C struct pointer - how to return struct pointer from module?

Question

I have 3 different files: main.c, module.h and module.c

The module.c should "transmit" 2 text messages to the main:

One "info" message
And one "error" message.

Those 2 messages are generated within the module.c

The idea is passing both messages using pointer to struct. Unfortunately I am missing something about pointer because only the first message ("This is info") goes through... The second one gets lost somewhere in between.

/*file:main.c (gcc -o test main.c module.c)*/
#include <stdio.h>
#include <stdlib.h>
#include "module.h"

static struct message *text = NULL;

int main(int argc, char **argv)
{
  text = (struct message *) malloc(sizeof(struct message));
  text->info_text="toto";
  text->error_text="tutu";
  text->id = 55;

  text = moduleFcn();

  printf("message->info_text: %s\n", text->info_text);
  printf("message->error_text: %s\n", text->error_text);
  printf("message->id: %u\n", text->id);
  return 0;
}

And the module

/*module.h*/
struct message
{
  char *info_text;
  char *error_text;
  int id;
};
extern struct message* moduleFcn(void);

/*module.c*/
#include <stdio.h>
#include "module.h"

static struct message *module_text = NULL;

struct message* moduleFcn(void)
{
  struct message dummy;

  module_text = &dummy;

  module_text->info_text = "This is info";
  module_text->error_text = "This is error";
  module_text->id = 4;

  return module_text;
}

Thank you in advance for helping me. Stephane

First, you cannot return a pointer to a local variable. A local variable disappears after you return from the function, and a pointer to it makes no sense. Second, why do you spend 4 lines on creating text and then instantly overwrite it with the value returned from moduleFun? — DYZ
– DYZ, Commented Jan 27, 2017 at 18:46
Possible duplicate of C - function returning a pointer to a local variable — DYZ
– DYZ, Commented Jan 27, 2017 at 18:48
@DYZ, I used the 4 lines at the same time as commenting the line //text = moduleFcn() => only for "understanding" what went wrong... — Steph
– Steph, Commented Jan 27, 2017 at 21:16

user7375520 · Accepted Answer · 2017-01-27 20:13:23Z

1

Make changes in your module code and main functions. Allocate struct on heap in module section and return that structure. In main function why you're allocating a struct and overwriting it with return struct from moduleFcn()?

/*module.h*/
struct message
{
  char *info_text;
  char *error_text;
  int id;
};
extern struct message* moduleFcn(void);

/*module.c*/
#include <stdio.h>
#include "module.h"

struct message* moduleFcn(void)
{
  struct message *dummy = (struct message*)malloc(sizeof(struct message));

  dummy->info_text = "This is info";
  dummy->error_text = "This is error";
  dummy->id = 4;

  return dummy;
}

In main() do the following changes.

/*file:main.c (gcc -o test main.c module.c)*/
#include <stdio.h>
#include <stdlib.h>
#include "module.h"

int main(int argc, char **argv)
{
  struct message *text = moduleFcn();
  printf("message->info_text: %s\n", text->info_text);
  printf("message->error_text: %s\n", text->error_text);
  printf("message->id: %u\n", text->id);
  free(text);
  return 0;
}

edited Jan 27, 2017 at 20:13

answered Jan 27, 2017 at 20:01

user7375520

2834 silver badges16 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Steph Over a year ago

Thank you very much. SOLVED :-)

Collectives™ on Stack Overflow

C struct pointer - how to return struct pointer from module?

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related