NAME
fdopen — associate
a stream with a file descriptor
SYNOPSIS
FILE *fdopen(int fildes, const char *mode);
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(3), 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]
If fildes refers to a shared memory object, the result of
the fdopen () function is unspecified.
[TYM]
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.
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:
EXAMPLES
None.
APPLICATION USAGE
File descriptors are obtained from calls like open(3), dup(3), creat(3), or pipe(3), which open files but do not return streams.
RATIONALE
The file descriptor may have been obtained from open(3), creat(3), pipe(3), dup(3), fcntl(3), or socket(3); inherited through fork(3), posix_spawn(3), or exec; or perhaps obtained by other means.
The meanings of the mode arguments of
fdopen () and fopen(3) 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(3). 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.
FUTURE DIRECTIONS
None.
SEE ALSO
V2_chap02(3), fclose(3), fmemopen(3), fopen(3), open(3), open_memstream(3), posix_spawn(3), socket(3)
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:
- In the DESCRIPTION, the use and setting of the mode argument are changed to include binary streams.
- In the DESCRIPTION, text is added for large file support to indicate setting of the offset maximum in the open file description.
- All errors identified in the ERRORS section are added.
- In the DESCRIPTION, text is added that the
fdopen() function may cause st_atime to be updated.
The following changes were made to align with the IEEE P1003.1a draft standard:
- Clarification is added that it is the responsibility of the application to ensure that the mode is compatible with the open file descriptor.
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 (“POSIX.1”)/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.