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Segmentation Fault

drumgirl67
P: 4
I am getting a segmentation fault in a function in a C++ program. "fields" is a two dimensional array that was passed to the function. Each "row" in fields is a 32 character array, and the total number of "rows" is numfields, also passed to the function. I have double checked the number and it's correct.

The segmentation fault is occurring at the strncpy. I have also tried strcpy and the commented out code.

Expand|Select|Wrap|Line Numbers
  1.     for(int x = 0; x < numfields; x++) {
  2.         int mismatch = 1;
  3.         int y = 0;
  4.  
  5.         while( y < numselect && mismatch != 0 ) {
  6.             char fieldname[11];
  7.             cout << "before for loop" << endl;
  8.             cout.flush();
  9.  
  10. //            for(int z = 0; z < 10; z++)
  11. //                fieldname[z] = fields[x][z];
  12. //            fieldname[10] = '\0';
  13.             strncpy(fieldname, fields[x], 10);
  14.  
  15.             mismatch = strcmp(fieldname, selFields[y]);
  16.             if(mismatch == 0)
  17.                 select[x] = true;
  18.             y++;
  19.         }
  20.     }
Please let me know if you can see something that I don't, or if I haven't given enough info.

The segmentation fault occurs on the first iteration of the loops when x and y are both 0.

Thanks.
Dec 13 '07 #1
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6 Replies


gpraghuram
Expert 100+
P: 1,275
I am getting a segmentation fault in a function in a C++ program. "fields" is a two dimensional array that was passed to the function. Each "row" in fields is a 32 character array, and the total number of "rows" is numfields, also passed to the function. I have double checked the number and it's correct.

The segmentation fault is occurring at the strncpy. I have also tried strcpy and the commented out code.

Expand|Select|Wrap|Line Numbers
  1.     for(int x = 0; x < numfields; x++) {
  2.         int mismatch = 1;
  3.         int y = 0;
  4.  
  5.         while( y < numselect && mismatch != 0 ) {
  6.             char fieldname[11];
  7.             cout << "before for loop" << endl;
  8.             cout.flush();
  9.  
  10. //            for(int z = 0; z < 10; z++)
  11. //                fieldname[z] = fields[x][z];
  12. //            fieldname[10] = '\0';
  13.             strncpy(fieldname, fields[x], 10);
  14.  
  15.             mismatch = strcmp(fieldname, selFields[y]);
  16.             if(mismatch == 0)
  17.                 select[x] = true;
  18.             y++;
  19.         }
  20.     }
Please let me know if you can see something that I don't, or if I haven't given enough info.

The segmentation fault occurs on the first iteration of the loops when x and y are both 0.

Thanks.
Hi,
Can you post the calling function which is calling this.
I am asking for a asample code which is calling this in which the seg error is happening,.

Raghuram
Dec 13 '07 #2

drumgirl67
P: 4
Hi,
Can you post the calling function which is calling this.
I am asking for a asample code which is calling this in which the seg error is happening,.

Raghuram
I ended up going with a new algorithm to avoid the segmentation fault. Thanks for the reply, though.
Dec 13 '07 #3

P: 35
Passing 2-D array to a function. I think you need to check it out yourself.

I don't think you need to specify the 1st dimension in the calling function, which u mentioned u passed in to the function. The compiler would just ignore it , the important one is the 2nd dimension. Segmentation fault might be encountered due to the improper passing of argument to your function which then access those memory location that you are not allow to.

i am not sure this would help you, just try to give some opinion ^.^
Dec 13 '07 #4

drumgirl67
P: 4
Well I came across the problem again...

Here is what is relevant in main:
Expand|Select|Wrap|Line Numbers
  1. #include <iostream>
  2. #include <fstream>
  3. #include <ctime>
  4. using namespace std;
  5.  
  6. int getInput(char*, char*, char*, char**);
  7. void getFieldInfo(char**, fstream &, int, bool*, char**, int);
  8.  
  9. int main() {
  10.     char** selectfields;
  11.     char** fieldnames;
  12.     bool* select;
  13.         int numselect;
  14.  
  15.     numselect = getInput(tablename, wherefield, wherevalue, selectfields);
  16.  
  17.         // some code that determines headersize, not really important
  18.     int numfields = ((headersize-1)/32)-1;
  19.  
  20.     getFieldInfo(fieldnames, file, numfields, select, selectfields, numselect);
  21.  
  22.     return 0;
  23. }
Now in getInput (selectNum is getting the correct value, I have checked that):
Expand|Select|Wrap|Line Numbers
  1. int getInput(char* tablename, char* wherefield, char* wherevalue, char** selectfields) {
  2.     char command[81];
  3.  
  4.     cout << "Command >> ";
  5.     cin.getline(command,80);
  6.  
  7.     int numSelect = 1;
  8.     int x = 0;
  9.  
  10.     while( x < 81 && command[x] != '\0' ) {
  11.         if( command[x] == ',' )
  12.             numSelect++;
  13.         x++;
  14.     }
  15.  
  16.     selectfields = new char*[numSelect];
  17.     for( x = 0; x < numSelect; x++ ) {
  18.         selectfields[x] = new char[20];
  19.     }
  20.  
  21.     int position = 7;
  22.     int fieldpos = 0;
  23.     x = 0;
  24.     while( x < numSelect ) {
  25.         if(command[position] == ' ' || command[position] == ',') {
  26.             selectfields[x][fieldpos] = '\0';
  27.             position++; 
  28.             x++; 
  29.             fieldpos = 0;
  30.             while(x < numSelect && (command[position] == ' ' || command[position] == ','))
  31.                 position++;
  32.         }
  33.         else {
  34.             selectfields[x][fieldpos] = command[position];
  35.             fieldpos++;
  36.             position++;
  37.         }
  38.     }
  39.  
  40.     return numSelect;
  41. }
The command I am entering now only has 1 select field, and when I cout selectfields[0] in the previous function it works.

Now for whatever reason, when I go to the next function that's called in main, selectfields[0] causes a segmentation fault.

Expand|Select|Wrap|Line Numbers
  1. void getFieldInfo(char** fieldnames, fstream &file, int numfields, bool* select, char** selFields, int numselect) {
  2.  
  3.     cout << "0: " << selFields[0] << endl;
  4.     cout.flush();
  5.  
  6. // some commented out stuff
  7.  
  8. }
Dec 13 '07 #5

Expert 100+
P: 849
A char** is not a 2D array. A char*[] is, and you do still have to supply a size here, I believe. There's a very good article on this in the Howtos section that you probably ought to read.
Dec 13 '07 #6

drumgirl67
P: 4
A char** is not a 2D array. A char*[] is, and you do still have to supply a size here, I believe. There's a very good article on this in the Howtos section that you probably ought to read.
Thanks, I was under the impression that they were the same thing. I have fixed it now.
Dec 13 '07 #7

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