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

Question

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().

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()

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 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, i.e., fprintf() and family.

There are no performance issues using either.

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.

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 *.