C - Linked List Segmentation Fault During Display - c

Question

Edit 2: I realized that I did not have a "Not found" result for any query not in the database. Changes have been made to introduce this feature. Here is the current test and test output:
Input:
3
sam
99912222
tom
11122222
harry
12299933
sam
edward
harry
Output:
Not found
=0
Not found
=0
Not found
=0
Not found
=0
sam
=99912222
Not found
=0
Not found
=0
Not found
[Infinite loop continues]
Edit: I have changed a few things in the while loop in display(). I am now getting an infinite loop printing "=0" except for the third or fourth cycle through the search. Hmmm...
By the way, thanks for the reminder of testing strings with ==. Seems like a no-brainer now.
I have done some searching and have yet to be able to understand where I have gone wrong with my code. I am working on a challenge which will result in a simple phone-book program. It will take input of a number (the number of entries to be added) then the names and associated phone numbers (no dashes or periods). After the entries have been added then the user can search for entries by name and have the number displayed in the format "name=number".
The code throws a segmentation fault with the while loop in the display() function. I assume that I am attempting to print something assigned as NULL, but I cannot figure out where I have gone wrong. Any help would be very appreciated.
Lastly, the challenge calls for me to read queries until EOF; however, this confuses me since I am to accept user input from stdin. What does EOF look like with stdin, just a register return (\n)?
(PS: This is my first attempt at linked lists, so any pointers would be greatly appreciated.)
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
void add_entry(void);
void display(void);
struct phonebook {
char name[50];
int number;
struct phonebook *next;
};
struct phonebook *firstp, *currentp, *newp;
char tempname[50];
int main() {
int N;
firstp = NULL;
scanf("%d", &N);
for (int i = 0; i < N; i++) {
add_entry();
}
display();
return 0;
}
void add_entry(void) {
newp = (struct phonebook*)malloc(sizeof(struct phonebook));
if (firstp == NULL) firstp = currentp = newp;
else {
currentp = firstp;
while (currentp->next != NULL)
currentp = currentp->next;
currentp->next = newp;
currentp = newp;
}
fgets(currentp->name, 50, stdin);
scanf("%d", &currentp->number);
currentp->next = NULL;
}
void display(void) {
while (strcmp(tempname, "\n") != 0) {
currentp = firstp;
fgets(tempname, 50, stdin);
while (strcmp(currentp->name, tempname) != 0) {
if (currentp->next == NULL) {
printf("Not found\n");
break;
}
currentp = currentp->next;
}
printf("%s=%d\n", currentp->name, currentp->number);
}
}

Answer 1

Your problem is that you never find the entry you're looking for. The expression currentp->name != tempname will always be true, since those are always unequal. In C, this equality test will not compile into a character-by-character comparison, but into a comparison of pointers to currentp->name and tempname. Since those are never at the same addresses, they will never be equal.
Try !strcmp(currentp->name, tempname) instead.
The reason you crash, then, is because you reach the end of the list, so that currentp will be NULL after your loop, and then you try to print NULL->name and NULL->number, actually causing the crash.
Also, on another note, you may want to start using local variables instead of using global variables for everything.

Answer 2

Not sure if this solves the problem, but you can't directly compare strings with != in C. You need to use if( strcmp( string1, string2 ) == 0 ) to check.
fgets doesn't take EOF (= -1) like getchar does, but it does include '\n' and pad the rest with NULL (= 0) so checking for EOF is not really helpful, but yes you can stop after \n or NULL.