Question

I’ve been stuck on a segmentation fault of a long time. I declared a struct with a pointer to a string. I wrote two functions, create and remove to manipulate values. The struct is as follows:

#include "filename.h"  
//*in filename.h:* typedef struct linkNode linkNode_t;

struct linkNode{
    struct linkNode *next;
    char *value;
};

The create function will first allocate memory for the node, then allocate memory for the value, and then copy the input value into the value field:

linkNode_t* create(char* stuff){
    linkNode_t *ptr=malloc(sizeof(linkNode_t));
    if(ptr==NULL){
        printf("malloc failure");
        return NULL;
    }
    char* tempvalu=malloc(sizeof(char)*strlen(stuff)+1);
    if(tempvalu==NULL){
        printf("malloc failure");
        return NULL;
    }
    strcpy(tempvalu,stuff);
    ptr->next=NULL;
    ptr->value=tempvalu;
    return ptr;
}

A function is used to insert a node into the linked list:

linkNode_t* insertLast(linkNode_t* start, linkNode_t* newNode){
    linkNode_t* current=start;
    while(current->next!=NULL){
        current=current->next;
    }
//now current points to the last element in the linked list
    current->next=newNode;
    return start;
}

The part causing me problem is as follows:

linkNode_t* removebyValue(linkNode_t* start, char* valu){
/**removes the first instance of a node with a certain value. Return *start after     removing.
    if linked list becomes empty, return NULL*/

    linkNode_t *current=start;
    linkNode_t *previous=start;
    while(current!=NULL){
        if(strcmp(valu,current->value)==0) {//found the node to delete
            if(current==start){//removing the head
                linkNode_t* retvalue= current->next;
                free(current->value);
                free(current);
                return retvalue;
            }
            else{   //removing other elements in the linked list
                previous->next=current->next;
                free(current->value);
                free(current);
                return start;
            }
        }
        else{
            previous=current;
            current=current->next;
        }
    }
    return start;
}

In the Main I created a linked list of two elements,1 and 2, and tried to free element 1 when segmentation fault occured.

int main(){
    linkNode_t *pt1=create("1");
    pt1=insertLast(pt1,create("2"));
    removebyValue(pt1,"1"); //Causes seg fault. If I replace "1" by "2" nothing happens 

Can someone give some suggestions on this? Thanks in advance

EDIT: I put all the code that could be related since someone said the sections I put on didn’t have an error

Answer 1

I think you’re over-thinking the removal of a node while maintaining the start pointer properly. Consider a hopefully simpler approach.

typedef struct node_t 
{
    struct node_t* next;
    char* value;
} node_t;

node_t* remove(node_t *start, const char* valu)
{
    node_t* current=start;
    node_t* prev=NULL;

    while(current && strcmp(current->value, valu))
    {
        prev = current;
        current = current->next;
    }

    if (current)
    {
        if (prev) // we're not deleting start node
            prev->next = current->next;

        else      // we *are* deleting start node
            start = current->next;

        // now the node is unlinked. remove it.
        free(current->value);
        free(current);
    }
    return start;
}