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The C++ Standard, [filebuf], paragraph 2 , states [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 FILEs.

The C Standard, subclause 7.19.5.3, paragraph 66 [ISO/IEC 9899:1999], places the following restrictions on FILE objects opened for both reading and writing [ISO/IEC 9899:1999]:

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.

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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().

Note that calling 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 IO I/O operation.

Noncompliant Code Example

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#include <fstream>
#include <string>

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

Compliant Solution

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#include <fstream>
#include <string>

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

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

Bibliography

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Clause 27, "Input/Output Library"

[ISO/IEC 9899:1999]	Subclause 7.19.5.3, "The fopen Function"
[ISO/IEC 14882-2014]	Clause 27, "Input/Output Library"

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