Thread: Very odd segmentation fault/core dump.

So I'm pulling out my hair here, as I can't figure this out. I'm getting a segmentation fault ( core dump) in my program and I don't understand why. It's a clean compile and when I run the program and it asks me for a valid choice, the scanf statement causes the seg fault. The code is below, but I'm totally baffled here.

Code:

void MenuOne ( int numStrings, char **strArray )
{
   int i, inc = 1, userInput = 1, constant = 5;

   printf("\nMenuOne function opened. %d\n", numStrings );


      for ( i = 0; i < numStrings; i++ )
      {
            printf("strArray[%2d] - %s\n", i, strArray[i] );
      }

      printf ("Please select which string you'd like to analyze, or 0 to quit.\n");
      scanf ("%d", &userInput );
      printf("%d is the user's input\n", userInput);

      while( userInput < 0 ||  userInput > numStrings )
      {
         fprintf(stderr, "\nPlease enter a valid response. " );
         scanf("%d", &userInput );
      }

   MenuTwo ( userInput, strArray );
}

I just compiled and successfully executed your code and it works for me (although the array holds null values for me).

Here is my output:

MenuOne function opened. 5
strArray[ 0] - (null)
strArray[ 1] - (null)
strArray[ 2] - (null)
strArray[ 3] - (null)
strArray[ 4] - (null)
Please select which string you'd like to analyze, or 0 to quit.
3
3 is the user's input

Are you sure the seg. fault is during the scanf call?

I found I had a semicolon right after my for loop which was obviously a mistake, but that wasn't causing the seg fault. I have to think it's from that since it doesn't make it to my probe statement RIGHT after it.

Here's my pal.c file

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include "pal.h"

/* Display greeting message */
/* name: PrintGreeting      */
/* inputs: none             */
/* returned value: none     */
void PrintGreeting ()
{
   printf( "\nWelcome to the Strings program.");
   printf( "\nThis program will allow you to anaylze any of the strings");
   printf( "\nfrom the input file in several different ways.");
   printf( "\n\nAlso: a palindrome is defined as any word or phrase that is");
   printf( "\nread the same way backwards and forwards.  For example, the word");
   printf( "\nracecar is a palindrome.");

}

/* Display instructions         */
/* name: PrintInstructions      */
/* inputs: none                 */
/* returned value: none         */
void PrintInstructions ()
{
   printf( "\n\nTo use this program, simply select one of the strings you");
   printf( "\nwant to analyze, or 0 to quit.  After selecting a string");
   printf( "\na menu will appear which will enable you to test that string");
   printf( "\nin a number of different ways.  Select the corresponding");
   printf( "\nletter, or Q to quit.\n");
}

/* Reads in number of strings         */
/* name: ReadNumStrings               */
/* inputs: File pointer               */
/* returned values: number of strings */
int ReadNumStrings ( FILE *data )
{
   int numStrings = 0;
   char buffer[101];

   while ( fgets ( buffer, 101, data ) != NULL )
   {
      numStrings++;
   }

   return numStrings;
}

/* Fills the array with the strings from the data file */
/* name: FillArray                                     */
/* inputs: File pointer, array of strings              */
/* returned values: none                               */
void FillArray ( FILE *data, char **strArray, int numStrings )
{
   char buf[100];
   int counter = 0;
   int strLen;
   int i;

   rewind ( data );

   while ( fgets ( buf, 100, data ) != NULL )
   {
      strLen = strlen ( buf );
      strArray[counter] = (char *) malloc (strLen * sizeof (char));
      strcpy ( strArray[counter], buf );
      printf("%2d = %s\n", counter, strArray[counter]);
      counter++;
   }

   for ( i = 0; i < numStrings; i++ )
   {
      printf("strArray[%2d] = %s\n", i, strArray[i]);
   }

   printf("\nBeginning MenuOne function\n");
   MenuOne ( numStrings, strArray );
}

/* Controls the first level menu                 */
/* name: MenuOne                                 */
/* inputs: Number of strings, array of strings   */
/* returned values: none                         */
void MenuOne ( int numStrings, char **strArray )
{
   int i, userInput = -41;

   printf("\nMenuOne function opened. %d\n", numStrings );


      for ( i = 0; i < numStrings; i++ )
      {
            printf("strArray[%2d] - %s\n", i, strArray[i] );
      }

      printf ("Please select which string you'd like to analyze, or 0 to quit.\n");
      scanf ("%d", &userInput );
      printf("%d is the user input\n", userInput);

      while( userInput < 0 ||  userInput > numStrings )
      {
         fprintf(stderr, "\nPlease enter a valid response. " );
         scanf("%d", &userInput );
      }

   MenuTwo ( userInput, strArray );
}


/* Controls the second level menu                    */
/* Name: MenuTwo                                     */
/* inputs: user integer selection, array of strings  */
/* returned values: true/false ending condition      */
int MenuTwo ( int input, char **strArray )
{
   char select = 'Q';
   int end, numVowels, numCons, length, numWords, truefalse = 0;

   printf("\nMenuTwo just opened.\n");

   for ( end = 0; end < 19999; end++ )
   {
      printf("     \n   Analysis of the string :");
      printf("     \n   %s\n", strArray[input - 1] );
      printf("     \n   P - Is this string a palindrome ?");
      printf("     \n   V - How many vowels ?");
      printf("     \n   C - How many consonants ?");
      printf("     \n   L - What's its length ?");
      printf("     \n   W - How many words ?");
      printf("\n\n");
      printf("     \n   Q - Quit ");
      printf("\nEnter choice : ");
      scanf("%c", &select );
      
if( select == 'Q' || select == 'q' )
      {
         end = 20000;
      }
      else if( select == 'P' || select == 'p' )
      {
         truefalse = IsPalindrome( input, strArray );
         if ( truefalse == 1 )
         {
            printf( "\nThe following string is a palindrome : %s\n", strArray[input-1]);
         }
         else
         {
            printf(" \nThe following string is not a palindrome : %s\n", strArray[input-1]);
         }
      }
      else if( select == 'V' || select == 'v' )
      {
         numVowels = NumVowels ( input, strArray );
         printf( "\nThere are %d vowels in : %s ", numVowels, strArray[input-1]);
      }
      else if( select == 'C' || select == 'c' )
{
         numCons = NumCons ( input, strArray );
         printf( "\nThere are %d consonants in : %s ", numCons, strArray[input-1]);
      }
      else if( select == 'L' || select == 'l' )
      {
         length = strlen ( strArray[input-1] );
         printf("\n%d is the length of : %s", length, strArray[input-1] );
      }
      else if( select == 'W' || select == 'w' )
      {
         numWords = NumWords ( input, strArray );
         printf("\nThere's %d word(s) in : %s ", numWords, strArray[input-1] );
      }

   }

   return 1;
}


/* Tests to see if the string is a palindrome   */
/* name: IsPalindrome                           */
/* inputs: user input, array of strings         */
/* returned values: true/false statement        */
int IsPalindrome ( int input, char **strArray )
{
   int output;



   return output;
}


/* Determines the number of vowels in the string   */
/* name: NumVowels                                 */
/* inputs: user input, array of strings            */
/* returned values: number of vowels in string     */
int NumVowels ( int input, char **strArray )
{
   int numVowels;



   return numVowels;
}


/* Determines the number of consonants in the string   */
/* name: NumCons                                       */
/* inputs: user input, array of strings                */
/* returned values: number of consonants in the string */
int NumCons ( int input, char **strArray )
{
   int numCons = 0;
   int i;

   if ( strArray[input-1][i] == 'n' )
   {
      numCons++;
   }

   return numCons;
}


/* Determines how many words are in the string     */
/* name: NumWords                                  */
/* inputs: user input, array of strings            */
/* returned values: number of words in the string  */
int NumWords ( int input, char **strArray )
{
   int numWords;



   return numWords;
}

Here's my main file

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include "pal.h"

int main( int argc, char* argv[] )
{
   FILE *data;
   int numStrings;
   char** strArray;

   /* Ensures one data file is being entered at the command line  */
   if ( argc != 2 )
   {
      fprintf (stderr, "\nYou must enter the name of only one data file on the");
      fprintf (stderr, "\ncommand line to run this program." );
      fprintf (stderr, "\nExample: a.out filename\n");
      exit(-2);
   }

   data = fopen (argv[1],"r");

   /* Ensures the file is valid, and can be opened */
   if (data == NULL)
   {
      fprintf (stderr, "\nError opening %s\n", argv[1]);
      exit (-2);
   }

   PrintGreeting();
   PrintInstructions();

   numStrings = ReadNumStrings( data );

   FillArray ( data, strArray, numStrings );

   return 1;
}

And my header file:

Code:

/* Display greeting message */
/* name: PrintGreeting      */
/* inputs: none             */
/* returned value: none     */
void PrintGreeting() ;

/* Display greeting message    */
/* name: PrintInstructions     */
/* inputs: none                */
/* returned value: none        */
void PrintInstructions() ;

/* Reads in number of strings         */
/* name: ReadNumStrings               */
/* inputs: File pointer               */
/* returned values: number of strings */
int ReadNumStrings ( FILE *data ) ;

/* Fills the array with the strings from the data file */
/* name: FillArray                                     */
/* inputs: Number of strings, file pointer             */
/* returned values: none                               */
void FillArray ( FILE *data, char **strArray, int numStrings ) ;

/* Controls the first level menu                 */
/* name: MenuOne                                 */
/* inputs: Number of strings, array of strings   */
/* returned values: none                         */
void MenuOne ( int numStrings, char **strArray ) ;

/* Controls the second level menu               */
/* name: MenuTwo                                */
/* inputs: integer input, array of strings      */
/* returned values: true/false ending condition */
int MenuTwo ( int input, char **strArray ) ;

/* Tests to see if the string is a palindrome   */
/* name: IsPalindrome                           */
/* inputs: user input, array of strings         */
/* returned values: true/false statement        */
int IsPalindrome ( int input, char **strArray ) ;

/* Determines the number of vowels in the string   */
/* name: NumVowels                                 */
/* inputs: user input, array of strings            */
/* returned values: true/false statement           */
int NumVowels ( int input, char **strArray ) ;

/* Determines how many words are in the string     */
/* name: NumWords                                  */
/* inputs: user input, array of strings            */
/* returned values: number of words in the string  */
int NumWords ( int input, char **strArray ) ;

/* Determines how many consonants are in the string     */
/* name: NumCons                                        */
/* inputs: user input, array of strings                 */
/* returned values: number of words in the string       */
int NumCons ( int input, char **strArray ) ;

I get the follow results when I run the program:

Code:

strArray[13] - A man, a plan, a cat, a canal: Panama?

Please select which string you'd like to analyze, or 0 to quit.
0
Segmentation fault (core dumped)

The code is in the MenuOne function:

Code:

printf ("Please select which string you'd like to analyze, or 0 to quit.\n");
scanf ("%d", &userInput );
printf("%d is the user input\n", userInput);

Your compiler should be warning something like:

Code:

Warning: Possible usage of strArray before initialization in function main

That should be giving you a hint on the cause of the problem.

So strArray has to be initialized in main?

Edit: It compiles cleanly, no warnings/errors.

Tried that. Didn't seem to have any effect.

I did that. I'm not familiar with fflush, so I don't know what to expect but I didn't see any change.

OK, to put it more simply: You must allocate memory for strArray before trying to store objects in it.

Code:

   char ** strArray;
   /* At this point strArray points to a random memory address. Trying to dereference
      or write into random memory will lead to non-optimal results. Usually a crash. */

   ...

   numStrings = ReadNumStrings( data );

   /* Allocate memory for strArray before trying to use it */
   strArray = malloc(numStrings * sizeof(char *));

   if (strArray != NULL)
      FillArray ( data, strArray, numStrings );

OR, on the stack:

Code:

char * strArray[MAX_NUMBER_OF_STRINGS];

Just like you allocate memory for your character arrays before trying to store characters in them.