The C Standard, 7.21.5.3, paragraph 7 [ISO/IEC 9899:2011], places the following restrictions on update streams:

When a file is opened with update mode . . ., both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.

The following scenarios can result in undefined behavior. (See undefined behavior 151.)

Noncompliant Code Example

This noncompliant code example appends data to a file and then reads from the same file:

#include <stdio.h>
 
enum { BUFFERSIZE = 32 };

extern void initialize_data(char *data, size_t size);
 
void func(const char *file_name) {
  char data[BUFFERSIZE];
  char append_data[BUFFERSIZE];
  FILE *file;

  file = fopen(file_name, "a+");
  if (file == NULL) {
    /* Handle error */
  }
 
  initialize_data(append_data, BUFFERSIZE);

  if (fwrite(append_data, 1, BUFFERSIZE, file) != BUFFERSIZE) {
    /* Handle error */
  }
  if (fread(data, 1, BUFFERSIZE, file) < BUFFERSIZE) {
    /* Handle there not being data */
  }

  if (fclose(file) == EOF) {
    /* Handle error */
  }
}

Because there is no intervening flush or positioning call between the calls to fread() and fwrite(), the behavior is undefined.

Compliant Solution

In this compliant solution, fseek() is called between the output and input, eliminating the undefined behavior:

#include <stdio.h>
 
enum { BUFFERSIZE = 32 };
extern void initialize_data(char *data, size_t size);
 
void func(const char *file_name) {
  char data[BUFFERSIZE];
  char append_data[BUFFERSIZE];
  FILE *file;

  file = fopen(file_name, "a+");
  if (file == NULL) {
    /* Handle error */
  }

  initialize_data(append_data, BUFFERSIZE);
  if (fwrite(append_data, BUFFERSIZE, 1, file) != BUFFERSIZE) {
    /* Handle error */
  }

  if (fseek(file, 0L, SEEK_SET) != 0) {
    /* Handle error */
  }

  if (fread(data, BUFFERSIZE, 1, file) != 0) {
    /* Handle there not being data */
  }

  if (fclose(file) == EOF) {
    /* Handle error */
  }
}

Risk Assessment

Alternately inputting and outputting from a stream without an intervening flush or positioning call is undefined behavior.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

FIO39-C

Low

Likely

Medium

P6

L2

Automated Detection

Tool

Version

Checker

Description

Astrée

Supported, but no explicit checker
Compass/ROSE

Can detect simple violations of this rule

LDRA tool suite

84 D

Fully implemented
Parasoft C/C++test

CERT_C-FIO39-a

Do not alternately input and output from a stream without an intervening flush or positioning call

Polyspace Bug Finder

CERT C: Rule FIO39-CChecks for alternating input and output from a stream without flush or positioning call (rule fully covered)
PRQA QA-C

5029

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT CFIO50-CPP. Do not alternately input and output from a file stream without an intervening positioning callPrior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961:2013Interleaving stream inputs and outputs without a flush or positioning call [ioileave]Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-6642017-07-10: CERT: Rule subset of CWE

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-664 and FIO39-C

CWE-664 = Union( FIO39-C, list) where list =



This CWE is vague on what constitutes “improper control of a resource”. It could include any violation of an object’s method constraints (whether they are documented or not). Or it could be narrowly interpreted to mean object creation and object destruction (which are covered by other CWEs).

Bibliography

[ISO/IEC 9899:2011]7.21.5.3, "The fopen Function"