Kochan InsertString segmentation fault - c

Question

I am working through Kochan's programming in C book and I am working on an exercise which requires a function to insert one character string inside another string, with the function call including where the string is to be inserted.
I have written the below code but I receive a segmentation fault whenever I enter the inputs. I think it's because the 'input' string is defined to the length of the user's input and then the insertString function tries to add additional characters to this string. I just can't see a way of defining the string as large enough to be able to take in additional characters. Do you think that this is the reason I am receiving a segmentation fault? Are there any other ways to go about this problem?
#include<stdio.h>
#include <string.h>
insertString(char input[], const char insert[], int position)
{
int i, j;
char temp[81];
j = strlen(input);
for(i = 0; i < position - 1; i++)
{
temp[i] = input[i];
}
for(j = 0; insert != '\0'; i++, j++)
{
temp[i] = insert[j];
}
for(j = i - j; input != '\0'; i++, j++)
{
temp[i] = input[j];
}
for(i = 0; temp[i] != '\0'; i++)
{
input[i] = temp[i];
}
input[i] = '\0';
}
void readLine(char buffer[])
{
char character;
int i = 0;
do
{
character = getchar();
buffer[i] = character;
i++;
}
while(character != '\n');
buffer[i - 1] = '\0';
}
int main(void)
{
char input[81];
char insert[81];
int position;
printf("Enter the first string: ");
readLine(input);
printf("Enter the insert string: ");
readLine(insert);
printf("Enter placement position int: ");
scanf("%i", &position);
insertString(input, insert, position);
printf("The adjusted string is %s\n", input);
return 0;
}

Answer 1

There might be other reasons as well, but the following fragment will crash for sure:
for(j = 0; insert != '\0'; i++, j++)
{
temp[i] = insert[j];
}
The reason is that - since insert will not be increased or manipulated - this is an endless loop writing "indefinitely" long into temp. Once exceeding its length 80 (or a bit later) it will crash. I suppose you meant for(j = 0; insert[j] != '\0'; i++, j++), right?

Answer 2

Check all for loop conditions in insertString function. For example:
for(j = 0; insert != '\0'; i++, j++)
{
temp[i] = insert[j];
}
is infinite loop. Because of it you access memory out of temp array bounds. It causes UB and segmentation fault. Looks like you need insert[j] != '\0' condition here.

Answer 3

I'm familiar with this book. The author, Stephen Kochan, has a website with answers to the odd-numbered end of chapter exercises.
The website is at classroomm.com but you'll need to look around some to find the information.
Here is the info from that site related to this exercise:
Programming in C, exercise 10-7 (3rd edition) and 9-7 (4th edition)
/* insert string s into string source starting at i
This function uses the stringLength function defined
in the chapter.
Note: this function assumes source is big enough
to store the inserted string (dangerous!) */
void insertString (char source[], char s[], int i)
{
int j, lenS, lenSource;
/* first, find out how big the two strings are */
lenSource = stringLength (source);
lenS = stringLength (s);
/* sanity check here -- note that i == lenSource
effectively concatenates s onto the end of source */
if (i > lenSource)
return;
/* now we have to move the characters in source
down from the insertion point to make room for s.
Note that we copy the string starting from the end
to avoid overwriting characters in source.
We also copy the terminating null (j starts at lenS)
as well since the final result must be null-terminated */
for ( j = lenSource; j >= i; --j )
source [lenS + j] = source [j];
/* we've made room, now copy s into source at the
insertion point */
for ( j = 0; j < lenS; ++j )
source [j + i] = s[j];
}

Answer 4

There's an error somewhere in your insertString function where it goes out of bounds. By the way your insertString function doesn't start with the word void.
If I substitute the insertString function which I wrote for the exercise then the program works.
#include<stdio.h>
#include <string.h>
void insertString (char source[], const char s[], int start)
{
int stringLength (const char s[]);
int lenSource = strlen (source);
int lenString = strlen (s);
int i;
if ( start > lenSource ) {
printf ("insertion point exceeds string length\n");
return;
}
// move the characters in the source string which are above the
// starting point (including the terminating null character) to make
// room for the new characters; to avoid overwriting characters the
// process begins at the end of the string
for ( i = lenSource; i >= start; --i )
source[i + lenString] = source[i];
// insert new characters
for ( i = 0; i < lenString; ++i )
source[start + i] = s[i];
}
void readLine(char buffer[])
{
char character;
int i = 0;
do
{
character = getchar();
buffer[i] = character;
i++;
}
while(character != '\n');
buffer[i - 1] = '\0';
}
int main(void)
{
char input[81];
char insert[81];
int position;
printf("Enter the first string: ");
readLine(input);
printf("Enter the insert string: ");
readLine(insert);
printf("Enter placement position int: ");
scanf("%i", &position);
insertString(input, insert, position);
printf("The adjusted string is %s\n", input);
return 0;
}