Segmentation fault (core dumped) error when making a linked list

Question

I'm learing C for school, and one of the assignments was to create a database. Now I'm trying to add some of the inputs I give it to the list, but I keep getting a segmentation error. What am I doing wrong?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct carinfo_t {
    char* carbrand;
    char* carmodel;
    int caryear;
    float carvalue;
    struct carinfo_t * next;
};

struct carinfo_t * carbase;

struct carinfo_t * tempcar;

struct carinfo_t * tempcar2;

struct carinfo_t * tempprint;

void freeCarinfo(struct carinfo_t * carinfo){
    free(carinfo->carbrand);
    free(carinfo->carmodel);
    free(carinfo);    
}

struct carinfo_t * createCarinfo(char *carbrand, char *carmodel, int caryear, float carvalue){
    struct carinfo_t * newcar;
    newcar = (struct carinfo_t *)malloc(sizeof (struct carinfo_t));
    newcar->carbrand=(char *)malloc(sizeof(char)*(strlen(carbrand) + 1));
    strcpy(newcar->carbrand, carbrand);

    newcar->carmodel=(char *)malloc(sizeof(char)*(strlen(carmodel) + 1));
    strcpy(newcar->carmodel, carmodel);

    newcar->caryear=caryear;

    newcar->carvalue=carvalue;

    newcar->next= NULL;

    return newcar;
}

struct carinfo_t * addCarinfo(struct carinfo_t *carbase, struct carinfo_t *newcar){
    if(carbase=NULL){
        carbase = newcar;
        return carbase;
    }

    else{
        tempcar2->next=carbase;
        carbase=tempcar2;
        return carbase;

    }

}

void printCarbase(struct carinfo_t *carbase){

    struct carinfo_t *tempprint = carbase;

    if (carbase == NULL){
        printf("The database contains no cars\n");
    }

    else{
        while (tempprint != NULL){
            printf("Car:\n");
            printf("- brand: %s\n", carbase->carbrand);
            printf("- model: %s\n", carbase->carmodel);
            printf("- year: %d\n", carbase->caryear);
            printf("- value: %7.2f\n", carbase->carvalue);
            tempprint = tempprint->next;
        }
    }
}


void main(void){
    struct carinfo_t * carbase;
    carbase = NULL;

    struct carinfo_t * tempcar;
    tempcar = createCarinfo("Opel", "Manta", 1965, 20000);

    struct carinfo_t * tempcar2 = createCarinfo("Ford", "Focus", 1999, 350.25);
    addCarinfo(carbase, tempcar);
}

Also, if you see any way to improve my code please tell me, I'm very new to programming, and I'd like to be able to do this properly.

edit: Thanks to everyone who responded, I figured out how to use GDB. Now that the original issue is fixed, I got the same error but this time it's "tempcar2" that seems to be the issue:

Program received signal SIGSEGV, Segmentation fault.
0x000000000040072a in addCarinfo (carbase=0x602010, newcar=0x602080)
    at database.c:56
56              tempcar2 = tempcar2->next;
(gdb) bt
#0  0x000000000040072a in addCarinfo (carbase=0x602010, newcar=0x602080)
    at database.c:56
#1  0x0000000000400869 in main () at database.c:98

Answer 1

I ran your program in gdb it points to the line 52

Program received signal SIGSEGV, Segmentation fault.
0x00000000004007b6 in addCarinfo (carbase=0x0, newcar=0x602010) at so.c:52
(gdb) bt 
#0  0x00000000004007b6 in addCarinfo (carbase=0x0, newcar=0x602010) at so.c:52
#1  0x00000000004008e7 in main () at so.c:89

Here its mistake, instead of == your using =.

// Your assigning carbase struct pointer to NULL instead of validating pointer is null. 
    if(carbase=NULL){ //change to carbase == NULL
        carbase = newcar;
        return carbase;
    }

---

How to debug using GDB:

1. compile your C code using -g flag if you use gcc. example: gcc -g file.c
2. gdb ./a.out
3. bt(backtrace if will pointout the callstack)

Answer 2

Some problems in your code:

• some variable are not initialized ( tempcar2 for instance, which is global)
• some local and global variable have the same name
• some comparison are done with = instead of == operator
• in your print function, you always print carbase instead of tempprint
• malloc return is casted, it should not

Some problems could have been detected by the compiler: turn on compiler warnings ( -Wall for most compilers)

When your code is compiled with warning on, your problems are revealed:

.code.tio.c: In function ‘addCarinfo’:
.code.tio.c:46:8: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
     if(carbase=NULL){
        ^~~~~~~
.code.tio.c: At top level:
.code.tio.c:81:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]
 void main(void){
      ^~~~
.code.tio.c: In function ‘main’:
.code.tio.c:88:24: warning: unused variable ‘tempcar2’ [-Wunused-variable]
    struct carinfo_t * tempcar2 = createCarinfo("Ford", "Focus", 1999, 350.25);
                        ^~~~~~~~

A corrected version could be:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct carinfo_t
{
    char* carbrand;
    char* carmodel;
    int caryear;
    float carvalue;
    struct carinfo_t * next;
};

void freeCarinfo(struct carinfo_t * carinfo)
{
    free(carinfo->carbrand);
    free(carinfo->carmodel);
    free(carinfo);    
}

struct carinfo_t * createCarinfo(char *carbrand, char *carmodel, int caryear, float carvalue)
{
    struct carinfo_t * newcar;
    /* malloc cast is not recommender */
    newcar = malloc(sizeof (struct carinfo_t));

    /* strdup can be used here */
    newcar->carbrand = strdup(carbrand);    
    newcar->carmodel = strdup(carmodel);

    newcar->caryear=caryear;
    newcar->carvalue=carvalue;

    newcar->next= NULL;

    return newcar;
}

struct carinfo_t * addCarinfo(struct carinfo_t *carbase, struct carinfo_t *newcar)
{
    if(carbase==NULL)
    {
        carbase = newcar;
        return carbase;
    }
    else
    {
        /* find for the last element */
        struct carinfo_t * tempcar2 = carbase;
        while(tempcar2->next)
        {
            tempcar2 = tempcar2->next;
        }
        /* add the new car to the list */
        tempcar2->next=newcar;

        return carbase;
    }
}

void printCarbase(struct carinfo_t *carbase)
{
    struct carinfo_t *tempprint = carbase;

    if (carbase == NULL)
    {
        printf("The database contains no cars\n");
    }
    else
    {
        while (tempprint != NULL)
        {
            printf("Car:\n");
            printf("- brand: %s\n", tempprint->carbrand);
            printf("- model: %s\n", tempprint->carmodel);
            printf("- year: %d\n", tempprint->caryear);
            printf("- value: %7.2f\n", tempprint->carvalue);
            tempprint = tempprint->next;
        }
    }
}

int main(void)
{
   struct carinfo_t * carbase;
   carbase = NULL;

   struct carinfo_t * tempcar;
   tempcar = createCarinfo("Opel", "Manta", 1965, 20000);

   struct carinfo_t * tempcar2 = createCarinfo("Ford", "Focus", 1999, 350.25);
   carbase = addCarinfo(carbase, tempcar);
   carbase = addCarinfo(carbase, tempcar2);
   printCarbase(carbase);

   return 0;       
}

The result with this code is:

Car: 
- brand: Opel
- model: Manta
- year: 1965
- value: 20000.00
Car: 
- brand: Ford
- model: Focus
- year: 1999
- value:  350.25