What is the reason of file open failure?

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.

PHP Code:

---

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!

Check this FAQ
[17] Exceptions and error handling

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';
}

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?

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

Code:

---

#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:

Code:

---

      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

---

Quote:

---

Originally Posted by treuss

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::open 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. :(