What is the reason of file open failure?

[infrared]

hello,
I'm programming a simulation of a disk.
I was told to use the try/catch mechanism to catch I/O errors.
This is a part of a function i'm using,
dskfl is an instance of fstream.

void disk::mountdisk(String sFilename)
{
    dskfl.open(sFilename.GetBuffer(),ios::in);
    
    if (!dskfl.is_open())
    {
        //what to put here?
    }

    Sector sec;
    dskfl >> sec; 

What do i need to put in the !is_open(), if i want to throw an exception (with the reason of the error!!!) to the caller of the function?

Thanks!

[sreehari]

Check this FAQ
[17] Exceptions and error handling

[satchi gowda]

You will be using try/catch if you are expecting something to go wrong. (Like trying to divide a number by 0) . In your case you can expect an error to occur while you try to open the file itself. But if you want to throw an exception then try this

try
{
dskfl.open(sFilename.GetBuffer(),ios::in);
if (!dskfl.is_open())
{
throw "Put your message here";
}
}
catch(char *str)
{
cout << "Exception raised: " << str << '\n';
}

[infrared]

Thanks, but how I determine what the problem was in the open operation?

let's say i'm opening a file the errors that can happend:
1. file not found
2. file already opened
3. some kind of other I/O errors...

how can I throw the correct reason?

[treuss]

Check the variable "errno" (#include <cerrno>) for the reason code or use strerror to get a textual explanation.

#include <fstream>
#include <iostream>
#include <cerrno>

using namespace std;

int main( int argc, char* argv[] )
{
  ifstream f;
  f.open( argv[1] );
  if ( ! f.is_open() ) {
    cout << "Error opening file: " << strerror(errno) << endl;
  }
}

So, either you just create an exception that includes the errno, or you make a big switch statement based on errno and throw specific exceptions like FileNotFound and PermissionDenied. The list of error codes may differ on different OSes, but below is an example:

       E2BIG  Arg list too long

       EACCES Permission denied

       EADDRINUSE
              Address in use

       EADDRNOTAVAIL
              Address not available

       EAFNOSUPPORT
              Address family not supported

       EAGAIN Resource temporarily unavailable

       EALREADY
              Connection already in progress

       EBADF  Bad file descriptor

       EBADMSG
              Bad message

       EBUSY  Resource busy

       ECANCELED
              Operation canceled

       ECHILD No child processes

       ECONNABORTED
              Connection aborted

       ECONNREFUSED
              Connection refused

       ECONNRESET
              Connection reset

       EDEADLK
              Resource deadlock avoided

       EDESTADDRREQ
              Destination address required

       EDOM   Domain error

       EDQUOT Reserved

       EEXIST File exists

       EFAULT Bad address

       EFBIG  File too large

       EHOSTUNREACH
              Host is unreachable

       EIDRM  Identifier removed

       EILSEQ Illegal byte sequence

       EINPROGRESS
              Operation in progress

       EINTR  Interrupted function call

       EINVAL Invalid argument

       EIO    Input/output error

       EISCONN
              Socket is connected

       EISDIR Is a directory

       ELOOP  Too many levels of symbolic links

       EMFILE Too many open files

       EMLINK Too many links

       EMSGSIZE
              Inappropriate message buffer length

       EMULTIHOP
              Reserved

       ENAMETOOLONG
              Filename too long

       ENETDOWN
              Network is down

       ENETRESET
              Connection aborted by network

       ENETUNREACH
              Network unreachable

       ENFILE Too many open files in system

       ENOBUFS
              No buffer space available

       ENODATA
              No message is available on the STREAM head read queue

       ENODEV No such device

       ENOENT No such file or directory

       ENOEXEC
              Exec format error

       ENOLCK No locks available

       ENOLINK
              Reserved

       ENOMEM Not enough space

       ENOMSG No message of the desired type

       ENOPROTOOPT
              Protocol not available

       ENOSPC No space left on device

       ENOSR  No STREAM resources

       ENOSTR Not a STREAM

       ENOSYS Function not implemented

       ENOTCONN
              The socket is not connected

       ENOTDIR
              Not a directory

       ENOTEMPTY
              Directory not empty

       ENOTSOCK
              Not a socket

       ENOTSUP
              Not supported

       ENOTTY Inappropriate I/O control operation

       ENXIO  No such device or address

       EOPNOTSUPP
              Operation not supported on socket

       EOVERFLOW
              Value too large to be stored in data type

       EPERM  Operation not permitted

       EPIPE  Broken pipe

       EPROTO Protocol error

       EPROTONOSUPPORT
              Protocol not supported

       EPROTOTYPE
              Protocol wrong type for socket

       ERANGE Result too large

       EROFS  Read-only file system

       ESPIPE Invalid seek

       ESRCH  No such process

       ESTALE Reserved

       ETIME  STREAM ioctl() timeout

       ETIMEDOUT
              Operation timed out

       ETXTBSY
              Test file busy

       EWOULDBLOCK
              Operation would block (may be same value as EAGAIN)

       EXDEV  Improper link

[SuperKoko]

Check the variable "errno" (#include <cerrno>) for the reason. The list of error codes may differ on different OSes, but below is an example:

fstream:pen may not modify the errno variable (even if it may on some implementations).

However, it is still possible to search for file existence if the file has not been opened.
fstreams does not provide much information on the type of error which occured.