segmentation fault when executing C code in ubuntu

Question

When I compile the c file, gcc returns no errors, but a segmentation fault occurs at runtime. The debugger showed that the fault occurs in this part of the code:

      int q=size[current];
      int *temp;
      temp = malloc ( sizeof(int)*q);
      for(i=0;i<size[current];i++)
        temp[i]=arr[current][i];

arr is defined :

      int arr[20][200];

current by default is 0.

I am trying to copy the contents of an array inside the 2d array (arr) to a temporary array, I've tried not using malloc but still the same problem.

ps: the program runs fine on windows.

What's `arr`? Can you give us something that compiles and exhibits the problem? — nmichaels, Dec 30 '11 at 15:10
Could you add definitions of current and arr? Also, I suggest you compile with all warnings enabled, if you don't yet, as it may give you a hint. — gnometorule, Dec 30 '11 at 15:11
i suspect the erro is in `arr[current][i]` ... make sure that the 1st dimmention of arr has `current` or more elements and the second dimmention of arr has `i` or more elements — A4L, Dec 30 '11 at 15:12
i defined arr as follows : int arr[20][200]; current is by default = 0 — Marwan Tushyeh, Dec 30 '11 at 15:14
how do you fill `size[current]` witch is the upper bound of `i` in your loop ? — A4L, Dec 30 '11 at 15:16
Where in your code is `current` modified? Where is `size[current]` set? — nmichaels, Dec 30 '11 at 15:21
Learn to compile with `gcc -g -Wall` and to use the `gdb` debugger. — Basile Starynkevitch, Dec 30 '11 at 15:45
You might explicitly check (and/or log) wether `size[current]` **does** contain a value `<=200` though. — alk, Dec 30 '11 at 16:43
Did you add code to check the return from `malloc()`? You should, because if it returns 0 because it was unable to allocate memory, then you will get a seg fault when you access the null pointer. — Jonathan Leffler, Dec 30 '11 at 17:18

score 3 · Answer 1 · answered Dec 30 '11 at 16:58

There is nothing wrong with the code you have shown (that I can see).

The first step you should take is run your program under Valgrind, and fix any errors it reports.

If Valgrind reports errors you don't understand, update your question with Valgrind output.

If Valgrind does not report anything, run your program under GDB, print values of i, current, size[current], etc. and update your question with your GDB session.

score 2 · Answer 2 · edited Dec 30 '11 at 20:04

2

I suspect that size[current] is greater than 200.

edited Dec 30 '11 at 20:04

Udo Held

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answered Dec 30 '11 at 19:53

user1123450

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segmentation fault when executing C code in ubuntu

2 Answers2