Segmentation Fault, but I have no infinite loops... I think - c

Question

New to C syntax, so perhaps I'm just making a dumb error. I'm trying to implement the strcat() function myself by using the exact same process. My copycat function is strcat406().
I keep getting segmentation fault as an error when trying to run the program.
EDIT: The first while loop in strcat406() is my attempt at getting around the strlen() function. I'm trying to avoid using the built in operations.
EDIT2: Okay, so as people pointed out, I replaced the '\n' to '\0'. Stupid mistake. Then I fixed the whole thing by removing the 'string1[i] = string[i]'. That way the first loop just iterates to determine i (the length of string1), then string2 is added to string1 in the second while loop. Corrections in code below.
#include <stdio.h>
char *strcat406(char string1[ ], char string2[ ]) {
int i = 0, j = 0;
while (string1[i] != '\0') { //replaced '\n' with '\0'
//removed: string1[i] = string1[i];
i++;
}
while (string2[j] != '\0') { //replaced '\n' with '\0'
string1[i+j] = string2[j];
j++;
}
string1[i+j] = '\0';
return string1;
}
int main() {
char str1[81], str2[81];
char again = 'y', newline;
while (again == 'y') {
printf("Enter a string\n");
scanf("%s", str1);
printf("Enter another string\n");
scanf("%s", str2);
printf("The concatention is %s\n", strcat406(str1, str2));
printf("Second test: The concatenation is %s\n", str1);
printf("The second string is still %s\n", str2);
printf("Again? (y/n)\n");
scanf("%c%c", &newline, &again);
}
}

Answer 1

The problem is that your while loops are looking for a newline character to terminate, but scanf("%s", ...) won't include the ending newline in the scanned string. You should look for '\0' to terminate those loops.
By the way... the title of this question reflects a misunderstanding. You said you're getting a segfault, but "have no infinite loops". Segfaults are not normally caused by infinite loops. They are commonly caused by dereferencing a null pointer, or a pointer which is "bad" in some other way. Note that array indexing is a form of pointer dereference, so using a "bad" array index is just the same thing.

Answer 2

A string is an array of characters containing a terminating null character '\0', not a newline character '\n'. Therefore, in strcat406 function, you should check the value for the null byte and not the newline character. Note that str2 must be large enough for string2 to be appended to it else it will cause buffer overflow invoking undefined behaviour. Also note that the lengths of both the strings string1 and string2 should each be less than 81 and the sum of their lengths should be less than 81 + 81 == 162.
#include <stdio.h>
char *strcat406(char string1[], char string2[]) {
int i = 0, j = 0;
// increment i till the terminating null byte is reached
while(string1[i++]) ; // the null statement
i--; // reset i to the index of the null byte
// copy the characters from string2 to string1 till and
// including the terminating null byte of string2
while((string1[i++] = string2[j++])) ; // the null statement
return string1;
}
int main(void) {
char str1[81], str2[81];
char again = 'y';
while(again == 'y') {
printf("Enter a string\n");
scanf("%s", str1);
printf("Enter another string\n");
scanf("%s", str2);
printf("The concatention is %s\n", strcat406(str1, str2));
printf("Second test: The concatenation is %s\n", str1);
printf("The second string is still %s\n", str2);
printf("Again? (y/n)\n");
// note the leading space in the format string of scanf.
// this reads and discards the newline left in the buffer in
// the previous scanf call
scanf(" %c", &again);
}
return 0;
}

Answer 3

The problem is that you are writing the result of the concatenation on top of one of the input strings. At the moment the first loop (over string1) does nothing; it is just copying string1 on top of itself character by character. I think you might be getting confused between the newline character '\n' and the string termination character '\0' as well.
The reason you are getting segfaults is that in the second loop you start writing onto the memory that follows the first input string. Only 81 characters of memory are reserved for it, but you might write more than that for the concatenation.
I think the answer would be to create a new string to contain the result of your function. You need to find out how long it needs to be by looping over the input strings first, counting their length. Then when you copy the two strings into the result you will only use the space that you have reserved for it in memory. Also you won't change the input as part of the function, which is another problem with the current method.