Am I passing structures by reference wrong?

Question

#include<stdio.h>

struct mystruct{ 
    int age;                    //age of child
    int flag;                   //flag: if 1 then change the age, if 0 then don't
};

///////////////////////////////////////////////////////////////////////////////

void Change_Age_Of_First_Child(struct mystruct *child[]){ //parameter is a structure passed by reference
    if (child[0]->flag==1){     //checking if flag of first child is 1 
        child[0]->age=0;        //if it is 1 then change the age to 0
    }   
}

///////////////////////////////////////////////////////////////////////////////

int main(){
    int x,i;                    //x is used to store the integer version of ch, i is loop counter
    char ch;                    //used to store character from file
    FILE *test;

    test=fopen("test.txt","r+");
    struct mystruct child[7];

    for(i=0;i<7;i++){
        ch=fgetc(test);         //this ch is the age
        x= ch - '0';            //saving integer version of ch
        child[i].age=x;         //saving x into age part of structure
        ch=fgetc(test);         //this ch is the flag
        x= ch - '0';            //saving integer version of ch
        child[i].flag=x;        //saving x into flag part of structure
        fseek(test,2,SEEK_CUR); //moving to next line of file for the consecutive loops
    }

    Change_Age_Of_First_Child(&child);
    fclose(test);
    printf("%d",child[0].age);
}

File "test.txt":

I am trying to learn passing structures by reference. I made a simple test program to read the age and flag from a file and store it into the array called child which is a structure. Then I pass child as reference into a function. In that function, to make things simpler, I only check the first element of child (which contains data for the first line of file). If the flag is 1 then the set the age to 0.

However when I print the age of the first child in the main program it doesn't work. In fact, the console shows me nothing. It runs for a while then simply says "process exited" which means that the program is finished. Help would be appreciated.
Note: the first character of every line of file is the age and second is the flag.

Turn warnings of your compiler on! It wil tell you not to pass `&child`. — Paul Ogilvie, Nov 17 '19 at 16:30
...and `*child[]` is an array of pointers to structs, not a pointer to an array of structs, which would be just `child[]` — Paul Ogilvie, Nov 17 '19 at 16:32
@PaulOgilvie OP doesn't need to pass any warning flags to get the warning, see my answer. :) — gsamaras, Nov 17 '19 at 16:39
Yes, you are very much passing it wrong. If your compiler does not tell you about that, you should throw it away and buy, borrow, or steal a modern one. [Enable warnings and treat them as errors](https://stackoverflow.com/questions/57842756/why-should-i-always-enable-compiler-warnings). — n. m. could be an AI, Nov 17 '19 at 17:01
There is no "enabling warnings". A conforming compiler **must** produce warnings for this in any case, unless you laboriously *suppress them*. Are you using the notoriously bad Visual Studio Code by happenstance? — Antti Haapala -- Слава Україні, Nov 17 '19 at 17:17
@AnttiHaapala in this case it is more about treating warnings as errors. — n. m. could be an AI, Nov 19 '19 at 07:34

gsamaras · Accepted Answer · 2019-11-17T16:45:53.413

The compiler should already have generated a warning, like this:

Georgioss-MBP:~ gsamaras$ gcc main.c 
main.c:36:31: warning: incompatible pointer types passing
      'struct mystruct (*)[7]' to parameter of type 'struct mystruct **'
      [-Wincompatible-pointer-types]
    Change_Age_Of_First_Child(&child);
                              ^~~~~~
main.c:10:49: note: passing argument to parameter 'child' here
void Change_Age_Of_First_Child(struct mystruct *child[]){ //parameter i...
                                                ^
1 warning generated.

Why make your life difficult? Simply pass the array as is, without any extra pointer. So, change this:

void Change_Age_Of_First_Child(struct mystruct *child[]) {
  // method's body
}

to this:

void Change_Age_Of_First_Child(struct mystruct child[]) {
  if (child[0].flag==1){     //checking if flag of first child is 1 
    child[0].age=0;        //if it is 1 then change the age to 0
  }   
}

and call the method like this:

Change_Age_Of_First_Child(child);

PS: Interesting read What is array decaying?

ah..im so stupid, i was trying to pass an array by reference when it already is that way. Thanks — Arzaan Ul, Nov 17 '19 at 20:25

Am I passing structures by reference wrong?

1 Answers1