How to get error message when ifstream open fails
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...
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";
}