This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       File descriptors are obtained from calls like open(), dup(), creat(), or pipe(), which open files but  do
       not return streams.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee  document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                        FDOPEN(3POSIX)

       The fdopen() function shall associate a stream with a file descriptor.

       The mode argument is a character string having one of the following values:

       r or rb       Open a file for reading.

       w or wb       Open a file for writing.

       a or ab       Open a file for writing at end-of-file.

       r+ or rb+ or r+b
                     Open a file for update (reading and writing).

       w+ or wb+ or w+b
                     Open a file for update (reading and writing).

       a+ or ab+ or a+b
                     Open a file for update (reading and writing) at end-of-file.

       The  meaning  of these flags is exactly as specified in fopen(), except that modes beginning with w shall
       not cause truncation of the file.

       Additional values for the mode argument may be supported by an implementation.

       The application shall ensure that the mode of the stream as expressed by the mode argument is allowed  by
       the  file  access  mode  of the open file description to which fildes refers. The file position indicator
       associated with the new stream is set to the position indicated by the file offset  associated  with  the
       file descriptor.

       The  error  and  end-of-file indicators for the stream shall be cleared.  The fdopen() function may cause
       the last data access timestamp of the underlying file to be marked for update.

       If fildes refers to a shared memory object, the result of the fdopen() function is unspecified.

       If fildes refers to a typed memory object, the result of the fdopen() function is unspecified.

       The fdopen() function shall preserve the offset maximum previously set  for  the  open  file  description
       corresponding to fildes.

       The fdopen() function shall fail if:

       EMFILE {STREAM_MAX} streams are currently open in the calling process.

       The fdopen() function may fail if:

       EBADF  The fildes argument is not a valid file descriptor.

       EINVAL The mode argument is not a valid mode.

       EMFILE {FOPEN_MAX} streams are currently open in the calling process.

       ENOMEM Insufficient space to allocate a buffer.

       Thefollowingsectionsareinformative.

       None.

       None.

       fdopen — associate a stream with a file descriptor

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       The  file  descriptor  may  have been obtained from open(), creat(), pipe(), dup(), fcntl(), or socket();
       inherited through fork(), posix_spawn(), or exec; or perhaps obtained by other means.

       The meanings of the mode arguments of fdopen() and fopen() differ. With fdopen(), open for  write  (w  or
       w+)  does  not  truncate,  and append (a or a+) cannot create for writing. The mode argument formats that
       include a b are allowed for consistency with the ISO C standard function fopen().  The b has no effect on
       the resulting  stream.  Although  not  explicitly  required  by  this  volume  of  POSIX.1‐2017,  a  good
       implementation of append (a) mode would cause the O_APPEND flag to be set.

       Upon  successful completion, fdopen() shall return a pointer to a stream; otherwise, a null pointer shall
       be returned and errno set to indicate the error.

Section2.5.1,  InteractionofFileDescriptorsandStandardI/OStreams, fclose(), fmemopen(), fopen(),
       open(), open_memstream(), posix_spawn(), socket()

       The Base Definitions volume of POSIX.1‐2017, <stdio.h>

       #include <stdio.h>

       FILE *fdopen(int fildes, const char *mode);