Error when concatenating strings in C

Question

I am new to C and I am trying to join 2 strings in a char array, using the following code:

char url[300];
strcpy(url, "http://example.com/script.php?email=example@example.com");
char * mail = lpObj->Email;
strcat(url, mail);

I am running this on a VPS with Visual Studio, and my program always crashes and it goes to a file with asm code. lpObj->Email; is assigned and it has a well-formed string.

Does somebody knows why this happens?

Answer 1

There are at least two cases that will cause error, the first case is when mail is too large for the destination url . I was able to get this code to seg fault but the length required to achieve the overflow may vary:

char url[50];
strcpy(url, "http://example.com/script.php?email=example@example.com");
char  *mail = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
strcat(url, mail);

The second case is if mail is NULL :

char  *mail = NULL ; 
strcat(url, mail) ;