How to get error message when ifstream open fails

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

User · Accepted Answer

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...

User · Answer

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 amp  e        std  cerr  lt  lt  e code   message    lt  lt  std  endl      This prints No such file or directory  if fileName doesn t exist

User · Answer

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 amp  e      std  cerr  lt  lt  e what    lt  lt    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 amp  e      if   e code      std  make error condition std  io errc  stream        std  cerr  lt  lt   Stream error  n      else     std  cerr  lt  lt   Unknown failure opening file  n

User · Answer

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     quot failed to open quot   filename    Or use errno and std  generic category    eg throw new std  system error errno  std  generic category     quot failed to open quot   filename

User · Answer

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  lt exception gt      lt -- requires this  include  lt fstream gt   include  lt iostream gt   void process const std  string amp  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  lt  lt   opened    lt  lt  fileName  lt  lt  std  endl     int main int argc  char  argv          try           process argv 1          catch  const std  system error amp  e            std  clog  lt  lt  e what    lt  lt        lt  lt  e code    lt  lt       lt  lt  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

[c++] How to get error message when ifstream open fails

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