Help - builds, runs but crashes out at strcat

[buckgb]

Hello all, my first post ::) ???


I am running through some basic tutorials on C, using Code::Blocks. Can anyone help me out with the following code, with some explanation? It builds but crashes when run.

---------------
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define MAX_LEN 40

main()
{
int pos;
char *line,*aster,*blank;

line=(char *)malloc(MAX_LEN);
aster=(char *)malloc(1);
blank=(char *)malloc(1);
line=" ";
aster="*";
blank="";

printf("Enter position of star (1-40): ");
scanf("%d",&pos);

if(pos>0&&pos<41)
{
printf("\n");
printf(" 1 2 3 4\n");
printf("1234567890123456789012345678901234567890\n");
puts(strcat(strncat(blank,line,pos-1),aster));
}
else
printf("Out of range");
}
-------------------


The problem seems to lie within the strcat line of code and I guess the use of pointers?

Thanks,
Simon

[Chervil]

The destination field blank is too small. Given that it is only one byte long, it has just enough room to store the null terminator, that is, a zero length string.

Attempting to do any string operation with a 1-byte field as the destination is bound to fail.

Also, rather than using the = operator here: aster="*"; you need the strcpy() function. Or just declare char aster[] = "*";

I think line should probably be the destination of your strcat operation.

[buckgb]

Thanks Chervil. You were spot on with the blank pointer allocated length being too long, changed it for MAX_LEN+1.

I also encountered a problem when inserting "" into the pointers target as I now know this eliminates/overwrites the pointer address. I got around this by using blank[0]=0;

It compiled and worked following that. I didn't adjust aster. Its an early tutorial program, strcpy is upcoming so I will review that once I read through it.

Thanks again ;)