How to malloc linked list node while reading file - c

Question

I've posted a question similar to this, but I was able to further narrow down the issue. I'm pretty sure I know what the problem is, but I've been stuck figuring out how to solve it.
//assuming typedef struct person person;
struct person{
char first_n[100];
char last_n[100];
char middle_n[100];
struct person* next;
};
void open_file_and_read(char* file){
FILE* fp = fopen(file_name, "r");
if (fp != NULL){
while (!feof(fp)){
person* person = malloc(sizeof(person));
person->next = NULL;
while (fscanf(fp, "%s %s %s", person->first_n, person->last_n, person->middle_n) == 3){
add_to_contacts(person);
}
}
}
}
void open_write_file(char* file){
FILE* filep = fopen(file, "w");
person* copy;
for (copy = front; copy != NULL; copy=copy->next){
fprintf(filep, "%s %s %s\n", copy->last_n, copy->first_n, copy->middle_n);
}
fclose(filep);
}
void add_to_contacts(person* person){
printf("last_name: %s", person->last_name);
if (head == NULL){
person->next = head;
head = person;
else{
person->next = head;
head = person;
}
}
int main(int argc, char * argv[])
{
char* inputFilename = argv[1];
if (inputFilename == NULL){
inputFilename = "myRolodex";
}
open_read_file(inputFilename);
char command;
while (command != 'Q' && command != 'q'){
printf("STARTING TO READ COMMAND\n");
command = read_command(inputFilename);
evaluate_command(command);
}
open_write_file(inputFilename);
clear_rolodex();
The printf prints out everything in the file as one node. Ex: file contains:
Bob Lee Steve
Mike Steven Noel first, last, middle
James Nguyen Lee
the printf prints out LeeStevenNguyen. These should be separate.
I believe the problem has to do with malloc only being called once, when what I really need is for it to be called each time I scanf. However I can't move person* person = malloc(sizeof(person)); under the while...fscanf loop because fscanf is dependent on having a person already created. So how do I go about mallocing each new person. A similar assignment is due in a couple of hours and I've spent the last 10 hours trying to get everything to work and I believe this is the big part.
Thank you
EDIT for more detail: I have a print statement in my add_to_contacts function that prints out the node->lasts_name. I also added a print statement before my sorting function(not posted) and it prints infinitely

Answer 1

There are several ways to resolve this.
The one that I think most directly matches what you want is:
person* person = malloc(sizeof(person));
person->next = NULL;
while (fscanf(fp, "%s %s %s", person->first_n, person->last_n, person->middle_n) == 3){
add_to_contacts(person);
person = malloc(sizeof(person));
person->next = NULL;
}
free(person);
This assumes that add_to_contacts inserts the malloc'd
pointer into a data structure, so we need the next fscanf to put its
data in a different location.
Having done this, there will always be one person* allocated
at the end that is not actually ever passed to add_to_contacts, so we
can and should free that pointer when the loop exits.
A slight variation is
person* person = malloc(sizeof(person));
while (fscanf(fp, "%s %s %s", person->first_n, person->last_n, person->middle_n) == 3){
person->next = NULL;
add_to_contacts(person);
person = malloc(sizeof(person));
}
free(person);
(The idea there is that the only thing that needs person->next = NULL;
here is presumably add_to_contacts(person), and by postponing
person->next = NULL; until the very last moment we don't have to call
it outside the loop as well as inside the loop.)
An alternative (quite different) approach is to
declare local variables, let fscanf write to them,
and copy them to a newly allocated person inside the loop.
A third approach is to make person a stack-allocated (not malloc'd) variable,
and make add_to_contacts make its own malloc'd copy of it in order
to insert the copy in the data structure.
But these last two options involve fscanf writing the data one place
and then having to copy it to another in order to put it in the larger structure. It seems you may want to avoid the copy.

Answer 2

regarding this part of the question:
believe the problem has to do with malloc only being called once, when what I really need is for it to be called each time I scanf. However I can't move person* person = malloc(sizeof(person)); under the while...fscanf loop because fscanf is dependent on having a person already created
suggest fscanf() into local variables (remember to use length modifiers on the format parameters to avoid buffer overflow.
Then, within the loop, malloc, error check, copy variables, call function to insert malloc memory into linked list.