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First chance exception and string concatenation: two related questions

mrwad99
mrwad99 asked
on
Medium Priority
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Last Modified: 2010-04-15
Hello.

Please look at the following code, intended to read 10 chars from a file and then append it to a char array.  The end result is that the multiple calls to fgets should be represented as array y:

char y[30];
memset(y, ' ', 30);

FILE* ifp = fopen("input.txt", "r");

char sz[10];

while (strlen(y) != 30 && (!feof(ifp)) ) {
      fgets(sz, 10, ifp);
      strcat(y, sz);
}

fputs(y, stdout);

The input file is

line 1
line 2
line 3
line 4
line 5
line 6

and the output I get is

                              @line 1
line 2
line 3
line 4
line 5
line 6
line 6

So,

1) Why do I have the rubbish at the start of line 1 of the output ?
2) I get a first chance exception when run in debug mode *when the input file has one blank line at the end*.  This does not happen in release mode, and does not happen at all without the final blank line.  The code crashes at

if ( wchar > 255 )  /* validate high byte */

(XWCTOMB.c, line 116).

Any ideas ?  I know that in general first chance exceptions can be ignored, but just wondered why this is happening.

(25 points per question here)

TIA !
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Data Warehouse / Database Architect
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Commented:
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Author

Commented:
1) Right thanks Kent, it was the issue with initialising y to hold ' ' not '\0' that was causing the problem.
2) I am not testing for any type of character; the code I posted in the original question is all the code I have (less main() and a few #includes).  The line the code crashes on is in XWCTOMB.c, line 116: I found this out by running the debugger.  Anyway that is redundant now because with the new changes that does not happen.

So what I think y will look like is:

line 1\0line 2\0line 3\0line 4\0line5\0line 6\0

Or would it look like

line 1line2line 3line 4line 5line 6\0

After reading all the file in (with an appropriately sized char[])

TIA !
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Commented:
And I take it that is because when using strcat(), the first character of the string to be concatenated is placed over any preceeding '\0' ??
Yes.
And '\0' is placed at the end of the concatenated string.

Author

Commented:
Points split in favour of Kdo since he answered the primary questions; ankuratvb you get 20 of the 50 since you sorted my last query.

Thanks a lot both !
BTW,strcat() searches for the first '\0' and concatenates the second string from here.

Try this:

#include <string.h>
#include <stdio.h>

int main(void)
{
   char destination[25]="a\0b";
   char *blank = " ", *c = "C++", *turbo = "Turbo";

   strcat(destination, turbo);

   printf("%s\n", destination);
   return 0;
}

The result is "aTurbo".

Author

Commented:
Yep; thanks for solidifying that ankuratvb !  Greatly appreciated.
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