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MISRA C++:2023 Rule 30.0.2

Reads and writes on the same file stream shall be separated by a positioning operation

Since R2024b

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Description

Rule Definition

Reads and writes on the same file stream shall be separated by a positioning operation.

Rationale

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

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 single 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.

Polyspace Implementation

The rule checker reports a violation 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. The checker detects issues in file operations using one of these file pointers:

  • 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())

Depending on whether you use a FILE* pointer or an std::basic_filebuf object, you can reset the position in the file stream using one of these functions:

  • For std::basic_filebuf objects, you can reposition a file stream using methods such as std::fstream<>::seekp() or std::fstream<>::seekg().

  • For FILE* pointers, you can call 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)..

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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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: Input/Output Library
Category: Required

Version History

Introduced in R2024b

See Also

Topics