ERR04-A. Choose an appropriate termination strategy

Some errors, such as a value out of range, might be the result of erroneous user input. If the input is interactive, the program can prompt the user for an acceptable value. With other errors, such as a resource allocation failure, the system may have little choice other than to shutdown.

exit()

The C standard exit() function is typically used to end a program. It takes one argument, which should be either EXIT_SUCCESS or EXIT_FAILURE indicating normal or abnormal termination. It never returns.

#include <stdlib.h>
/* ... */

if (/* something really bad happened */) {
  exit(EXIT_FAILURE);
}

Calling exit()

• Flushes unwritten buffered data.
• Closes all open files.
• Removes temporary files.
• Returns an integer exit status to the operating system.

The C standard atexit() function to customize exit() to perform additional actions at program termination.

For example, calling:

atexit(turn_gizmo_off);

registers the turn_gizmo_off() function so that a subsequent call to exit() will invoke turn_gizmo_off() as it terminates the program. According to C99, atexit() can register up to 32 functions.

atexit() is only called by exit() or upon normal completion of main().

_Exit()

A more abrupt function, _Exit() also takes one argument and never returns. The standard specifies that _Exit() also closes open file descriptors, but does not specify if _Exit() flushes file buffers or deletes temporary files. Functions registered by atexit() are not executed.

#include <stdlib.h>
/* ... */

if (/* something really bad happened */) {
  _Exit(EXIT_FAILURE);
}

The _exit() function is a synonym for _Exit().

abort()

The quickest and most abrupt way to terminate a program, abort() takes no arguments, and always signifies abnormal termination to the operating system.

{code:bgColor=#ccccff}
#include <stdlib.h>
/* ... */

if (/* something really bad happened */) {
  abort();
}

The abort() function causes abnormal program termination to occur, unless the signal SIGABRT is caught and the signal handler does not return.

Whether open streams with unwritten buffered data are flushed, open streams are closed, or temporary files are removed is implementation-defined. Functions registered by atexit() are not executed.

Summary

Non-Compliant Code Example

In this example, abort() is called after data is sent to an open file descriptor. The data may or may not actually get written to the file.

#include <stdlib.h>
#include <stdio.h>

int write_data() {
  const char* filename = "hello.txt";
  FILE *f = fopen(filename, "w");
  if (f == NULL) {
    /* handle error */
  }
  fprintf(f, "Hello, World\n");
  /* ... */
  abort(); /* oops! data might not get written! */
  /* ... */
  return 0;
}

int main() {
  write_data();
  return 0;
}

Compliant Solution

In this solution, the call to abort() is replaced with exit(), which guarantees that buffered I/O data is flushed to the file descriptor and the file descriptor is properly closed.

#include <stdlib.h>
#include <stdio.h>

int write_data() {
  const char* filename = "hello.txt";
  FILE *f = fopen(filename, "w");
  if (f == NULL) {
    /* handle error */
  }
  fprintf(f, "Hello, World\n");
  /* ... */
  exit(EXIT_FAILURE); /* writes data & closes f. */
  /* ... */
  return 0;
}

int main() {
  write_data();
  return 0;
}

While this particular example benefits from calling exit() over abort(), there will be situations where abort() is the better choice. Usually this will occur if one does not want to close any file descriptors or call any handlers registered with atexit(), for instance, if the speed of terminating the program is critical.

Risk Analysis

An usage of abort() or _Exit() in place of exit() may leave files written in an inconsistent state. It may also leave sensitive temporary files on the filesystem.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

ERR03-A. Use runtime-constraint handlers when calling functions defined by TR24731-1      12. Error Handling (ERR)       ERR05-A. Application-independent code must provide error detection without dictating error handling