FIO42-C. Close files when they are no longer needed

A call to the fopen() or freopen() function must be matched with a call to fclose() before the lifetime of the last pointer object that stores the return value of the call has ended.

In general, this rule can also be applied to other functions with open and close resources, such as the POSIX open() and close() functions, or the Microsoft Windows CreateFile() and CloseHandle() functions.

Noncompliant Code Example

This code example is noncompliant because the resource allocated by the call to fopen() is not closed before function func() returns.

#include <stdio.h>
 
int func(const char *filename) {
  FILE *f = fopen(filename, "r"); 
  if (NULL == f) {
    return -1;
  }

  /* ... */

  return 0;
}

Compliant Solution

In this compliant solution, f is closed before returning to the caller:

#include <stdio.h>
 
int func(const char *filename) {
  FILE *f = fopen(filename, "r"); 
  if (NULL == f) {
    return -1;
  }

  /* ... */

  if (fclose(f) == EOF) {
    return -1;
  }
  return 0;
}

Noncompliant Code Example (POSIX)

This code example is noncompliant because the resource allocated by the call to open() is not closed before function func() returns.

#include <stdio.h>
#include <fcntl.h>
 
int func(const char *filename) {
  int fd = open(filename, O_RDONLY, S_IRUSR);
  if (-1 == fd) {
    return -1
  }

  /* ... */

  return 0;
}

Compliant Solution (POSIX)

In this compliant solution, fd is closed before returning to the caller:

#include <stdio.h>
#include <fcntl.h>
 
int func(const char *filename) {
  int fd = open(filename, O_RDONLY, S_IRUSR);
  if (-1 == fd) {
    return -1
  }

  /* ... */

  if (close(fd) == -1) {
    return -1;
  }
  return 0;
}

Noncompliant Code Example (Windows)

In this noncompliant code example, a file is opened using the Microsoft Windows CreateFile() API, but it is not subsequently closed before func() returns.

#include <Windows.h>
 
int func(LPCTSTR filename) {
  HANDLE hFile = CreateFile(filename, GENERIC_READ, 0, NULL,
                            OPEN_EXISTING,
                            FILE_ATTRIBUTE_NORMAL, NULL);
  if (INVALID_HANDLE_VALUE == hFile) {
    return -1;
  }
 
  /* ... */
 
  return 0;
}

Compliant Solution (Windows)

In this compliant solution, hFile is closed using the CloseHandle() API before returning to the caller.

#include <Windows.h>
 
int func(LPCTSTR filename) {
  HANDLE hFile = CreateFile(filename, GENERIC_READ, 0, NULL,
                            OPEN_EXISTING,
                            FILE_ATTRIBUTE_NORMAL, NULL);
  if (INVALID_HANDLE_VALUE == hFile) {
    return -1;
  }
 
  /* ... */
 
  if (!CloseHandle(hFile)) {
    return -1;
  }
 
  return 0;
}

Risk Assessment

Failing to properly close files may allow an attacker to exhaust system resources and increases the risk that data written into in-memory file buffers will not be flushed in the event of abnormal program termination. .

Automated Detection

This rule is stricter than rule [fileclose] in TS 17961. Analyzers that conform to the TS may not detect all violations of this rule.

Related Vulnerabilities

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

Related Guidelines

Bibliography