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 (http://www.parashift.com/c++-faq-lite/exceptions.html)

[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::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. :(