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The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008
Copyright © 2001-2008 The IEEE and The Open Group

#### NAME

> fmemopen - open a memory buffer stream

#### SYNOPSIS

> ``[CX] #include <stdio.h>
> FILE *fmemopen(void *restrict` buf`, size_t` size`,
> const char *restrict` mode`);``
> `
> `

#### DESCRIPTION

> The fmemopen() function shall associate the buffer given by the buf and size arguments with a stream. The buf argument shall be either a null pointer or point to a buffer that is at least size bytes long.

> The mode argument is a character string having one of the following values:

> r or rb
>   Open the stream for reading.
> w or wb
>   Open the stream for writing.
> a or ab
>   Append; open the stream for writing at the first null byte.
> r+ or rb+ or r+b
>   Open the stream for update (reading and writing).
> w+ or wb+ or w+b
>   Open the stream for update (reading and writing). Truncate the buffer contents.
> a+ or ab+ or a+b
>   Append; open the stream for update (reading and writing); the initial position is at the first null byte.

> The character `'b'` shall have no effect.

> If a null pointer is specified as the buf argument, fmemopen() shall allocate size bytes of memory as if by a call to malloc(). This buffer shall be automatically freed when the stream is closed. Because this feature is only useful when the stream is opened for updating (because there is no way to get a pointer to the buffer) the fmemopen() call may fail if the mode argument does not include a `'+'` .

> The stream maintains a current position in the buffer. This position is initially set to either the beginning of the buffer (for r and w modes) or to the first null byte in the buffer (for a modes). If no null byte is found in append mode, the initial position is set to one byte after the end of the buffer.

> If buf is a null pointer, the initial position shall always be set to the beginning of the buffer.

> The stream also maintains the size of the current buffer contents. For modes r and r+ the size is set to the value given by the size argument. For modes w and w+ the initial size is zero and for modes a and a+ the initial size is either the position of the first null byte in the buffer or the value of the size argument if no null byte is found.

> A read operation on the stream cannot advance the current buffer position beyond the current buffer size. Reaching the buffer size in a read operation counts as ``end-of-file''. Null bytes in the buffer have no special meaning for reads. The read operation starts at the current buffer position of the stream.

> A write operation starts either at the current position of the stream (if mode has not specified `'a'` as the first character) or at the current size of the stream (if mode had `'a'` as the first character). If the current position at the end of the write is larger than the current buffer size, the current buffer size is set to the current position. A write operation on the stream cannot advance the current buffer size beyond the size given in the size argument.

> When a stream open for writing is flushed or closed, a null byte is written at the current position or at the end of the buffer, depending on the size of the contents. If a stream open for update is flushed or closed and the last write has advanced the current buffer size, a null byte is written at the end of the buffer if it fits.

> An attempt to seek a memory buffer stream to a negative position or to a position larger than the buffer size given in the size argument shall fail.

#### RETURN VALUE

> Upon successful completion, fmemopen() shall return a pointer to the object controlling the stream. Otherwise, a null pointer shall be returned, and errno shall be set to indicate the error.

#### ERRORS

> The fmemopen() function shall fail if:

> [EINVAL]
>   The size argument specifies a buffer size of zero.

> The fmemopen() function may fail if:

> [EINVAL]
>   The value of the mode argument is not valid.
> [EINVAL]
>   The buf argument is a null pointer and the mode argument does not include a `'+'` character.
> [ENOMEM]
>   The buf argument is a null pointer and the allocation of a buffer of length size has failed.
> [EMFILE]
>   {FOPEN_MAX} streams are currently open in the calling process.

The following sections are informative.

#### EXAMPLES

>     #include <stdio.h>
>     #include <string.h>

>     static char buffer[] = "foobar";

>     int
>     main (void)
>     {
>         int ch;
>         FILE *stream;

>         stream = fmemopen(buffer, strlen (buffer), "r");
>         if (stream == NULL)
>             /* handle error */;

>         while ((ch = fgetc(stream)) != EOF)
>             printf("Got %c\n", ch);

>         fclose(stream);
>         return (0);
>     }

> This program produces the following output:

>     Got f
>     Got o
>     Got o
>     Got b
>     Got a
>     Got r

#### APPLICATION USAGE

> None.

#### RATIONALE

> This interface has been introduced to eliminate many of the errors encountered in the construction of strings, notably overflowing of strings. This interface prevents overflow.

#### FUTURE DIRECTIONS

> None.

#### SEE ALSO

> fdopen , fopen , freopen , malloc , open_memstream

> XBD <stdio.h>

#### CHANGE HISTORY

> First released in Issue 7.

End of informative text.

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Copyright © 2001-2008 The IEEE and The Open Group, All Rights Reserved
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