How Can I Retrieve the Error Message When an `ifstream` Fails to Open a File?

Coping with ifstream Open Failures: Retrieve the Error Message

Accessing error messages in C when opening a file using ifstream can be challenging. To address this issue, let's delve into the techniques for extracting the error reason.

The standard way to check for file opening errors is:

ifstream f;
f.open(fileName);

if ( f.fail() )
{
    // I need error message here, like "File not found" etc. -
    // the reason of the failure
}

Using errno for Error Messages

In C , when a system call fails, it updates the global variable errno. Therefore, you can obtain additional details about ifstream open failures using errno:

ifstream f;
f.open(fileName);

if ( f.fail() )
{
    cerr << "Error: " << strerror(errno);
}

Thread Safety Considerations

Note that errno is a global variable. In multithreaded applications, you need to be cautious. If another system call triggers an error between the execution of f.open() and the use of errno, this could affect the accuracy of the error message.

POSIX Standard Systems

On POSIX-compliant systems, errno is thread-local. This implies that setting errno in one thread does not impact its value in any other thread.

Implementation-Dependent e.what() Function

Originally, e.what() seemed like a more C -appropriate approach for retrieving error information. However, this function returns an implementation-specific string. In G 's libstdc , this string often provides little meaningful information about the failure.

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