O, 1, "
O, 1, )
O, 1, ;
S, 1, white space
S, 1, white space
O, 1, *
S, 1, white space
O, 1, =
S, 1, white space
O, 1, (
O, 1, [
O, 1, ]
O, 1, ,
S, 1, white space
O, 1, "
O, 1, "
O, 1, )
O, 1, ;
T, 1, new line character
S, 1, white space
O, 1, _
S, 1, white space
O, 1, =
S, 1, white space
O, 1, ,
S, 1, white space
S, 1, white space
O, 1, =
S, 1, white space
O, 1, ;
S, 1, white space
S, 1, white space
O, 1, _
O, 1, [
O, 1, ]
O, 1, ;
S, 1, white space
S, 1, white space
O, 1, [
O, 1, ]
O, 1, ;
S, 1, white space
O, 1, (
S, 1, white space
O, 1, !
O, 1, =
S, 1, white space
O, 1, )
O, 1, {
S, 1, white space
O, 1, (
O, 1, !
O, 1, (
O, 1, )
O, 1, )
T, 1, new line character
O, 1, {
S, 1, white space
O, 1, (
O, 1, ,
S, 1, white space
O, 1, "
O, 1, %
O, 1, "
O, 1, ,
S, 1, white space
O, 1, &
O, 1, _
O, 1, )
O, 1, ;
S, 1, white space
O, 1, _
O, 1, [
O, 1, ]
S, 1, white space
O, 1, =
S, 1, white space
O, 1, _
S, 1, white space
S, 1, white space
O, 1, ;
S, 1, white space
S, 1, white space
S, 1, white space
S, 1, white space


UNKNOWN linked list:
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character

Now delete half the objects in list UNKNOWN
Deleted 8-th element in list UNKNOWN.
Deleted 7-th element in list UNKNOWN.
Deleted 6-th element in list UNKNOWN.
Deleted 5-th element in list UNKNOWN.

New UNKNOWN linked list:
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character
U, 1, unknown character

Press any key to continue . . .

[rpiper138]

What did your core file tell you?

I don't have a linux machine to test it on and it is fine on my Sun, but it is probably a different architecture than you are using.

Generally Unix segmentation faults on a free are caused by over running a dynamic allocation. The failure will occur on a call to realfree. Once you find the allocation that is failing, you have to trace it back to the point where the array bounds were over written.

Try running Rational Purify on it if you can get a copy. It will show up as an array bounds write (ABW) problem.

[ArkM]

Regrettably I have no time now to inspect your code more carefully but your list() initialization function is obviously wrong. It does initialize nothing because of it modifies by value copies of head/tail pointers only. It must have double indirection parameters to modify its arguments.

[stilllearning]

You could try using valgrind on Linux to see if you have any memory errors. See http://valgrind.org/ .

[ahamed101]

Hi,
I am not able to run the program in Windows using Eclipse, it give me error...
To get more info on core file, comile your sources with -g option like
cc P3.c -c -g P3.o
and then when the core is generated run the command
gdb a.out core
It will give you the details as in where the segmentation fault occured... You may want to use where command when in gdb mode...

Will analyse your code and let you know whats the issue...