What's the difference between a file descriptor and file pointer?

Question

I want to know the difference between a file descriptor and file pointer.

Also, in what scenario would you use one instead of the other?

Answer 1

A file descriptor is a low-level integer "handle" used to identify an opened file (or socket, or whatever) at the kernel level, in Linux and other Unix-like systems.

You pass "naked" file descriptors to actual Unix calls, such as read(), write() and so on.

A FILE pointer is a C standard library-level construct, used to represent a file. The FILE wraps the file descriptor, and adds buffering and other features to make I/O easier.

You pass FILE pointers to standard C functions such as fread() and fwrite().

Answer 2

One is buffered (FILE *) and the other is not. In practice, you want to use FILE * almost always when you are reading from a 'real' file (ie. on the drive), unless you know what you are doing or unless your file is actually a socket or so..

You can get the file descriptor from the FILE * using fileno() and you can open a buffered FILE * from a file descriptor using fdopen()

Answer 3

A file descriptor is just an integer which you get from the Posix' open()call. Using the standard C fopen() you get a FILE struct back. The FILE struct contains the this file descriptor amongst other things such as end-of-file and error indicator, stream position etc.

So using fopen() gives you a certain amount of abstraction compared to open(). In general you should be using fopen() since that is more portable and you can use all the other standard C functions that uses the FILE struct, ie fprintf() and family.

There are no performance issues using either or.

Answer 4

File descriptor vs File pointer

File descriptor:

File Descriptor is an integer value returned by open() system call.

int fd = open (filePath, mode);

1. Low/Kernel level handler.
2. passe to read() and write() of UNIX System Calls.
3. Doesn't include buffering and such features.
4. Less portable and lacks efficiency.

File pointer:

File Pointer is a pointer to a C structure returned by fopen() library function, which is used to identifying a file, wrapping the file descriptor, buffering functionality and all other functionality needed for I/O operation.The file pointer is of type FILE, whose definition can be found in "/usr/include/stdio.h". This definition may vary from one compiler to another.

FILE *fp = fopen (filePath, mode);

// A FILE Structure returned by fopen 
    typedef struct 
    {
        unsigned char   *_ptr;
        int     _cnt;
        unsigned char   *_base;
        unsigned char   *_bufendp;
        short   _flag;
        short   _file;
        int     __stdioid;
        char    *__newbase;
#ifdef _THREAD_SAFE
        void *_lock;
#else
        long    _unused[1];
#endif
#ifdef __64BIT__
        long    _unused1[4];
#endif /* __64BIT__ */
    } FILE;

1. It is high level interface.
2. Passed to fread() and fwrite() functions.
3. Includes buffering,error indication and EOF detection,etc.
4. Provides higher portability and efficiency.

Answer 5

Want to add points which might be useful.

ABOUT FILE *

1. can't be used for interprocess communication(IPC).
2. use it when you need genral purpose buffered I/O.(printf,frpintf,snprintf,scanf)

3. I use it many times for debug logs. example,

                FILE *fp;
                fp = fopen("debug.txt","a");
                fprintf(fp,"I have reached till this point");
                fclose(fp);
   

ABOUT FILE DESCRIPTOR

1. It's generally used for IPC.

2. Gives low-level control to files on *nix systems.(devices,files,sockets,etc), hence more powerfull than the FILE *.

Answer 6

FILE * is more useful when you work with text files and user input/output, because it allows you to use API functions like sprintf(), sscanf(), fgets(), feof() etc.

File descriptor API is low-level, so it allows to work with sockets, pipes, memory-mapped files (and regular files, of course).

Answer 7

Just a note to finish out the discussion (if interested)....

fopen can be insecure, and you should probably use fopen_s or open with exclusive bits set. C1X is offering x modes, so you can fopen with modes "rx", "wx", etc.

If you use open, you might consider open(..., O_EXCL | O_RDONLY,... ) or open(..., O_CREAT | O_EXCL | O_WRONLY,... ).

See, for example, Do not make assumptions about fopen() and file creation.

Answer 8

System calls are mostly using file descriptor, for example read and write. Library function will use the file pointers ( printf , scanf). But, library functions are using internally system calls only.