fdopen.3p fdopen

The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


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 st_atime
field 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 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.
[EMFILE]
STREAM_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 IEEE Std 1003.1-2001, 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() , fopen() , open() , posix_spawn() , socket() , the Base Definitions volume of
IEEE Std 1003.1-2001, <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/Cor 2-2004, item XSH/TC2/D6/30 is applied, making corrections to the RATIONALE.

End of informative text.


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