How to get error message when ifstream open fails - c++

Question

ifstream f;
f.open(fileName);
if ( f.fail() )
{
// I need error message here, like "File not found" etc. -
// the reason of the failure
}
How to get error message as string?

Answer 1

Every system call that fails update the errno value.
Thus, you can have more information about what happens when a ifstream open fails by using something like :
cerr << "Error: " << strerror(errno);
However, since every system call updates the global errno value, you may have issues in a multithreaded application, if another system call triggers an error between the execution of the f.open and use of errno.
On system with POSIX standard:
errno is thread-local; setting it in one thread does not affect its
value in any other thread.
Edit (thanks to Arne Mertz and other people in the comments):
e.what() seemed at first to be a more C++-idiomatically correct way of implementing this, however the string returned by this function is implementation-dependant and (at least in G++'s libstdc++) this string has no useful information about the reason behind the error...

Answer 2

You could try letting the stream throw an exception on failure:
std::ifstream f;
//prepare f to throw if failbit gets set
std::ios_base::iostate exceptionMask = f.exceptions() | std::ios::failbit;
f.exceptions(exceptionMask);
try {
f.open(fileName);
}
catch (std::ios_base::failure& e) {
std::cerr << e.what() << '\n';
}
e.what(), however, does not seem to be very helpful:
I tried it on Win7, Embarcadero RAD Studio 2010 where it gives "ios_base::failbit set" whereas strerror(errno) gives "No such file or directory."
On Ubuntu 13.04, gcc 4.7.3 the exception says "basic_ios::clear" (thanks to arne)
If e.what() does not work for you (I don't know what it will tell you about the error, since that's not standardized), try using std::make_error_condition (C++11 only):
catch (std::ios_base::failure& e) {
if ( e.code() == std::make_error_condition(std::io_errc::stream) )
std::cerr << "Stream error!\n";
else
std::cerr << "Unknown failure opening file.\n";
}

Answer 3

Following on #Arne Mertz's answer, as of C++11 std::ios_base::failure inherits from system_error (see http://www.cplusplus.com/reference/ios/ios_base/failure/), which contains both the error code and message that strerror(errno) would return.
std::ifstream f;
// Set exceptions to be thrown on failure
f.exceptions(std::ifstream::failbit | std::ifstream::badbit);
try {
f.open(fileName);
} catch (std::system_error& e) {
std::cerr << e.code().message() << std::endl;
}
This prints No such file or directory. if fileName doesn't exist.

Answer 4

You can also throw a std::system_error as shown in the test code below. This method seems to produce more readable output than f.exception(...).
#include <exception> // <-- requires this
#include <fstream>
#include <iostream>
void process(const std::string& fileName) {
std::ifstream f;
f.open(fileName);
// after open, check f and throw std::system_error with the errno
if (!f)
throw std::system_error(errno, std::system_category(), "failed to open "+fileName);
std::clog << "opened " << fileName << std::endl;
}
int main(int argc, char* argv[]) {
try {
process(argv[1]);
} catch (const std::system_error& e) {
std::clog << e.what() << " (" << e.code() << ")" << std::endl;
}
return 0;
}
Example output (Ubuntu w/clang):
$ ./test /root/.profile
failed to open /root/.profile: Permission denied (system:13)
$ ./test missing.txt
failed to open missing.txt: No such file or directory (system:2)
$ ./test ./test
opened ./test
$ ./test $(printf '%0999x')
failed to open 000...000: File name too long (system:36)

Answer 5

The std::system_error example above is slightly incorrect. std::system_category() will map the error codes from system's native error code facility. For *nix, this is errno. For Win32, it is GetLastError(). ie, on Windows, the above example will print
failed to open C:\path\to\forbidden: The data is invalid
because EACCES is 13 which is the Win32 error code ERROR_INVALID_DATA
To fix it, either use the system's native error code facility, eg on Win32
throw new std::system_error(GetLastError(), std::system_category(), "failed to open"+ filename);
Or use errno and std::generic_category(), eg
throw new std::system_error(errno, std::generic_category(), "failed to open"+ filename);