Getting gibberish instead of numbers using memcpy and strtoul

Question

I have the following piece of code compiling under gcc:

int parseMsg(const char *msg_to_parse, unsigned long *exp_input, unsigned long *sysTicks )
{
   int l_msg_size = strlen(msg_to_parse);
   if(l_msg_size <10)
          return -1;
    char l_exp_input_arr[10];
    char l_sys_ticks_arr[10];
    memcpy(l_sys_ticks_arr,msg_to_parse+12,10);

    memcpy(l_exp_input_arr,msg_to_parse,10);
   //l_msg_size = strlen(msg_to_parse);
    *sysTicks = strtoul(l_sys_ticks_arr,NULL,10);

   *exp_input = strtoul(l_exp_input_arr,NULL,10);



   return 0;
}

And I'm trying to test it in the following manner:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int parseMsg(const char *msg_to_parse, unsigned long *exp_input, unsigned long *sysTicks );

int main(void) {
char msg[] = "1234567890  59876543213";
unsigned long along1, along2;
along1 =0;
along2=0;
parseMsg(msg,&along1, &along2 );
printf("result of parsing: \n \t Along 1 is %lu \n \t Along 2 is %lu \n",along1, along2);
return 0;
}

But, I'm getting the following result:

result of parsing: Along 1 is 1234567890 Along 2 is 4294967295

Why is the second unsigned long wrong?

Thanks

Answer 1

The second integer you provide is too big to be represented in memory on your architecture. So, according to its API, strtoul is just returning you ULONG_MAX (=4294967295 on your architecture), along with setting errno to ERANGE

strtoul API is here : http://www.cplusplus.com/reference/clibrary/cstdlib/strtoul/

BUT it may also fail if you gave a smaller integer, because strtoul only stops parsing when it encounters a non-numerical character. Since you didn't ensure that, you cannot be sure that strtoul will not try to parse whatever is in memory just after your strings. (So assuming random, you have 10 chance out of 256 to have a conversion error)

Terminate your strings with \0, it will be ok then :

char l_exp_input_arr[11]; // +1 for \0
char l_sys_ticks_arr[11];

memcpy(l_sys_ticks_arr, msg_to_parse+12, 10);
l_sys_ticks_arr[10] = '\0';

memcpy(l_exp_input_arr, msg_to_parse, 10);
l_exp_input_arr[10] = '\0';

Answer 2

You need to make your two temporary char[] variables one char longer and then make the last character NULL.