fopen, freopen, fdopen - open a stream

     #include <stdio.h>

     FILE *fopen(const char *filename, const char *type)
     FILE *freopen(const char *filename, const char *type, FILE *stream)
     FILE *fdopen(int fildes, const char *type)

     Fopen opens the file named by filename and associates a stream  with  it.
     Fopen  returns  a pointer to be used to identify the stream in subsequent

     Type is a character string having one of the following values:

     "r"  open for reading

     "w"  create for writing

     "a"  append: open for writing at end of file, or create for writing

     In addition, each type may be followed by a "+" to have the  file  opened
     for  reading  and writing.  "r+" positions the stream at the beginning of
     the file, "w+" creates or truncates it, and "a+" positions it at the end.
     Both  reads  and  writes  may  be  used  on  read/write streams, with the
     limitation that an fseek, rewind, or reading an end-of-file must be  used
     between a read and a write or vice-versa.

     Freopen substitutes the named file in  place  of  the  open  stream.   It
     returns the original value of stream.  The original stream is closed.

     Freopen is typically used to attach the preopened constant names,  stdin,
     stdout, stderr, to specified files.

     Fdopen associates a stream with a file  descriptor  obtained  from  open,
     dup,  creat, or pipe(2).  The type of the stream must agree with the mode
     of the open file.

     open(2), fclose(3).

     Fopen and freopen return the pointer NULL if filename cannot be accessed,
     if  too  many files are already open, or if other resources needed cannot
     be allocated.

     Fdopen is not portable to systems other than UNIX.

     The read/write types do not exist on all systems.  Those systems  without
     read/write  modes  will  probably  treat  the  type as if the "+" was not
     present.  These are unreliable in any event.