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

About our authors: Team EQA

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