This rule is part of MISRA C++:2023.
MISRA Rule 30.0.2
Category: Required
Analysis Type: Undecidable, System
Amplification
An explicit, interleaving stream positioning operation shall be used between input operations and output operations on a
std::basic_filebuf.
This rule applies to direct and indirect calls (e.g. from std::fstream) to std::basic_filebuf.
Note: for the purposes of this rule, a call to fflush after an output operation is considered to be an explicit file
positioning operation.
Rationale
The C FILE * abstraction, used as the underlying system file input/output for std::basic_filebuf, holds a single file
position that is used when reading from or writing to the file. Using an input operation on a FILE * immediately after an output
operation (or vice versa) results in undefined behaviour, unless an interleaving file positioning operation is used to update the file’s
position.
In addition, a streambuf object keeps separate buffer positions for reading and writing characters from its internal buffer. A
basic_filebuf object is only guaranteed to synchronize the separate internal streambuf read and write positions that it
maintains when a positioning operation is called when alternating between reading and writing (and vice versa). Failure to include such a positioning
operation leads to undefined behaviour.
The accessible positioning operations for streambuf are pubseekoff and pubseekpos, whilst for file streams
they are tellg, seekg, tellp, and seekp. One of these functions shall be called when switching
from output to input, or vice versa.
Example
void show_fstream_non_compliant()
{
std::fstream f { "hello.txt" };
f << "Hello world!\n" << std::flush; // flush is not a positioning operation
std::string s {};
std::getline( f, s ); // Non-compliant - undefined behaviour
}
void show_fstream_compliant()
{
std::fstream f { "hello.txt" };
f << "Hello world!\n";
std::string s {};
f.seekg( 0, std::ios_base::beg );
std::getline(f, s); // Compliant - s holds "Hello world!"
}
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