[SOLVED] Char pointer on string malloc-ed, strcpy causes crash and burn

809areacode · 07-07-2012, 12:35 AM

Code:

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

struct junk {
   char * data;
};

int main() {
   struct junk * ljunk = malloc(sizeof(struct junk));
   strcpy(ljunk->data, "GREENLAND!");
   printf("%s\n", ljunk->data);
   return 0;
}

Segmentation fault.

What is happening? How do I fix this?

414N · 07-07-2012, 02:28 AM

You need to malloc ljunk->data too after the struct allocation.
Add this line before the strcpy:

Code:

ljunk->data = malloc(sizeof(*(ljunk->data)) * 100);

and the program should be working ok.
I put 100 but it's an overkill for the "GREENLAND!" string

dwhitney67 · 07-07-2012, 08:23 AM

A simpler approach is to use strdup(), not malloc() followed by strcpy(). Also, don't forget to free() allocated memory.

Something like:

Code:

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

struct junk {
   char * data;
};

int main() {
   struct junk * ljunk = malloc(sizeof(struct junk));
   ljunk->data = strdup("GREENLAND!");
   printf("%s\n", ljunk->data);

   
   free(ljunk->data);
   free(ljunk);
   

   return 0;
}

809areacode · 07-07-2012, 12:16 PM

No dice. Whenever I print the string I get segfault.

Code:

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

struct junk {
   char * data;
};

int main() {
   struct junk * ljunk = malloc(sizeof(struct junk));

   ljunk->data = malloc(sizeof(char) * 32); // I forgot to post this which is not a minor detail

   memcpy(ljunk->data, "GREENLAND!", strlen("GREENLAND!"));
   /* This doesn't blow it up */

   strcpy(ljunk->data, "GREENLAND!");
   /* This doesn't blow it up */

   ljunk->data = strdup("GREENLAND!");
   /* This doesn't blow it up */

   printf("%s\n", ljunk->data);
   /* But no matter what, this does */

   free(ljunk->data);
   free(ljunk);

   return 0;
   /* So we never get here */
}

Thanks for any help.

809areacode · 07-07-2012, 12:32 PM

NVM, I found the problem. It was another variable in the struct

struct junk {
double otherdata; /* this one */
char * data;
};

That was causing the sigsev in printf
...
printf("otherdata: %d\tdata: %s\n", ljunk->otherdata, ljunk->data);

Thanks for all help, though.

dwhitney67 · 07-07-2012, 02:15 PM

When printing 'otherdata', which has been declared as a double, you would want to use the "%g" format option, not the "%d" option.

ntubski · 07-08-2012, 11:34 AM

By the way, if you pass the right options (-Wformat, also enabled by -Wall) to gcc it will warn you about incorrect format strings.