The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group

NAME

fdopen - associate a stream with a file descriptor

SYNOPSIS

[CX] [Option Start] #include <stdio.h>

FILE *fdopen(int
fildes, const char *mode); [Option End]

DESCRIPTION

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.

[SHM] [Option Start] If fildes refers to a shared memory object, the result of the fdopen() function is unspecified. [Option End]

[TYM] [Option Start] If fildes refers to a typed memory object, the result of the fdopen() function is unspecified. [Option End]

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

RETURN VALUE

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

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.

EXAMPLES

None.

APPLICATION USAGE

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

RATIONALE

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.

FUTURE DIRECTIONS

None.

SEE ALSO

Interaction of File Descriptors and Standard I/O Streams, fclose, fmemopen, fopen, open, open_memstream, posix_spawn, socket

XBD <stdio.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Realtime Extension.

Large File Summit extensions are added.

Issue 6

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The following changes were made to align with the IEEE P1003.1a draft standard:

The DESCRIPTION is updated for alignment with IEEE Std 1003.1j-2000 by specifying that fdopen() results are unspecified for typed memory objects.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/30 is applied, making corrections to the RATIONALE.

Issue 7

SD5-XSH-ERN-149 is applied, adding the {STREAM_MAX} [EMFILE] error condition.

Changes are made related to support for finegrained timestamps.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0121 [409] is applied.

End of informative text.

 

return to top of page

UNIX ® is a registered Trademark of The Open Group.
POSIX ™ is a Trademark of The IEEE.
Copyright © 2001-2018 IEEE and The Open Group, All Rights Reserved
[ Main Index | XBD | XSH | XCU | XRAT ]