How to pass a pointer of a struct to a function

This is the code I have... how do I make it so I can access the info in the read_file function in my print_report function?

Code:

---

#include <stdio.h>
#define MAXRECORDS 4

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

void read_file();
void print_report();

int main()
{
  read_file();
  print_report();
}

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

  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);
}

void print_report()
{
  FILE *fpout;

  fpout = fopen("surveyreport.txt", "w");

  if(fpout == NULL)
  {
      printf("Error opening file");
  }
  else
  {
      printf("%c  %d %d", rec[0].gender, rec[0].age, rec[0].lead);
  }
}

---

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

Quote:

---

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);
}

---

Quote:

---

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

Quote:

---

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;
}

---

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

Quote:

---

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

oh i forgot the record before rec[] lol, thanks

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.