segmentation fault error [help]

[blaze505]

Hey, I'm still in the process of learning C and need some help. So I'm working with data structures and having trouble making a tree of strings where every node has a passengers name, number of the city they are at and number of the the city they want to go to. the tree is aligned by the passengers names.
My problems are how to deal with the name string and inserting the passenger that i read into the tree.

This is what I came up with till now.

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

#define MAX 50

struct passengerNode{   
    char *name;
    int currentCity;
    int destination;
    struct passengerNode *leftPtr;
    struct passengerNode *rightPtr;
};
typedef struct passengerNode PassengerNode;
typedef PassengerNode *PassengerNodePtr;

PassengerNodePtr readPassenger(void);
void insertPassenger(PassengerNodePtr *passengerPtr,char *name);


int main(int argc, char*argv[]){
    PassengerNodePtr newPass;
    newPass = readPassenger();
    insertPassenger(&newPass,newPass->name);
    return 0;
}

PassengerNodePtr readPassenger(void){
    
    PassengerNodePtr pass = malloc(sizeof(struct passengerNode));

    printf("Name: ");
    fgets(pass->name,MAX,stdin);

    printf("Source city: ");
    scanf("%d",&(pass->currentCity));

    printf("destination city: ");
    scanf("%d",&(pass->destination));

    return pass;
}       

void insertPassenger(PassengerNodePtr *passengerPtr, char *name){
    if(*passengerPtr == NULL){
        *passengerPtr = malloc(sizeof(struct passengerNode));
        if (*passengerPtr !=NULL){
            (*passengerPtr )->leftPtr = NULL;
            (*passengerPtr )->rightPtr = NULL;
        }else{
            if(strcmp(name,(*passengerPtr)->name)== -1){
                insertPassenger(&(*passengerPtr)->leftPtr, name);
            }else if(strcmp(name,(*passengerPtr)->name) == 1 || strcmp(name,(*passengerPtr)->name) == 0){
                insertPassenger(&(*passengerPtr)->rightPtr,name);
            }
        }
    }
}

[cas]

If you get a segfault, your first step should always be to run the program through a debugger.

If it's available for your platform, I recommend Valgrind. It can often tell you where the problem is instead of where the fault occurred.

Otherwise, try something like gdb, or whatever your platform's native debugger is.

[GReaper]

Doesn't even compile!, how did you manage to get a runtime error?!

EDIT: Ok, you corrected it...

[GReaper]

The segfault occurs inside "fgets()". Check your code around there. My guess is a wrong memory allocation.

[anduril462]

PassengerNodePtr pass = malloc(sizeof(struct passengerNode));
 
printf("Name: ");
fgets(pass->name,MAX,stdin);

You allocate space for a passengerNode object. That has a member called name which is a char pointer. That pointer is uninitialized, and doesn't point to memory you own, so you can't read into it. You need to malloc some space for pass->name before reading into it, or make it a char array in the structure declaration.

[blaze505]

Yes thank you, I just made it a char array in the structure and now it works.

[Salem]

> strcmp(name,(*passengerPtr)->name)== -1
According to the standard, you should only test for < 0 , == 0 and > 0
strcmp() is allowed to return any negative value if s1 is before s2 (not specifically -1)

[CommonTater]

Hey, I'm still in the process of learning C and need some help. So I'm working with data structures and having trouble making a tree of strings where every node has a passengers name, number of the city they are at and number of the the city they want to go to. the tree is aligned by the passengers names.
My problems are how to deal with the name string and inserting the passenger that i read into the tree.

This is what I came up with till now.

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

#define MAX 50

struct passengerNode{   
    char *name;
    int currentCity;
    int destination;
    struct passengerNode *leftPtr;
    struct passengerNode *rightPtr;
};
typedef struct passengerNode PassengerNode;
typedef PassengerNode *PassengerNodePtr;

PassengerNodePtr readPassenger(void);
void insertPassenger(PassengerNodePtr *passengerPtr,char *name);


int main(int argc, char*argv[]){
    PassengerNodePtr newPass;
    newPass = readPassenger();
    insertPassenger(&newPass,newPass->name);
    return 0;
}

PassengerNodePtr readPassenger(void){
    
    PassengerNodePtr pass = malloc(sizeof(struct passengerNode));

    printf("Name: ");
    fgets(pass->name,MAX,stdin);

    printf("Source city: ");
    scanf("%d",&(pass->currentCity));

    printf("destination city: ");
    scanf("%d",&(pass->destination));

    return pass;
}       

void insertPassenger(PassengerNodePtr *passengerPtr, char *name){
    if(*passengerPtr == NULL){
        *passengerPtr = malloc(sizeof(struct passengerNode));
        if (*passengerPtr !=NULL){
            (*passengerPtr )->leftPtr = NULL;
            (*passengerPtr )->rightPtr = NULL;
        }else{
            if(strcmp(name,(*passengerPtr)->name)== -1){
                insertPassenger(&(*passengerPtr)->leftPtr, name);
            }else if(strcmp(name,(*passengerPtr)->name) == 1 || strcmp(name,(*passengerPtr)->name) == 0){
                insertPassenger(&(*passengerPtr)->rightPtr,name);
            }
        }
    }
}

At line 30, you allocate memory for the struct but you do not allocate memory for the passenger's name.

This leaves you two choices...

A call malloc() again to allocate memory for the name... keeping in mind that you will have to free it later.
OR
Modify your struct with a fixed size buffer built in...

struct passengerNode{   
    char name[MAX];
    int currentCity;
    int destination;
    struct passengerNode *leftPtr;
    struct passengerNode *rightPtr;
};

Personally I would favor the second approach as it's far less likely to leak memory.

[Salem]

Also, your recursive calls to insertPassenger() can basically never happen (unless you run out of memory, then things get really horrible!).

[CommonTater]

Also, your recursive calls to insertPassenger() can basically never happen (unless you run out of memory, then things get really horrible!).

I'd like to see the train that could handle a 16gb passenger list....