How to install cleanup handler during exit using atexit?

We often write large and complex programs. We allocate memory, open files/ resources and lots of other things. Most of the time our program executes in proper environment and encounters less/no error. We should handle this errors and exit properly.

Exiting from a logic or program involves freeing up memories and closing handles and files. We often write a cleanup handler which does this work. once cleanup is done we can exit gracefully. This cleanup ensures no memory and resource leak.

C runtime provides a atexit() API to register for cleanup handlers. 

int atexit(void (*function)(void));

User need to implement these callbacks and C runtime will call all registered callbacks in reversed order. Maximum 32 routines can be registered for one application.


#include <stdlib.h>
#include<string.h>
#include<stdio.h>
#include<malloc.h>
#include<unistd.h>

FILE* fp = NULL;
char *hostname = NULL;

void free_memory(void)
{
  printf ("Cleaning memory\n");
  if (hostname) {
    free (hostname);
  }
}

void close_files(void)
{
  printf ("Closing files\n");
  if (fp) {
    fclose (fp);
  }
}

int main(int argc, char* argv[])
{
  int err;
  err = atexit (free_memory);
  if (err != 0) {
    fprintf (stderr, "cannot set exit function\n");
    exit(-1);
  }
  err = atexit (close_files);
  if (err != 0) {
    fprintf(stderr, "cannot set exit function\n");
    exit(-1);
  }
  hostname = (char *) malloc(HOST_NAME_MAX);
  if (hostname == NULL) {
    fprintf(stderr, "Error in memory allocation, exiting.\n");
    exit(-1);
  }
  err = gethostname (hostname, HOST_NAME_MAX);
  if (err != 0) {
    fprintf(stderr, "Error %d executing gethostname(), exiting.\n", err);
    exit(-1);
  }
  fp = fopen ("hostinfo.txt", "w");
  if (fp == NULL) {
    fprintf (stderr, "Error writing to file, exiting.\n");
    exit (-1);
  }
  fprintf (fp, "Hostname : %s\r\n", hostname);
  /* try to open a non-existent file */ 
  if (fopen ("foo", "r") == NULL) {
    fprintf (stderr, "Oops some error happened, exiting!\n");
    exit(-1);
  }
    /* additional lines here to write more info */
  /* ........................................ */
  /* Finally close the file */
  fclose(fp);
  free (hostname);

}

Output:
Oops some error happened, exiting!
Closing files
Cleaning memory


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