FOPEN(3S)                                                            FOPEN(3S)



NAME
       fopen, freopen, fdopen - open a stream

SYNOPSIS
       #include <stdio.h>

       FILE *fopen(filename, type)
       char *filename, *type;

       FILE *freopen(filename, type, stream)
       char *filename, *type;
       FILE *stream;

       FILE *fdopen(fildes, type)
       char *type;

DESCRIPTION
       _F_o_p_e_n opens the file named by _f_i_l_e_n_a_m_e and associates a stream with it.
       _F_o_p_e_n returns a pointer to be used to identify the stream in subsequent
       operations.

       _T_y_p_e 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 _t_y_p_e 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  _f_s_e_e_k_,  _r_e_w_i_n_d_, or reading an end-of-file must be
       used between a read and a write or vice-versa.

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

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

       _F_d_o_p_e_n associates a stream with a file descriptor obtained  from  _o_p_e_n_,
       _d_u_p_,  _c_r_e_a_t_,  or  _p_i_p_e(2).   The _t_y_p_e of the stream must agree with the
       mode of the open file.

SEE ALSO
       open(2), fclose(3)

DIAGNOSTICS
       _F_o_p_e_n and _f_r_e_o_p_e_n  return  the  pointer  NULL  if  _f_i_l_e_n_a_m_e  cannot  be
       accessed,  if  too  many  files are already open, or if other resources
       needed cannot be allocated.

BUGS
       _F_d_o_p_e_n is not portable to systems other than UNIX.

       The read/write _t_y_p_e_s do not exist on all systems.  Those systems  with‐
       out read/write modes will probably treat the _t_y_p_e as if the "+" was not
       present.  These are unreliable in any event.

       In order to support the same number of open files as does  the  system,
       _f_o_p_e_n  must allocate additional memory for data structures using _c_a_l_l_o_c
       after 20 files have been opened.  This confuses some programs which use
       their  own memory allocators.  An undocumented routine, _f__p_r_e_a_l_l_o_c, may
       be called to force immediate allocation of all internal  memory  except
       for buffers.



4th Berkeley Distribution        May 27, 1986                        FOPEN(3S)