CERT C++: FIO50-CPP

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

Description

Rule Definition

Do not alternately input and output from a file stream without an intervening positioning call.¹

Polyspace Implementation

The rule checker checks for Alternating input and output from a stream without flush or positioning call.

Examples

Alternating input and output from a stream without flush or positioning call

Issue

Alternating input and output from a stream without flush or positioning call occurs when you perform an output operation on a file stream following an input operation or vice versa, without an intermediate repositioning of the file stream.

Risk

When you open a file stream using a function such as std::fstream(), an std::basic_filebuf object is created to track the position in the file stream where the next read or write operation will occur. The implementation of this object uses the C FILE* abstraction. According to the C standard, a FILE* pointer can be used to perform both input and output operations on a file stream. However, before you perform an output operation following an input (or vice versa), you must first reposition the pointer in the file stream; otherwise, the behavior is undefined.

Fix

If you open a file stream for both reading and writing, you must perform an intermediate stream repositioning operation between read and write operations on the stream.

You can reposition a file stream using methods such as std::fstream<>::seekp() or std::fstream<>::seekg().

Example – Input and Output Operations in Succession Without Intermediate Repositioning

In this example, in the function getSomeLineFromFile(), the file stream object file is used for both input and output operations:

• The input operation is done using the std::fstream<>::operator<<() method.

• The output operation is done using the std::getline() method.

However, there is no file stream repositioning operation between the output and input operation. This issue is remedied in the function getOtherLineFromFile(), which repositions the file stream using the std::fstream<>::seekg() method.

#include <fstream>
#include <string>

void getSomeLineFromFile() {
    std::fstream file { "hello.txt" };
    file << "Some line\n" << std::flush; 
    std::string line {};
    std::getline( file, line ); //Noncompliant
}

void getOtherLineFromFile() {
    std::fstream file { "hello.txt" };
    file << "Some line\n" << std::flush; 
    std::string line {};
    file.seekg( 0, std::ios_base::beg );
    std::getline( file, line ); //Compliant
}

Check Information

Group: 07. Input Output (FIO)

Version History

Introduced in R2019a

R2024a: Checker covers C++-specific file stream operations

The defect checker now detects successive read and write operations using std::basic_filebuf objects without an intermediate stream repositioning. std::basic_filebuf objects are created when you open a file stream using a function such as std::fstream().

Previously, the checker only covered file operations using C-style functions such as fopen(). These functions are now covered only by CERT C++: FIO39-C.