Why does some characters change? - c

Question

I'm programming with C language in CodeBlocks with GNU GCC compiler.I was writing a function to create some Link List consisting token as nodes.for example, for the below text file:
main ( )
{
int a ;
int b ;
}
the link list of tokens would be
main -> ( -> ) -> { -> int -> a -> ; -> int -> b -> ; -> }
for which the delimiter is the space character.
Then i decided to make some other link list called line. Each line consisting successive tokens separated by space finished by ; character. For example , at the same text file with the relevant tokens the lines would be:
main ( ) { int a ; -> int b ;-> }
you see my code below:
//including the related Header files
typedef struct token {
char *tok;
struct token *next;
int tp;
} token;
typedef struct line {
char *ls;
struct line *next;
int parent;
} line;
token *start;
line *lstart;
void addline (line * a);
void showline (void);
void setline (void);
int main (void ) {
int i = 0;
// the next 4 lines allocates some space for start(pointer of type token)
// and lstart(pointer of type line) as the first node in the link
// list.The first meaningful data of each type are stored in the nodes
// after the start and lstart node
start = (token *) malloc (sizeof (token));
start->next = NULL;
lstart = (line *) malloc (sizeof (line));
lstart->next = NULL;
FILE *p;
p = fopen ("sample.txt", "r+");
if (p == NULL) {
printf ("Can Not Open File");
exit (1);
}
//calling some fnuction for making link list of tokens from the text
//file
setline ();
showline ();
return 0;
}
// the relevant add functions which adds a new token or
// link list at the end of the list
void showline ()
{
line *p;
p = lstart->next;
while (p != NULL) {
printf ("%s\n", p->ls);
p = p->next;
}
}
void setline (void)
{
int parent;
token *p;
p = start->next;
line *q;
q = (line *) malloc (sizeof (line));
q->ls = NULL;
while (p != NULL) {
if (p == NULL) {
break;
}
q->ls = strdup (p->tok);
strcat (q->ls, " ");
p = p->next;
while ((p != NULL)) {
if (strcmp (p->tok, ";") == 0) {
strcat (q->ls, "; ");
p = p->next;
break;
}
strcat (q->ls, p->tok);
strcat (q->ls, " ");
p = p->next;
}
printf ("%s\n", q->ls);
addline (q);
q->ls = NULL;
}
}
and I stored some data in the text file "sample.txt" :
#include <something.h>
int a , b ;
main ( )
{
int a ;
int b ;
}
I expected lines will be made correctly but something strange happened when I called showline() function (This function is used and can be seen in the main ):In some lines there was some strange characters.For example the ls of the second line node was expected to be int b ; But what Really happened was înt b ; in which the usual i character turned into î(a strange character) .Which mistake did i make when working with strings?

Answer 1

One of the problematic places:
q->ls=strdup(p->tok);
strcat(q->ls," ");
The strdup function allocates enough space for p->tok only, there's no space to append anything after the copies string. So calling strcat will of course write out of bounds and you will have undefined behavior.
If you want to append more characters, you need to allocate yourself (using malloc or calloc) with the size you need, and then manually copy the initial string.