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

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Note that 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 prior to the subsequent IO 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 &N) {
  std::fstream f(N);
  if (!f.is_open()) {
    // Handle error
    return;
  }
  
  f << "Output some data";
  std::string str;
  f >> str;
}

Compliant Solution

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

#include <fstream>
#include <string>

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

Bibliography

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