where am i going wrong?

nkhambal · #1 01-Aug-2004, 14:16

Hi,

I wrote following program to read a string from user containing words seperated by spaces and count the total no of words in the string and display each word separately .i.e.say a user entered string as

"my name is wilson" the programs should output.

total words:=4
word 1:=my
word 2:=name
word 3:=is
word 4:=wilson

following is my program.When I run it i get segmentation fault at the point mentioned in program.

CPP / C++ / C Code:

#include <stdio.h>
#include <ctype.h>

void search (char *str,char *tokens[]);
char *tokens[100]={"\0"};
int main()
{
	int i,j,k,count,total;
	char ch1,ch2;
	char list[200];
	i=0;
	printf("\n\nEnter the string with words seperated by white space now...\n\n");
	while ((ch1=getchar())>=0)
	{
		list[i]=ch1;
		i++;
	}
	list[i]='\\';
	count=0;
	total=0;
	printf("\n");
	search(list,tokens);
	return 0;
}

void search (char str[],char *tokens[])
{
	int i,j,count,total,wordcount;
	total=wordcount=count=0;
	for (j=0;j<(strlen(str));j++)
	{
		if (!(isspace(str[j])) && !(str[j] == '\\'))
		{
			/*printf("total= %d\tcount:= %d\t",total,count);
			printf("Str[%d]:= %c\n",j,str[j]);*/
			tokens[total][count]=str[j];			// Segmentation Fault at this point works fine if i comment this with above 2 printfs.
			/*printf("%c",tokens[total][count]);*/
			count++;
		} else {
			count=0;
			if ((isspace(str[j-1])) && str[j] == '\\')
			{
				break;
			} else {
			total++;
			}
		}
	}
	printf("\ntotal in function:= %d\n",total);
	
	for (i=0;i<=total;i++)
	{
		printf("Word[%d]:= ",i+1);
		for (j=0;j<(strlen(tokens[i]));j++)
		{
			tokens[i][j]=toupper(tokens[i][j]);
			printf("%c",tokens[i][j]);
		}
		printf("\n");
	}
}

Can some suggest what am doing wrong here ?

Thanks,

WaltP · #2 01-Aug-2004, 19:07

Quote:

Originally Posted by nkhambal

Hi,

I wrote following program to read a string from user containing words seperated by spaces and count the total no of words in the string and display each word separately .i.e.say a user entered string as

"my name is wilson" the programs should output.

total words:=4
word 1:=my
word 2:=name
word 3:=is
word 4:=wilson

following is my program.When I run it i get segmentation fault at the point mentioned in program.

Can some suggest what am doing wrong here ?

Thanks,

You have defined a buffer of pointers to tokens:

CPP / C++ / C Code:

char *tokens[100]={"\0"};

all pointing nowhere.

Then in the search function you use

CPP / C++ / C Code:

tokens[total][count] = str[j];

but there is no space defined at tokens[total][count] so you are placing str[j] in the middle of nowhere. And that causes your seg fault.

nkhambal · #3 02-Aug-2004, 11:38

Hello waltp,

I declared it the following way and it worked.

CPP / C++ / C Code:

char tokens[25][100];

Program gives expected output.

my name is wilson
Entered String is: my name is wilson

Total No of words:= 4
Word[1]:= MY
Word[2]:= NAME
Word[3]:= IS
Word[4]:= WILSON

Thanks,

davekw7x · #4 02-Aug-2004, 14:43

Without commenting on your style, here are my observations:

I see a couple of problems: one is obvious (and easy to fix); the other is extremely important, but maybe not so obvious.

The first problem: if you have found four words, for example, then your loop prints out five words. Maybe you have fixed it already.

If not, try changing

CPP / C++ / C Code:

 

  for (i=0;i<=total;i++)

to

CPP / C++ / C Code:


  for (i=0; i < total; i++)

Now, for the important one: in your search() function, you have

CPP / C++ / C Code:

 
  for (j=0;j<(strlen(str));j++)

In order to use strlen() properly, the character sequence must be null-terminated, and your input routine doesn't put the '\0' character in the buffer.

The following works for me:

upon exiting the while() loop, change

CPP / C++ / C Code:

  list[i]='\\';
  count=0;

to

CPP / C++ / C Code:

  
  list[i]='\\';
  list[i+i] = '\0';
  count=0;

Dave

nkhambal · #5 03-Aug-2004, 02:22

Quote:

Originally Posted by davekw7x

Without commenting on your style, here are my observations:

I see a couple of problems: one is obvious (and easy to fix); the other is extremely important, but maybe not so obvious.

The first problem: if you have found four words, for example, then your loop prints out five words. Maybe you have fixed it already.

If not, try changing

CPP / C++ / C Code:

 

  for (i=0;i<=total;i++)

to

CPP / C++ / C Code:


  for (i=0; i < total; i++)

Now, for the important one: in your search() function, you have

CPP / C++ / C Code:

 
  for (j=0;j<(strlen(str));j++)

In order to use strlen() properly, the character sequence must be null-terminated, and your input routine doesn't put the '\0' character in the buffer.

The following works for me:

upon exiting the while() loop, change

CPP / C++ / C Code:

  list[i]='\\';
  count=0;

to

CPP / C++ / C Code:

  
  list[i]='\\';
  list[i+i] = '\0';
  count=0;

Dave

Hi Dave,

Yes,I have fixed the problem with additional string being displayed.

And secondly,i use "\" to mark the end of the user entered string.Hence I read the string in search fucntion till i hit "\".So i actually dont need "\0" to end the string here.Anyway,thanks for the suggestion

davekw7x · #6 03-Aug-2004, 07:36

Quote:

Originally Posted by nkhambal

Hi Dave,

Yes,I have fixed the problem with additional string being displayed.

And secondly,i use "\" to mark the end of the user entered string.Hence I read the string in search fucntion till i hit "\".So i actually dont need "\0" to end the string here.Anyway,thanks for the suggestion

I can't tell you how important it is for you to understand that your use of strlen() is dangerous, since it is operating on something that is not a null-terminated sequence of characters.

The function strlen() looks at memory, starting at the pointer value that you give it and goes until it sees a '\0'. So --- it might go into never-never land (or maybe not, depending on what happens to be in the memory at the time the program is run). Never-never land is famous for giving exception errors from time to time; sometimes the program appears to work OK, and sometimes it crashes.

My point is: this is a Bad Thing to do, even if it looks like it works. Someday, somehow this kind of thing will leap up and bite you.

As a matter of programming style, why did you use '\\' to terminate the string; why not just use '\0'? Then there's no question of using strlen() or any other standard library C functions.

Regards,

Dave

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