FIO50-CPP. Do not alternately input and output from a file stream without an intervening positioning call

The C++ Standard, [filebuf], paragraph 2 [ ISO/IEC 14882-2014 ], states the following:

The restrictions on reading and writing a sequence controlled by an object of class basic_filebuf<charT, traits> are the same as for reading and writing with the Standard C library FILE s.

The C Standard, subclause 7.19.5.3, paragraph 6 [ ISO/IEC 9899:1999 ], places the following restrictions on FILE objects opened for both reading and writing:

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.

Consequently, the following scenarios can result in undefined behavior :

• Receiving input from a stream directly following an output to that stream without an intervening call to std::basic_filebuf<T>::seekoff() if the file is not at end-of-file
• Outputting to a stream after receiving input from that stream without a call to std::basic_filebuf<T>::seekoff() if the file is not at end-of-file

No other std::basic_filebuf<T> function guarantees behavior as if a call were made to a standard C library file-positioning function, or std::fflush() .

Calling std::basic_ostream<T>::seekp() or std::basic_istream<T>::seekg() eventually results in a call to std::basic_filebuf<T>::seekoff() for file stream positioning. Given that std::basic_iostream<T> inherits from both std::basic_ostream<T> and std::basic_istream<T> , and std::fstream inherits from std::basic_iostream , either function is acceptable to call to ensure the file buffer is in a valid state before the subsequent I/O operation.

Noncompliant Code Example

This noncompliant code example appends data to the end of a file and then reads from the same file. However, because there is no intervening positioning call between the formatted output and input calls, the behavior is undefined .

#include <fstream>
#include <string>

void f(const std::string &fileName) {
  std::fstream file(fileName);
  if (!file.is_open()) {
    // Handle error
    return;
  }
  
  file << "Output some data";
  std::string str;
  file >> str;
}

Compliant Solution

In this compliant solution, the std::basic_istream<T>::seekg() function is called between the output and input, eliminating the undefined behavior .

#include <fstream>
#include <string>

void f(const std::string &fileName) {
  std::fstream file(fileName);
  if (!file.is_open()) {
    // Handle error
    return;
  }
  
  file << "Output some data";
 
  std::string str;
  file.seekg(0, std::ios::beg);
  file >> str;
}

Risk Assessment

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

Automated Detection

Related Vulnerabilities

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

Related Guidelines

This rule supplements FIO39-C. Do not alternately input and output from a stream without an intervening flush or positioning call .

Bibliography