Passing struct to a function to assign values - c

Question

I am trying to pass a struct to a function residing in a separate file. When passing the struct as an argument, it throws errors.
Test.c
struct student{
int rollNumber;
unsigned char name[20];
int marks;
};
void func(struct student devanshu);
int main(){
struct student devanshu;
func(&devanshu);
printf("--------------------%d\n", devanshu.rollNumber);
printf("--------------------%d\n", devanshu.marks);
printf("--------------------%s\n", devanshu.name);
}
NewTest.c:
void func(struct student devanshu)
{
devanshu.rollNumber = 1;
devanshu.marks = 909;
strcpy(devanshu.name, "abc.xyz");
return;
}
And this is the output that I get:
In file included from test.c:6:0:
newtest.c:10:30: error: parameter 1 (‘devanshu’) has incomplete type
void func(struct student devanshu)
test.c: In function ‘main’:
test.c:23:7: error: incompatible type for argument 1 of ‘func’
func(&devanshu);
^
In file included from test.c:6:0:
newtest.c:10:6: note: expected ‘struct student’ but argument is of type ‘struct student *’
void func(struct student devanshu)
newtest.c:10:30: error: parameter 1 (‘devanshu’) has incomplete type
void func(struct student devanshu)
newtest.c:7:20: error: storage size of ‘devanshu’ isn’t known
struct student devanshu;
If I use the function in the same file i.e in test.c it does not throw any error and works just fine. But when keeping the functions in two different files, it gives me these errors.
Would be thankful if somebody could help me get through. Thanks in advance.

Answer 1

error: parameter 1 (‘devanshu’) has incomplete type
This means that the struct definition isn't visible to the file you use it inside. Unless this is intentional, you need to place the struct definition in a header file and include that by every .c file using the struct.
expected ‘struct student’ but argument is of type ‘struct student *’
You have written the functions incorrectly. It should be void func(struct student* devanshu); and inside the function you should access members with devanshu-> .... Otherwise you just pass a copy of the struct to the function and then change the local copy.

Answer 2

The errors are pretty self explanatory. Take this one:
test.c: In function ‘main’:
test.c:23:7: error: incompatible type for argument 1 of ‘func’
func(&devanshu);
It means you're passing something to func that has a different type to what you've told the compiler that func accepts. You've declared the function to take a struct student
void func(struct student devanshu);
but the function call is passing a pointer to struct student.
func(&devanshu);
The function call is correct as you want the function to have access to the struct you're passing and not a copy. So you'll need to change the function to take a struct student *.
The other errors are because you're not using an include file to store the definition of the struct.
Create a file called "student.h" (or whatever) and move the definition of the struct into it. And also the declaration of the function too.
struct student{
int rollNumber;
unsigned char name[20];
int marks;
};
void func(struct student *);
and at the top of both C files you put
#include "student.h"
which tells the compiler to copy the contents of that file into your C files when you're compiling. It means you have a consistent definition that can be shared by all the files that need it.

Answer 3

Ok, this will work: (Explanation below)
test.h: (New file!)
struct student{
int rollNumber;
char name[20];
int marks;
};
test.c:
#include <stdio.h>
#include <stdlib.h>
#include "test.h"
void func(struct student *devanshu);
int main(){
struct student devanshu;
func(&devanshu);
printf("--------------------%d\n", devanshu.rollNumber);
printf("--------------------%d\n", devanshu.marks);
printf("--------------------%s\n", devanshu.name);
}
NewTest.c:
#include <string.h>
#include "test.h"
void func(struct student *devanshu)
{
devanshu->rollNumber = 1;
devanshu->marks = 909;
strcpy(devanshu->name, "abc.xyz");
return;
}
This
unsigned char name[20];
will give you problems later on strcpy(). Change to just char.
This
void func(struct student devanshu);
is not wrong, but you fall into a trap. Structures in C are always passed by value (copy) of the entire struct. So, if you want func() to modify the original struct, you have to explicitly tell it so:
void func(struct student *devanshu);
Then, in func, you need to change the access syntax to using the -> operater insteadf of '.'
As you are using the struct student in both files, you need to move the declaration of the struct out into a header file. You could (as I did on the very first edit) also repeat the declaration because I was lazy, but the comments are 100% right: That is a very dangerous practice, so I changed that.