Question

I’m relatively new to C (and completely new to StackOverflow – hey guys!), and this segfault has been giving me no surcease of sorrow for the past few hours (DevC++ on a windows machine). It’s just a simple palindrome prime program, but it’s really giving me a hard time. I’m not generally a novice programmer like it seems here, but… Good god. Now I remember why I wanted to get away from C++ and to Python so quickly.

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

FILE *outputFile;                                           
char buffer[81];
char* strrev();                                            
int bytesWritten;                                           
char* strI = 0;   


char *strrev(char str[])
{
   char *p1 =NULL;
   char *p2 =NULL;  

  if (! str || ! *str)
        return str;
  for (p1 = str, p2 = str + strlen(str) - 1; p2 > p1; ++p1, --p2)
  {
        *p1 ^= *p2;
        *p2 ^= *p1;
        *p1 ^= *p2;
  }
  return str;
}

main()                                                       
{ 
    int isPrime(int);

    int i,j;                                                                     
    outputFile = fopen("DD:OUTPUT", "w");                       
    if (outputFile == NULL)                                     
    {                                                           
       printf("open error:   %d/%s\n", errno, strerror(errno));   
       exit(99);                                                  
    }                                                                                                                          
for (i=1; i<15000; i++)                                  
{   

if (isPrime(i)==1)                                    
{                                                      
 bytesWritten = sprintf(buffer,"%d is primepal!\n",i);    
 fwrite(buffer, 1, bytesWritten, outputFile);          
}                                                      
}                                                                         
fclose(outputFile);                                       

return 0;                                                
}    

int isPrime(int myInt)                              
{                                                    

int loop;                                           

for (loop = 2; loop < myInt/2+1; loop++)
  sprintf(strI, "%s%d", 10, myInt);           
  {                                                   
      if (myInt%loop==0 && (atoi(strrev(strI))-myInt)==0)
      {                              
      return 0;                                       
  }                                                   
  return 1;                                         
  }
}                                                    

I apologize ahead of time if this is a dumb question, and the answer is very obvious — but I’ve officially hit the limit where no matter how logical an answer, I’ve been coding the same problem for too long for it to make any sense. And also, segfaults are horrible beasts. Thank you ahead of time for anything you have to offer!

~ Jordan

Answer 1

The line sprintf(strI, "%s%d", 10, myInt); is likely crashing.

• You have not allocated any space for strI, it’s defined as char* strI = 0; Make it a char[64] , or a suitable size.
• You’re giving the wrong arguments to sprintf, “%s%d” says the first parameter should be a string (“%s”) , but you give it an int. Change %s to %d

Some other issues:

• Don’t use *p1 ^= *p2; hack to to swap variables, there’s many cases where this does not work. Do it properly with a temp variable.
• main() calls isPrime(), but there’s no prototype for isPrime at that time. Place int isPrime(int myInt); somewhere before main().
• The prototype for your strrev function should be char *strrev(char str[]); and not char *strrev()