creating a linkedlist of strings in C

So I wanted to make a hexadecimal to binary converting function in c and my plan of implementation was to change the characters in hex to a string of binary numbers
put that string of characater arrays into a linkedlist
and print out said linked list
however when I run it it segfaults and I don't really know how to debug using gdb considering it is in assembly
hence could someone point me out to where I went wrong?

#include <stdio.h>
#include <stdlib.h>
#include "h2b.h"
int main(int argc, char* argv[])
{
for (int i=1; i<argc; i++){
hexToBinary(argv[i]);
printf("\n");
}
return 0;
}
typedef struct hexString{
char* data;
struct hexString* next;
}hexString;
hexString* head= NULL;
void add(char* bits, hexString* ptr){
hexString* temp = (hexString*)malloc(sizeof(hexString));
temp->data = bits;
temp->next = NULL;
if(head == NULL){
head = temp;
}
else{
ptr = head;
while(ptr->next != NULL){
ptr = ptr->next;
}
ptr->next=temp;
}
free(temp);
}
void display(hexString* ptr){
if (head==NULL){
printf("There is nothing here");
}
else{
hexString* ptr;
while(ptr != NULL){
printf("%s", ptr->data);
}
ptr = ptr->next;
}
}
char* H2B_MAP(char hex){
switch(hex){
case'0':
return "0000";
case'1':
return "0001";
case '2':
return "0010";
case '3':
return "0011";
case '4':
return "0100";
case '5':
return "0101";
case '6':
return "0110";
case '7':
return "0111";
case '8':
return "1000";
case '9':
return "1001";
case 'A':
case 'a':
return "1010";
case 'B':
case 'b':
return "1011";
case 'C':
case 'c':
return "1100";
case 'D':
case 'd':
return "1101";
case 'E':
case 'e':
return "1110";
case 'F':
case 'f':
return "1111";
}
}
int hexToBinary(char* hex){ //no need for 0x formatting
hexString x;
hexString* x_ptr =&x;
for(int i=0; i<sizeof(hex); i++){ //internally it is a linked list storing strings so how would you do that, ie convert each number into a different number
add(H2B_MAP(hex[i]), x_ptr);
}
display(x_ptr);
return 0;
}

You're freeing memory after keeping a reference pointing to the block! Remove the call to free().

There are several problems/bugs in your code but let's start here:

> I don't really know how to debug using gdb considering it is in assembly

If you want to be a programmer you must learn how to use a debugger. So stop writing code for now. Instead spend your time learning how to use a debugger. There are several debuggers avaible. Text based debuggers like gdb but also gui based debuggers that are quite easy to use once they are setup correctly. Choose one and learn how to use it. For gdb it's not correct that you only have assembly. Most likely you didn't compile your code with -g. There are several post on stackoverflow about using gdb (e.g. https://stackoverflow.com/q/63647073/4386427). Just search and read until you master using gdb.

The next step is about compiler warnings. Your code have many statements that most compilers will warn about. Never ignore warnings !! Fix all warnings before executing the code.

If you didn't get any warnings, it's because you are compiling with too few compiler options. If you are using gcc at least do:

gcc -Wall -Wextra -pedantic h2b.c -o h2b

Even better do:

gcc -Wall -Wextra -pedantic -Werror h2b.c -o h2b

as -Werror turns all warnings into errors so that you are forced to fix all warnings.

Compiling your code like that gives me:

h2b.c: In function 'display':
h2b.c:40:25: error: unused parameter 'ptr' [-Werror=unused-parameter]
40 | void display(hexString* ptr){
|              ~~~~~~~~~~~^~~
h2b.c: In function 'hexToBinary':
h2b.c:100:19: error: comparison of integer expressions of different signedness: 'int' and 'long unsigned int' [-Werror=sign-compare]
100 |     for(int i=0; i<sizeof(hex); i++){ //internally it is a linked list storing strings so how would you do that, ie convert each number into a different number
|                   ^
h2b.c: In function 'H2B_MAP':
h2b.c:95:1: error: control reaches end of non-void function [-Werror=return-type]
95 | }
| ^
h2b.c: In function 'display':
h2b.c:46:19: error: 'ptr' may be used uninitialized [-Werror=maybe-uninitialized]
46 |         while(ptr != NULL){
|                   ^
h2b.c:45:20: note: 'ptr' was declared here
45 |         hexString* ptr;
|                    ^~~
In function 'display',
inlined from 'hexToBinary' at h2b.c:103:5,
inlined from 'main' at h2b.c:9:9:
h2b.c:46:19: error: 'ptr' may be used uninitialized [-Werror=maybe-uninitialized]
46 |         while(ptr != NULL){
|                   ^
h2b.c: In function 'main':
h2b.c:45:20: note: 'ptr' was declared here
45 |         hexString* ptr;
|                    ^~~
In function 'display',
inlined from 'hexToBinary' at h2b.c:103:5:
h2b.c:46:19: error: 'ptr' is used uninitialized [-Werror=uninitialized]
46 |         while(ptr != NULL){
|                   ^
h2b.c: In function 'hexToBinary':
h2b.c:45:20: note: 'ptr' was declared here
45 |         hexString* ptr;
|                    ^~~
cc1: all warnings being treated as errors

Go ahead and fix all these warnings/errors. Then you'll find most of the bugs in your code.

There are some bugs you won't find just by fixing warnings. So here are some extra tips:

free(temp); is wrong. Don't free the memory until you are done using it.

sizeof(hex) is not doing what you expect. Maybe consider strlen instead.

The code

    while(ptr != NULL){
printf("%s", ptr->data);
}

is an endless loop. ptr must be changed inside the while to avoid an endless loop.