fdopen(3p) — Linux manual page


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

PROLOG         top

       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.

NAME         top

       fdopen — associate a stream with a file descriptor

SYNOPSIS         top

       #include <stdio.h>

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

DESCRIPTION         top

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

       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

       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

       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

RETURN VALUE         top

       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.

ERRORS         top

       The fdopen() function shall fail if:

       EMFILE {STREAM_MAX} streams are currently open in the calling

       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

       ENOMEM Insufficient space to allocate a buffer.

       The following sections are informative.

EXAMPLES         top



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

RATIONALE         top

       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.



SEE ALSO         top

       Section 2.5.1, Interaction of File Descriptors and Standard I/O
       Streams, fclose(3p), fmemopen(3p), fopen(3p), open(3p),
       open_memstream(3p), posix_spawn(3p), socket(3p)

       The Base Definitions volume of POSIX.1‐2017, stdio.h(0p)

COPYRIGHT         top

       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(3P)

Pages that refer to this page: stdio.h(0p)fileno(3p)fmemopen(3p)fopen(3p)freopen(3p)open_memstream(3p)