CODEWITHSUNDEEP
fileassemblyhandle
KawaiKx
KawaiKx

Reputation: 9920

what is a file handle and where it is useful for a programmer?

I am learning assembly language along with C. this new chapter I started talks about 'file handles', file handles for screen display and file handles for keyboard input etc. I don't know what is a file handle? I am referring to IBM PC ASSEMBLY LANGUAGE PROGRAMMING by Peter Abel

Upvotes: 16

Views: 33854

Answers (3)

Heath Hunnicutt
Heath Hunnicutt

Reputation: 19477

There is a generic concept generally called a "handle" in the context of computer software APIs. In the comments you have probably found a link to the Wikipedia article on that subject.

You are dealing with a specific implementation of a handle data type -- the IBM PC/DOS file handles returned from the int 0x21 interface. If you would like to learn more about these specific file handles, you might want to consult the book Undocumented DOS, which details the in-memory data structures which allow you to investigate these handles further.

Another specific type of handle is the file descriptor returned from the POSIX-standard interface named open(). This function is implemented in the C run-time library on platforms such as Linux, Windows NT, Mac OS, and many other systems. The integer returned from a call to open() may not be a negative number.

Unless you are running under DOS, your file handles are probably provided by the Windows NT Operating System. These file handles are returned from CreateFile() (which is used to open as well as create files), and the only illegal value for a handle returned from this function is INVALID_HANDLE_VALUE. I.e., the Windows NT API may return what would be considered (via casting) a "negative" integer, although it has opened the file.

In all of these cases, the file handle is used to refer to some data structure that keeps track of how the file is open. One important thing which is tracked is the current file position. The position or pointer is set in POSIX by the lseek() function and is read by the tell() function. Any read() or write() takes place from the position of the current file pointer.

Your program can open the same file under two different handles. In this case, the file pointer for each handle is distinct. Updating the file pointer of one handle using lseek() will not affect the file pointer of the other handle to the same file.

Undocumented DOS

Upvotes: 15

James
James

Reputation: 9278

A handle is something the kernel uses internally to access some resource. Only the kernel really knows what it means, the user process is only told what value to use when it wants to access this resource. They have another advantage in that file handles can be shared among processes - whereas you can't do this with pointers.

Windows uses handles all over the place... files, bitmaps, device contexts, fonts, etc.

Upvotes: 4

user2100815
user2100815

Reputation:

A file handle is an integer value which is used to address an open file. Such handles are highly operating system specific, but on systems that support the open() call, you create a handle like this:

int handle = open( "foo.txt", OTHER_STUFF_HERE );

You can then use the handle with read/write calls. The non-portability of handles mean that most people avoid them and instead use the stream library functions in C, such as fopen, fread, fwrite etc.

Upvotes: 11

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