NAME

fdopen — associate a stream with a file descriptor

SYNOPSIS

[CX] #include <stdio.h>

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

DESCRIPTION

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

The mode argument points to a character string that is valid for fopen(). If the string begins with one of the following characters, then the stream shall be associated with fildes as specified. Otherwise, the behavior is undefined.

'r'

If mode includes '+', the associated stream shall be open for update (reading and writing); otherwise, the stream shall be open for reading only. If the open file description referenced by fildes has O_APPEND set, it shall remain set.

'w'

If mode includes '+', the associated stream shall be open for update (reading and writing); otherwise, the stream shall be open for writing only. The file shall not be truncated by the fdopen() call. If the open file description referenced by fildes has O_APPEND set, it shall remain set.

'a'

If mode includes '+', the associated stream shall be open for update (reading and writing); otherwise, the stream shall be open for writing only. If the open file description referenced by fildes has O_APPEND clear, it is unspecified whether O_APPEND is set by the fdopen() call or remains clear.

The presence of 'x' in mode shall have no effect. The FD_CLOEXEC flag of fildes shall be unchanged if 'e' is not present, and shall be set by the fdopen() call if 'e' is present.

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:

[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.

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(), write ('w') mode cannot create or truncate a file, and append ('a') mode cannot create a file. Inclusion of a 'b' in the mode argument is allowed for consistency with fopen(); the 'b' has no effect on the resulting stream. Implementations differ as to whether specifying append ('a') mode causes the O_APPEND flag to be set if it was clear, but they are encouraged to do so. Since fdopen() does not create a file, the 'x' mode modifier is silently ignored. The 'e' mode modifier is not strictly necessary for fdopen(), since FD_CLOEXEC must not be changed when it is absent; however, it is standardized here since that modifier is necessary to avoid a data race in multi-threaded applications using freopen(), and consistency dictates that all functions accepting mode strings should allow the same set of strings.

FUTURE DIRECTIONS

None.

SEE ALSO

2.5.1 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:

• 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.

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.

Issue 8

Austin Group Defects 411 and 1526 are applied, changing the requirements for the mode argument.