Passing struct data between files

**SuperMiguel** · 03-28-2012

Trying to learn how to use structs, i want the user to enter the phone number and the program to print that number back.

main.c

Code:

#include "phone.h"
#include <stdio.h>

int main()
{
phone d;
printf("Enter your phone number: ");
input_phone(&d);

print_phone(d);
return 0;

phone.h

Code:

#ifndef _PhoneH_
#define _PhoneH_

typedef struct {
int areacode;
int actualnumber;
} phone;

void input_phone(phone *dp);
void print_phone(phone d);

#endif

phone.c

Code:

#include "phone.h"
#include <stdio.h>

void input_phone(phone *dp)
{
phone phonein;
scanf("%d-%d",&phonein.areacode,&phonein.actualnumber);
}

void print_phone(phone d)
{
phone phonein;
printf("Your phone numer is %d-%d", phonein.areacode, phoneinactualnumber);
}

If i do a print within input_phone of either areacode or actualnumber it works fine, but the print inside print_phone just displays a ramdon number[FONT=Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace][/FONT]

**anduril462** · 03-28-2012

Ok, so what's your question? Does this code work the way you want it to, and you just need an explanation of why? Does it not work the way you expect it to? If so, what does it do that is wrong, and what should it do? Does it even compile? If not, what compile command did you use and what errors did you get?

Note, the syntax highlighter on our forum should help you locate at least one error, near the bottom of phone.c.

**SuperMiguel** · 03-28-2012

Originally Posted by anduril462

Ok, so what's your question? Does this code work the way you want it to, and you just need an explanation of why? Does it not work the way you expect it to? If so, what does it do that is wrong, and what should it do? Does it even compile? If not, what compile command did you use and what errors did you get?

Note, the syntax highlighter on our forum should help you locate at least one error, near the bottom of phone.c.

The code does compile, but If i do a print within input_phone of either areacode or actualnumber it works fine (display the right values), but the print inside print_phone just displays a ramdon number

**anduril462** · 03-28-2012

Well, you have a local variable called phonein inside input_phone. That's where you store the result of your scanf. You never copy those results into dp, so that they are in d in main, and get passed to print_phone.

**SuperMiguel** · 03-28-2012

how would i do that??

dp[0]=phonein.areacode
??

**anduril462** · 03-28-2012

dp is not an array (nor is it the address of an array in main), so I would not use array syntax. Just use the -> operator. It works just like the . operator for struct members, but it is used with pointers to structs:

Code:

dp->areacode = phonein.areacode;

Even better, you can just read directly into dp:

Code:

scanf("%d-%d", &dp->areacode, &dp->actualnumber);

**SuperMiguel** · 03-28-2012

Thank you very much for the info.
Then to print i just have to:

printf("Your phone number is: %d-%d", d.areacode, d.actualnumber);

**anduril462** · 03-28-2012

Yes. Though it is common in C to always pass a pointer to a struct, even when you don't really need to. If you just pass the struct, it has to copy the whole struct onto the stack as the parameter to the function. This can be quite inefficient if you have a large struct. If you pass it by pointer however, it only needs to copy the pointer (usually 4 or 8 bytes), no matter the size of the struct. Thus, you might do:

Code:

void print_phone(const phone *dp)
{
    printf("Your phone number is: %d-%d", dp->areacode, dp->actualnumber);
}

The 'const' is a good way to keep your print function from accidentally changing the contents of the struct pointed to by dp.

EDIT: And get rid of the phonein variable from both of your functions, it's not needed.

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