Using structs as parameters

Question

This time it's about using a struct as a parameter. In this assignment I have to create a FIFS queue system. The static person queue is of the size 10. It means that just 10 persons can be on the queue.

I also have variables head, tail and nbr_elem of type static. The tail variable is used to add the person in a specific position. And vice_versa for the variable head. nbr_elem is the amount of persons in the queue. I think you don't need more information to help me. Now to the question:

How can I use a struct as a parameter in a function? This is what I have done so far:

 static person queue[QUEUE_MAX_SIZE]; static int head = 0, tail = 0, nbr_elem = 0; struct person { char first_name[7]; char sur_name[10]; char pers_nbr[10]; }; void person_info(struct person p1){ /*printf("First name: ");*/ scanf("%s", &p1.first_name[7]); /* printf("Last name: ")*/; scanf("%s", &p1.sur_name[10]); /*printf("Id-number: ");*/ scanf("%s", &p1.pers_nbr[10]); }

everything is fine so far but now I have to put struct person p1 in the queue by first copying the inf using the strcpy-function and then putting it to the array person queue[QUEUE_MAX_SIZE] i guess(?)

 void enqueue( person queue[QUEUE_MAX_SIZE]) { queue[person_info(p1.first_name[7])]; }

I don't even know if this is a good start of the function enqueue so I need some help here. And how can I use the static person queue[QUEUE_MAX_SIZE] in the function?

I also have a header-file queue.h that contains the typedef person. The header-file is then called to queue.c which is this file.

You are updating a copy of your original structure. You need to return the modified structure from your function (or go with Joachim's answer). — haccks
– haccks, Commented Nov 9, 2013 at 17:57

Some programmer dude · Accepted Answer · 2013-11-09 17:50:49Z

With the function

void person_info(struct person p1)

the structure is passed by value, that is a copy is made and you modify only this copy. You want to pass it by reference, which in C is done by passing as a pointer:

void person_info(struct person *p1)

You scanf calls in the function is very wrong by the way. It will write beyond the arrays in the structure instance. Just use e.g.

scanf("%6s", p1.first_name);

Continuing reading your question, it seems you have some very basic misunderstandings of how to use arrays, so I suggest you go back and re-ready a chapter about arrays and array indexing in the nearest book or tutorial.

In fact, considering your basic misuse of arrays, I doubt you will understand a concept such as pointers immediately, so I recommend you go and read some books and tutorials about that subject as well.

yeah I realized now that the scanf is wrong, there shouldn't be any [7] at the end.

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