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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FDOPEN(3P) POSIX Programmer's Manual FDOPEN(3P)
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 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.
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.
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.
None.
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.1‐2017, a good implementation of append (a) mode would
cause the O_APPEND flag to be set.
None.
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)
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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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