FDOPEN(3POSIX)              POSIX Programmer's Manual             FDOPEN(3POSIX)

       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.

       fdopen — associate a stream with a file descriptor

       #include <stdio.h>

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

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

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

       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-

       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

       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.

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

       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.

       The following sections are informative.


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

       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.1‐2008, a good implementation of append
       (a) mode would cause the O_APPEND flag to be set.


       Section 2.5.1, Interaction of File Descriptors and Standard I/O Streams,
       fclose(), fmemopen(), fopen(), open(), open_memstream(), posix_spawn(),

       The Base Definitions volume of POSIX.1‐2008, <stdio.h>

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical
       and Electronics Engineers, Inc and The Open Group.  (This is POSIX.1-2008
       with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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                   2013                        FDOPEN(3POSIX)