Delete specific element linked list

Question

Can you help me to understand why this function doesn't delete a specific element in the linked list? What am I doing wrong?

typedef struct str_node{
    int data;
    struct str_node *next;
}node;

...
node *head;
head = malloc(sizeof(node));
...

void delete_spec(node *a){
    int num;
    node *tmp;

    tmp = a;
    printf("Insert number to eliminate: ");
    scanf("%d",&num);
    while(tmp!=NULL){
        if(tmp->data == num){
            tmp = tmp->next->next;
        }
        tmp = tmp->next;
    }
}

Answer 1

For starters it is unclear what the allocation is doing here

node *head;
head = malloc(sizeof(node));

The pointer head shall be initially set to NULL

node *head = NULL;

and new nodes shall be inserted in the list by the function that inserts new values in the list.

The function that deletes nodes from the list shall not issue any prompt. It is the caller of the function that will ask the user to specify the value that will be deleted from the list and then call the function passing the specified value. So the function should have two parameters: pointer to the pointer to the head node and the integer value that shall be deleted from the list.

The function can be defined the following way

void delete_spec( node **head, int data )
{
    while ( *head != NULL )
    {
        if ( ( *head )->data == data )
        {
            node *tmp = *head;
            *head = ( *head )->next;
            free( tmp );
        }
        else
        {
            head = &( *head )->next;
        }
    }
}     

Here is a demonstrative program.

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

typedef struct str_node
{
    int data;
    struct str_node *next;
} node;

int push_front( node **head, int data )
{
    node *new_node = malloc( sizeof( node ) );
    int success = new_node != NULL;

    if ( success )
    {
        new_node->data = data;
        new_node->next = *head;
        *head = new_node;
    }

    return success;
}

void delete_spec( node **head, int data )
{
    while ( *head != NULL )
    {
        if ( ( *head )->data == data )
        {
            node *tmp = *head;
            *head = ( *head )->next;
            free( tmp );
        }
        else
        {
            head = &( *head )->next;
        }
    }
}

void display( node *head )
{
    for ( ; head != NULL; head = head->next )
    {
        printf( "%d -> ", head->data );
    }

    puts( "null" );
}

int main(void) 
{
    node *head = NULL;
    int a[] = { 1, 3, 5, 7, 1, 2, 3, 1 };
    const size_t N = sizeof( a ) / sizeof( *a );

    for ( size_t i = 0; i < N; i++ )
    {
        push_front( &head, a[i] );
    }

    display( head );

    delete_spec( &head, 1 );

    display( head );

    return 0;
}

Its output is

1 -> 3 -> 2 -> 1 -> 7 -> 5 -> 3 -> 1 -> null
3 -> 2 -> 7 -> 5 -> 3 -> null