xref: /illumos-gate/usr/src/man/man3c/open_memstream.3c (revision cd62a92d4a964bfe61d35ba2301b69e65e22a509)

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.Dd March 25, 2020
.Dt OPEN_MEMSTREAM 3C
.Os
.Sh NAME
.Nm open_memstream ,
.Nm open_wmemstream
.Nd open a memory stream
.Sh SYNOPSIS
.In stdio.h
.Ft "FILE *"
.Fo open_memstream
.Fa "char **bufp"
.Fa "size_t *sizep"
.Fc
.In wchar.h
.Ft "FILE *"
.Fo open_wmemstream
.Fa "wchar_t **bufp"
.Fa "size_t *sizep"
.Fc
.Sh DESCRIPTION
The
.Fn open_memstream
and
.Fn open_wmemstream
functions create an I/O stream that is backed by a dynamic memory buffer
which will grow as needed.
The stream is seekable and writable.
The stream is not readable.
The stream is byte-oriented in the case of the
.Fn open_memstream
function or it is wide-oriented in the case of the
.Fn open_wmemstream
function.
.Pp
Memory for the stream is dynamically allocated and reallocated as though
a call to
.Xr malloc 3C .
As the stream is written to, flushed, or closed, the underlying buffer
will be grown automatically as required.
After the stream is flushed or closed, the
.Fa bufp
argument will be filled in with a pointer to the current buffer.
The
.Fa sizep
argument will be filled in with the smaller of the current buffer length
and the current position in bytes
.Pq Fn open_memstream
or wide characters
.Pq Fn open_wmemstream ,
excluding a NUL character.
Note, because the current position is taken into account, the actual
size of the buffer may be larger than is found in
.Fa sizep ;
however data should not be accessed beyond the indicated size.
The values stored in the
.Fa bufp
and
.Fa sizep
arguments are only valid until the next write operation on the stream
or a call to
.Xr fclose 3C .
.Pp
The stream maintains both the current position and the current length of
the stream.
Both the initial position and length of the buffer are set to zero.
Whenever a write at the current position exceeds the current length of the
buffer, the current length is increased and a NUL character,
.Sq \e0
.Pq Fn open_memstream
or NUL wide character,
.Sq L\e0
.Pq Fn open_wmemstream
will be added to the buffer.
If the stream is seeked beyond the current length and a subsequent write
occurs, intervening characters will be filled with the corresponding NUL
character for the stream.
.Pp
To release the stream, the caller must call the
.Xr fclose 3C
function.
The corresponding dynamically allocated memory will be disassociated
from the stream and will become owned by the caller.
The caller must eventually release the memory buffer pointed to in the
.Fa bufp
argument with a call to
.Xr free 3C .
.Ss Fn open_wmemstream , Xr fseek 3C , Xr fsetops 3C, and Xr ftell 3C
The specification for the
.Fn open_wmemstream
function causes the
.Xr fseek 3C
and
.Xr ftell 3C
families of functions to operate differently.
Traditionally, these functions always return units in bytes, regardless
of whether the underlying stream is byte- or wide-oriented.
However, when used against a stream created by the
.Fn open_wmemstream
function these now count in terms of units of wide characters.
While this is different from the traditional behavior, this does mean
that the units will be the same as tracked in
.Fa sizep .
.Ss Fn open_wmemstream and byte-oriented functions
Unlike other streams, streams created by
.Fn open_wmemstream
are not only wide-oriented, but operate in terms of the
.Vt wchar_t
data type.
When normal bytes are written to the stream, an implicit multi-byte
conversion state is maintained.
Writing byte sequences that don't correspond to valid byte sequences in
the locale can cause I/O errors to occur when using write functions such
as
.Xr fputc 3C
or
.Xr fwrite 3C ,
or when buffered data is flushed through functions like
.Xr fflush 3C
or
.Xr fclose 3C .
.Pp
The same problem can occur when explicitly using wide characters, for
example,
.Xr fputwc 3C ,
if the
wide character represents a code point that is not valid in the current
locale.
To avoid these errors when flushing or closing, one can disable
buffering by passing
.Dv _IONBF
as the buffering type with
.Xr setvbuf 3C .
.Pp
It is not recommended to change the locale of such a stream while
writing a byte sequence that represents valid wide characters.
The behavior of using byte-oriented functions is not standardized and
not all systems will behave the same way.
.Sh RETURN VALUES
Upon successful completion, the
.Fn open_memstream
and
.Fn open_wmemstream
functions returns a pointer to a stream.
Otherwise,
.Dv NULL
is returned and
.Dv errno
is set to indicate the error.
.Sh ERRORS
The
.Fn fmemopen
function will fail if:
.Bl -tag -width Er
.It Er EINVAL
Either of the
.Fa bufp
or
.Fa sizep
arguments are
.Dv NULL .
.It Er EMFILE
.Brq FOPEN_MAX
streams are currently open in the calling process.
.Pp
.Brq STREAM_MAX
streams are currently open in the calling process.
.It Er ENOMEM
The system was unable to allocate memory for the stream or its backing
memory buffer.
.El
.Sh MT-LEVEL
.Sy MT-Safe
.Sh INTERFACE STABILITY
.Sy Committed
.Sh SEE ALSO
.Xr fclose 3C ,
.Xr fflush 3C ,
.Xr fmemopen 3C ,
.Xr free 3C ,
.Xr malloc 3C ,
.Xr setvbuf 3C