Segmentation fault with input test file

Hi, ive got a program that runs fine except when I run an input test file to compare differences with an output test file. It then gives me a segmentation fault. What are some of the causes of this?

ie.

a.out <input3> tmp
Segmentation fault (core dumped)

ARe you trying to access memory that doesn't belong to you? Post some code.

if you really have those < and >'s in there then of course it's going to fault
your redirecting input and output from each other and I doubt your program has something meant to handle REDIR

Post some code, then we can help you better

> if you really have those < and >'s in there then of course it's going to fault
Nothing wrong with the command - looks like a standard way for a program to read from one file, and write to another

> and I doubt your program has something meant to handle REDIR
Redirection is handled by the shell, not the program
All the program still sees is stdin and stdout

But for sure, bentles has some bugs, and should post some code.

there are 4 files: a header file, a list file, a main file and a another file doing the operations:

main.c....

list.c...

I know the program looks dodgy, but it works...well... mostly:(

Haven't looked at ya code yet, but have you tried putting some printf's at key points within the code? This'll at least give you a clue about where it's going wrong.

Quote:

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Originally posted by Hammer
Haven't looked at ya code yet, but have you tried putting some printf's at key points within the code? This'll at least give you a clue about where it's going wrong.

---

The code works fine and there doesnt seem to be any problems whatsoever, but when using an input test file, it gives a segmentation fault. There are two input test files, one works, but i cant get the other working.

Would still suggest you put from printf's in there to try and determine which function is causing the failure. Just add something like this to the start of each of function.

Code:

---

fprintf(stderr, "Starting function <insertfunctionnamehere>\n");

---

Have had a quick look through the code. I see you do some scanf's and strcpy's. Their bounds checking is dogdy, are you sure you test data doesn't exceed the array bounds?

Be careful here too:

Code:

---

while(strcmp(units[low]->name, unit_name) !=0)
{
      low++;
}

---

The unit_name variable is read from an external source and is therefore unrealiable. If the unit_name does not exist within your array (remember it's a case sensative search too), the low variable will eventually exceed the bounds for the units variable, and you'll crash&burn.

Quote:

---

Originally posted by Hammer
Would still suggest you put from printf's in there to try and determine which function is causing the failure. Just add something like this to the start of each of function.

Code:

---

fprintf(stderr, "Starting function <insertfunctionnamehere>\n");

---

Have had a quick look through the code. I see you do some scanf's and strcpy's. Their bounds checking is dogdy, are you sure you test data doesn't exceed the array bounds?

Be careful here too:

Code:

---

while(strcmp(units[low]->name, unit_name) !=0)
{
      low++;
}

---

The unit_name variable is read from an external source and is therefore unrealiable. If the unit_name does not exist within your array (remember it's a case sensative search too), the low variable will eventually exceed the bounds for the units variable, and you'll crash&burn.

---

This is the test data so i cant see how it could be the size of the unit_name array thats the problem:

1
a
1
b
1
c
6
a
4
999999 a
4
2 a41 a
6
a
5
1 a
5
2 a
6
a
0

Ive inputted all this data outside the file and it works ok....one thing ive noticed is that if i do this..copy and paste all the file info....it usually prints the output from an operation a few lines down. ie. for the operation 6 that prints out all units in a, it usually prints the empty line after the operation 4 which adds 999999 to a. I dont know if this is important....

can you post the header file.....

here

another thing is that the file works if i remove the last one of the '4' operations, which adds a student, which suggests the problem lies in the add_student function.

OK, there's a bug here:

Code:

---

void add_student(int loopnum)
{
   
   
    char unit_name[101];
    int low, student_ID;
    Position P;
    low=0;
   
    scanf("%d %s", &student_ID, unit_name);

    units[loopnum] = (unit_struct*) malloc(sizeof(unit_struct));
    units[loopnum]->name = malloc(sizeof(unit_name)+1);
 
    while(strcmp(units[low]->name, unit_name) !=0)
    {
     
      low++;
    } 
 
    if(units[low]->IDs == NULL)
    {
      units[low]->IDs = MakeEmpty(NULL);
      P = Header(units[low]->IDs);
    }
   
    InsertInOrder(student_ID, units[low]->IDs);

    printf ("AddStud: pointer P pointing to: %p\n", P);

    P = Advance(P);
           
}

---

The problem is that P is only assigned a value if (units[low]->IDs == NULL). If that condition is not true, P does not get assigned.

In either case, you use P in the call : P = Advance(P); which is wrong!

I put a printf statement in to display the contents of the pointer to prove it:

Code:

---

1
a
4
1
a
AddStud: pointer P pointing to: 0x970c10
4
2
a
AddStud: pointer P pointing to: 0x0
Segmentation fault (core dumped)

---

Also, the Advance function does nothing for this variable as it's local to the add_student function. I'm not sure what you're trying to do with it.

Have fun fixing......

..but if i remove that if statement, then the list is just going to be emptied every time that function is called isn't it?

Quote:

---

Originally posted by bentles
..but if i remove that if statement, then the list is just going to be emptied every time that function is called isn't it?

---

Memory allocated via calls to malloc will remain in existance until you free() it. You have to keep a pointer that points to the start of the list, but that can exist globally or be passed from function to function.

I haven't checked to see if your memory management is correct or not!

That particular call to the advance func certainly does nothing. P is declared within add_student, and is therefore destroyed when you leave that function. If it was containing a value you needed, you would have to either assign it to an external variable, or return it to the caller.

As a quick fix, I run the prog without that line, and it appeared to work OK.

I dont even need the p variable, its not serving any purpose whatsoever..

....doesnt fix the problem tho..

Oh thank you lord 1!!:D I figured it out. Thanks Hammer ! I fixed it by making a global variable position P in the header and used the following: P=First(units[low]->IDs after the InsertInOrder call in add_student to move the position to the next position.
WOOHOO!!