Unable to get string using gets() in C

Question

I wanted to write a program to check which string has a greater length without using string functions. I tried to get the two strings as input using gets() but my program keeps on crashing. Please help! Thanks!

Here's my code:

#include <stdio.h>
int l1,l2;
char *st1,*st2;
void accept()
{
    gets(st1);
    gets(st2);
}
void length()
{
    int i = 0;
    while (st1[i] != '\0')
    {
        l1++; i++;
    }
    i = 0;
    while (st2[i] != '\0')
    {
        l1++; i++;
    }

}
int main()
{
    accept();
    length();
    if (l1 > l2)
        printf("String #1 is greater in length.\n");
    else
        printf("String #2 is greater in length.\n");
}

Answer 1

you have not allocated space to st1 or st2 nor have you initialized them... so they are both pointing to some unknown place in memory. Try...

char st1[1024];
char st2[1024];

That said, realize that gets is inherently unsafe as it is subject to buffer overrun attack; there's nothing to stop someone from entering a string longer than 1024 and crashing your program.

You can also greatly simplify the length() function as follows...

void length()
{
    for (l1 = 0; st1[l1] != '\0'; l1++ );
    for (l2 = 0; st2[l2] != '\0'; l2++ );
}

Expanding on this and your question about what's an alternative to gets(), the answer is to use something like fgets() -- for example...

int main( int argc, char** argv )
{
    if( fgets( st1, sizeof( st1 ), stdin ) != NULL )
    {
        if( fgets( st2, sizeof( st2 ), stdin ) != NULL )
        {
            length();

            if (l1 > l2) printf("String #1 is greater in length.\n");
            else if (l2 > l1) printf("String #2 is greater in length.\n");
            else printf( "Both strings are the same length.\n" );
        }
        else printf( "could not read second string\n" );
    }
    else printf( "could not read first string\n" );

    return( 0 );
}

In this case, fgets() will not allow the user to overflow st1 or st2 and it will ensure they are always null terminated strings.

Answer 2

use l2 in the second while loop,

    l1=0;
    while (st1[l1] != '\0')
    {
        l1++;
    }
    l2 = 0;
    while (st2[l2] != '\0')
    {
        l2++;
    }