Fstream Error String

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Ifstream Open Fail

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Std::ifstream::failure

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C++ Ifstream Exceptions

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 at 2:28 lpapp 35.4k134766 asked Jun 27 '13 at 7:51 0123456789 24.2k2191156 2 ifstream failbit 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 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. Ed

here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more about Stack Overflow the company Business c++ file error handling Learn more about hiring developers or posting ads with us Stack Overflow Questions Jobs Documentation c++ strerror Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just strerror_s like you, helping each other. Join them; it only takes a minute: Sign up Get std::fstream failure error messages and/or exceptions up vote 32 down vote favorite 5 I'm using fstream. Is there any way to get http://stackoverflow.com/questions/17337602/how-to-get-error-message-when-ifstream-open-fails the failure message/exception? For example if I'm unable to open the file? c++ share|improve this question edited Apr 17 '13 at 13:58 Jan Rüegg 3,03711860 asked May 8 '09 at 12:48 sofr 1,47041626 add a comment| 3 Answers 3 active oldest votes up vote 29 down vote Streams by default do not throw exceptions on error, they set flags. You can make them throw exceptions by using the stream's exception() member function: ifstream ifs; ifs.exceptions( http://stackoverflow.com/questions/839644/get-stdfstream-failure-error-messages-and-or-exceptions std::ios::failbit ); // throw if failbit get set Theoretically, you could then do something like this: try { int x; ifs >> x; } catch( const std::exception & ex ) { std::cerr << "Could not convert to int - reason is " << ex.what(); } Unfortunately, the C++ Standard does not specify that thrown exceptions contain any error message, so you are in implementation specific territory here. share|improve this answer edited May 8 '09 at 13:09 answered May 8 '09 at 13:03 anon add a comment| up vote 9 down vote Short answer: no. Even checking errno after you detect failure (using e.g. bad(), fail()) after various operations doesn't reliably work. Creating an ifstream/ofstream wrapping a file that can't be opened doesn't necessarily set a failure bit until you try to read, write, or close it. Long answer: you can call ios::exceptions(ios_base::iostate) to request that ios_base::ios_failure exceptions be thrown when a corresponding bit (badbit, failbit, eofbit) is set, but this (at least on GNU and Microsoft C++ libraries) doesn't get you any more information than manually checking the bits, and ends up being largely pointless, IMHO. share|improve this answer edited May 8 '09 at 13:13 answered May 8 '09 at 13:05 Doug 6,0301732 add a comment| up vote 2 down vote accepted From checking it out I found that also errno and

Motivated by this POV-Ray issue I tried to find a reliable way to read a file line by line in C++ using std::ifstream in combination with https://gehrcke.de/2011/06/reading-files-in-c-using-ifstream-dealing-correctly-with-badbit-failbit-eofbit-and-perror/ std::getline(). While doing so, the goal was to handle all underlying stream http://www.cplusplus.com/reference/fstream/ifstream/open/ errors as well as file opening errors, and to emit as precise error messages as possible. In a high-level programming language such as Python this level of reliability and usability is not difficult to obtain. However, in C++ this turned out to be a rather complex topic. Proper fstream error handling of the stream error bits eofbit, failbit, and badbit requires a tremendous amount of care, as discussed for example here, here, and here, and finally at cplusplus.com. It is worth mentioning that although cplusplus.com is a convenient reference, it does not provide us with a rock-solid solution for the above-stated problem and also does not mention all the important details. fstream error string When it comes to the idea of providing meaningful error messages, things become quite complicated. Proper evaluation of errno, respectively perror(), in response to the stream error bits is not a trivial task as can be inferred from discussions like this and this. From these discussions we learn that most of the related uncertainty comes from a lack of centralized documentation or even missing documentation. The exact behavior of C++ code with respect to file handling and stream manipulation is defined by an intertwining of language specification (C++ in this case), operating system interface (e.g. POSIX) and low-level APIs (provided by e.g. libc) -- they all are documented in different places and to a different extent. We for example expect that when fopen() returns NULL, errno is set to something meaningful. But where is this actually documented? In order to understand the relation between the language and operating system constructs involved, I performed quite some research and testing. Of course there are many obvious and non-obvious ways to write unreliable code. As expected, for writing reliable code, there also best p

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 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. 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 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 has both trunc and app set, the opening operation fails. It also fails if trunc is set but out is not. Return Value none If the function fails to open a file, the failbit state flag is set for the stream (which may throw ios_base::failure if that state flag was registered using member exceptions). Example 1
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