Thread: How to pass a pointer of a struct to a function

Your first step would have to be to move the rec array into main.
Since it is an array, you can just pass that as an argument of array/pointer type to the two functions that use the rec array. [Array arguments in C are automatically converted to a pointer to the first element, and pointers can be used in "array form", where the index behaves just like if you had an array of those type of elements].

--
Mats

Code:

struct record *read_file();
void print_report(struct record *ptr);

struct record *read_file()
{
   FILE *fpin;
   int i;
   struct *record rec[MAXRECORDS]=malloc(sizeof(rec)*MAXRECORDS);
  [....]
   return rec;

You want to pass a pointer to the beginning of the record. So in main:

Code:

int main()
{
   struct record *ptr=read_file();
   print_report(ptr);
   free(ptr);
}

The free() here is kind of superfluous but important to think about.

ps. matsp's idea may be better for allocating the space. The function prototypes would then be:

Code:

void read_file(struct record *ptr);
void print_report(struct record *ptr);

MK27 - why do you need to use dynamic memory all of a sudden - it was obviously enough stack for it to work in the original case... It shouldn't change much in the case of the array being placed in main().

--
Mats

Originally Posted by matsp

MK27 - why do you need to use dynamic memory all of a sudden - it was obviously enough stack for it to work in the original case... It shouldn't change much in the case of the array being placed in main().

--
Mats

Yes. Let me try and make this up:

Code:

int main()
{
   struct record rec[MAXRECORDS];
   read_file(&rec);
   print_report(&rec);
}

Originally Posted by MK27

Yes.

Sorry, I completely misread your post - I thought you put the malloc in main, and then called read_file() with it as an argument.

Aside from a syntactical error where you are assigning an array of pointers with a single pointer, I'd say I prefer to create a local array variable in main and call read_file() with it as an argument. It saves getting trouble with freeing it later on - imagine that add the filename as an argument, and then rewrite the program to parse through 4000 files's worth of data - then you'd probably run out of memory at some point before it's finished. Alocating memory in a function and then returning that to an upper layer is something that should be done VERY rarely - because the calling code won't know that it's supposed to free that memory, unless it's well documented.

--
Mats

Originally Posted by MK27

Yes. Let me try and make this up:

Code:

int main()
{
   struct record rec[MAXRECORDS];
   read_file(&rec);
   print_report(&rec);
}

Address operator is not needed (and may lead to error/warning messages of "incompatible type") if the compiler pays attention.

--
Mats

I read about passing arrays to functions and created this code but I get errors. On the read_file(rec); line I get "error: cannot convert 'record*' to 'rec' for argument '1' to 'void read_file(rec*)'" and on the fscanf line I get three errors of "error: expected primary-expression before '[' token"

Code:

#include <stdio.h>
#define MAXRECORDS 4

struct record
{
   char gender;
   int age;
   int lead;
};

void read_file(struct rec[]);

int main()
{
   struct record rec[MAXRECORDS];
   read_file(rec);
}

void read_file(struct rec[])
{
   FILE *fpin;
   int i;

   fpin = fopen("custsurvey.dat", "r");

   if(fpin == NULL)
   {
      printf("Error opening file");
   }
   else
   {
      for(i = 0; i < MAXRECORDS; i++)
         fscanf(fpin, " %c  %d %d", &rec[i].gender, &rec[i].age, &rec[i].lead);
   }

   fclose(fpin);

   return;
}

Originally Posted by tabstop

Surely you gave your function parameter a name of some kind?

Or rather, the name of the struct itself is missing.

--
Mats

Don't forget to put it into the prototype, as well.

Also note that if fpin is in fact NULL, you'll be calling fclose(NULL), which generally causes a segmentation fault on my machine.

Finally, you can generally leave whitespace out of *scanf() format strings; scanf() skips over any whitespace it sees under normal circumstances.