How Can I Get Error Messages from Failed `ifstream` Opens?

How to Retrieve Error Messages from Failed ifstream Openings

When attempting to open a file using ifstream and it fails, it's often desirable to retrieve the reason behind the failure. Here's how to do it:

Using errno and strerror

Every system call that fails updates the global errno value. This value can be used to obtain more information about the error:

ifstream f;
f.open(fileName);

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

Note that strerror provides the error message as a string.

Considerations for Multithreaded Applications

If using this method in a multithreaded application, it's important to consider the possibility of other system calls modifying errno between the f.open and errno retrieval.

Operating System Differences

On POSIX systems, errno is thread-local, meaning that changes made to it in one thread do not affect other threads. This isn't necessarily the case on all systems.

Avoiding Use of what()

Initially, it may seem like e.what() is a better approach, as it's more idiomatic in C . However, the string returned by this function is implementation-specific and may not always provide useful information about the failure.

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