Open the stream for update (reading and writing). Truncate the buffer contents.

Open the stream for update (reading and writing).

If the mode argument includes 'b', then the stream shall be in binary mode; otherwise the stream shall be in text mode.

If the mode argument begins with 'w' and max_size is not zero, the buffer contents shall be truncated by writing a null byte at the beginning. If the mode argument includes 'b', the results are implementation-defined.

The stream shall also maintain the size of the current buffer contents; use of fseek() or fseeko() on the stream with SEEK_END shall seek relative to this size. If mode starts with 'r' or includes 'b', the size shall be set to the value given by the size argument and shall not change. Otherwise, the stream is in text mode and writable, and the size shall be variable; for modes w and w+ the initial size shall be zero and for modes a and a+ the initial size shall be:

The stream shall also maintain the end position of the current buffer contents; use of fseek() or fseeko() on the stream with SEEK_END shall seek relative to this end position. If mode starts with 'r' the end position shall be set to the value given by the max_size argument and shall not change. Otherwise, the stream is writable and the end position shall be variable; for modes w and w+ the initial end position shall be zero and for modes a and a+ the initial end position shall be:

When a stream open for writing in text mode is flushed or closed, a null byte shall be 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 in text mode is flushed or closed and the last write has advanced the current buffer size, a null byte shall be written at the end of the buffer if it fits. If a stream is opened in binary mode, no additional null byte shall be written.

When a stream open for update (the mode argument includes '+') or for writing only is successfully written and the write advances the current buffer end position, a null byte shall be written at the new buffer end position if it fits.

Unlike fopen(), where a 'b' in the mode argument is required to have no effect, fmemopen() distinguishes between text and binary modes. Text mode guarantees that the underlying memory will always be null terminated after any write operation, and tracks the growth of the largest position written to up to that point; while binary mode only modifies the underlying buffer according to direct writes, while seeking relative to the full buffer size. The combination of append and binary modes is not commonly used, since any attempt to write to such a stream will necessarily fail because the stream does not dynamically grow beyond the initial size.

Implementations differ as regards how a 'b' in the mode argument affects the behavior. For some the 'b' has no effect, as is required for fopen(); others distinguish between text and binary modes.

Note that buf will not be null terminated if max_size bytes are written to the memory stream. Applications wanting to guarantee that the buffer will be null terminated need to call fmemopen() with max_size set to one byte smaller than the actual size of buf and set buf[max_size] to a null byte.