AUTOSAR C++14 Rule A27-0-3

Alternate input and output operations on a file stream shall not be used without an intervening flush or positioning call

Since R2020b

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Description

Rule Definition

Alternate input and output operations on a file stream shall not be used without an intervening flush or positioning call.

Rationale

Using a file stream for alternating input and output without using stream repositioning statements between read and write operations may lead to undefined behavior.

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 a C FILE* pointer. According to the C standard, a single FILE* pointer can be used to perform both input and output operations on a file stream. However, before you perform an output operation followed by an input operation or an input operation followed by an output operation, you must first reposition the pointer in the file stream. Failing to do so can result in undefined behavior.

Polyspace Implementation

The rule checker reports a violation when you perform an output operation on a file stream followed by an input operation or an input operation followed by an output operation, without an intermediate repositioning of the file stream. The checker detects issues in file operations using one of the following file stream representations:

A FILE* pointer for files opened using functions such as fopen()
An std::basic_filebuf object for files opened using functions such as std::fstream()

Reset the position in the file stream in one of these ways:

For FILE* pointers, use a file positioning function such as fseek() or fsetpos() between output and input operations or call the fflush() function to flush the output buffer between an output and a subsequent input operation.
For std::basic_filebuf objects, use file stream positioning methods such as seekp() or seekg().

Troubleshooting

If you expect a rule violation but Polyspace^® does not report it, see Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

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Input and Output Operations in Succession Without Intermediate Repositioning

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

The << operator performs the input operation.
The std::getline() function performs the output operation.

However, there is no file stream repositioning operation between the output and input operations. The function getOtherLineFromFile() remedies this issue by repositioning the file stream using the 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: Input/output library

Category: Required, Automated

Version History

Introduced in R2020b