获取C ++分段错误

Question

I'm in a linux server and when I try to execute the program it's returning a segmentation fault. when i use gdb to try and find out why, it returns..

Starting program: /home/cups/k

Program received signal SIGSEGV, Segmentation fault.
0x0000000000401128 in search(int) ()
Missing separate debuginfos, use: debuginfo-install glibc-2.12-1.192.el6.x86_64 libgcc-4.4.7-17.el6.x86_64 libstdc++-4.4.7-17.el6.x86_64

I couldn't quite interpret this. In my program i have a function called "search()" but i don't see anything that would cause a seg fault. here's the function def:

int search (int bit_type) {                                               // SEARCH FOR A CONSEC NUMBER (of type BIT_TYPE) TO SEE IF ALREADY ENCOUNTERED

    for (int i = 1; i <= MAX[bit_type]; i++) {               //GO THRU ALL ENCOUNTERED CONSEC NUMBERS SO FAR (for type BIT_TYPE)
        if (consec == r[bit_type][i])                           // IF: FOUND
        return i;                                                                       //                      -----> RETURN INDEX OF RECORDED CONSEC_NUM
    }
    // IF: NOT FOUND
    r[bit_type][++MAX[bit_type]] = consec;                          //                      -----> INCREMENT MAX[bit_type]  &  RECORD NEW CONSEC_NUM -------> ARRAY[MAX]
    n[bit_type][MAX[bit_type]] = 1;
    return (MAX[bit_prev]);                                                          //                      -----> RETURN THE NEWLY FILLED INDEX 
}

global functions:

int MAX[2];
int r[2][200];
int n[2][200];

The comments are pretty useless to you guys since you don't have the rest of the program.. but you can just ignore them.

But do you guys see anything I missed?

Answer 1

From the link to your code here , here is just one error:

 int *tmp = new int[MAX[0]];
 for (int y = 0; y <= MAX[0]; y++) {
     tmp[y] = 1;
}

You are going out-of-bounds on the last iteration. You allocated an array with MAX[0] items, and on the last iteration you're accessing tmp[MAX[0]] .

That loop should be:

 int *tmp = new int[MAX[0]];
 for (int y = 0; y < MAX[0]; y++) {
     tmp[y] = 1;
}

or better yet:

 #include <algorithm>
    //...
    std::fill(tmp, tmp + MAX[0], 1);  // no loop needed

or skip the dynamic allocation using new[] and use std::vector :

  #include <vector>
  //...
  std::vector<int> tmp(MAX[0], 1);

---

In general, you have multiple loops that do this:

for (int i = 1; i <= number_of_items_in_array; ++i )

and then you access your arrays with array[i] . It is the <= in that for loop condition that is suspicious since it will try to access the array with an out-of-bounds index on the last iteration.

Another example is this:

long sum(int arr_r[], int arr_n[], int limit)
{
    long tot = 0;
    for (int i = 1; i <= limit; i++)
    {
        tot += (arr_r[i])*(arr_n[i]);
    }
    return tot;
}

Here, limit is the number of elements in the array, and you access arr_r[i] on the last iteration, causing undefined behavior.

Arrays are indexed starting from 0 and up to n - 1 , where n is the total number of elements. Trying to fake 1-based arrays as you're attempting to do almost always results in these types of errors somewhere inside of the code base.