Fstream Error Code

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Ifstream Open Fail

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Std::ifstream::failure

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C++ Ifstream Exceptions

up How to get error message when ifstream open fails up vote 39 down vote favorite 7 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? c++ error-handling stream std share|improve this question edited Mar 7 '14 ifstream failbit at 2:28 lpapp 35.4k134766 asked Jun 27 '13 at 7:51 0123456789 24.2k2191156 2 possible duplicate of C++ ifstream Error Checking –Matthieu Rouget Jun 27 '13 at 8:19 1 possible duplicate of Can you get a specific error condition when a C++ stream open fails? –arne Jun 27 '13 at 8:28 3 @Alex Farber: Sure. cerr << "Error code: " << strerror(errno); // Get some info as to why seems relevant to the question. –Matthieu Rouget Jun 27 '13 at 8:28 @MatthieuRouget: Check the possible duplicate I posted -- it seems this is non-standard behaviour only implemented by gcc. –arne Jun 27 '13 at 8:29 @MatthieuRouget: strerror(errno) works. Post this as answer, I will accept it. –0123456789 Jun 27 '13 at 8:37 add a comment| 3 Answers 3 active oldest votes up vote 30 down vote accepted 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 err

for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of c++ file error handling this site About Us Learn more about Stack Overflow the company Business Learn c++ strerror more about hiring developers or posting ads with us Code Review Questions Tags Users Badges Unanswered Ask Question _ strerror_s Code Review Stack Exchange is a question and answer site for peer programmer code reviews. Join them; it only takes a minute: Sign up Here's how it works: Anybody can ask http://stackoverflow.com/questions/17337602/how-to-get-error-message-when-ifstream-open-fails a question Anybody can answer The best answers are voted up and rise to the top Better option than “errno” for file IO error handling up vote 10 down vote favorite 3 I have the following method for opening a file: void TankFile::OpenForReading(const std::string & filename) { assert(!filename.empty()); errno = 0; file.exceptions(0); // Don't throw file.open(filename, (std::fstream::in | std::fstream::binary)); if (!file.is_open() || !file.good()) { http://codereview.stackexchange.com/questions/57829/better-option-than-errno-for-file-io-error-handling const char * errorStr = strerror(errno); throw TankFileError(Format("Failed to open Tank file \"%s\": '%s'", filename.c_str(), errorStr)); } } The objective here is to attempt to open a file and throw TankFileError with a proper error description on failure. The caller will be expecting this exception type. Everything works fine and I get a nice error message like this if the exception is thrown: Failed to open Tank file "unexistent_file": 'No such file or directory' But what I don't like in that block is having to use the errno global and strerror(). A way around it would be to let the stream throw an exception, then catch it, get the error message from the what() member and re-throw with TankFileError, but I find this solution also a bit hackish, plus, in the tests I did, the resulting error message from std::fstream::failure was pretty cryptic: void TankFile::OpenForReading(const std::string & filename) { assert(!filename.empty()); try { file.exceptions(std::fstream::failbit); file.open(filename, (std::fstream::in | std::fstream::binary)); } catch (std::fstream::failure & err) { throw TankFileError(Format("Failed to open Tank file \"%s\": '%s'", filename.c_str(), err.what())); } } Produced the error message: Failed to open Tank file "unexistent_file": 'ios_base::clear: unspecified iostream_ca

open (const string& filename, ios_base::openmode mode = ios_base::in); Open file Opens the file identified by argument filename, associating it with the stream object, so that input/output operations are http://www.cplusplus.com/reference/fstream/ifstream/open/ performed on its content. Argument mode specifies the opening mode. If the stream is already associated with a file (i.e., it is already open), calling this function fails. The file association of a stream is kept by its internal stream buffer: Internally, the function calls rdbuf()->open(filename,mode) The function sets failbit in case of failure. fstream error The function clears the stream's state flags on success (setting them to goodbit). In case of failure, failbit is set. Parameters filename String with the name of the file to open. Specifics about its format and validity depend on the library implementation and running environment. mode Flags describing the requested i/o mode for the file. This fstream error code is an object of the bitmask member type openmode that consists of a combination of the following member constants: member constantstands foraccess in *inputFile open for reading: the internal stream buffer supports input operations. outoutputFile open for writing: the internal stream buffer supports output operations. binarybinaryOperations are performed in binary mode rather than text. ateat endThe output position starts at the end of the file. appappendAll output operations happen at the end of the file, appending to its existing contents. trunctruncateAny contents that existed in the file before it is open are discarded. These flags can be combined with the bitwise OR operator (|). * in is always set for ifstream objects (even if explicitly not set in argument mode). Note that even though ifstream is an input stream, its internal filebuf object may be set to also support output operations. If the mode has app set, the opening operation fails. It also fails if trunc is set but out is not. If the mode ha