The
fmemopen() function associates a stream with the given
buffer and
size. The
buffer can be either
NULL, or must be of the given
size. If the
buffer is
NULL, a
buffer of the given
size will be dynamically allocated using
malloc(3) and freed when
fclose(3) is called.
The
mode argument has the same meaning as in
fopen(3).
The stream treats the buffer as it would treat a file tracking the current position to perform I/O operations. For example, in the beginning the stream points to the beginning of the buffer, unless
a was specified in the
mode argument, and then it points to the first
NUL byte. If a
NULL buffer was specified, then the stream will always point at the first byte of the
buffer.
The stream also keeps track of the
size of the
buffer. The
size is initialized depending on the mode:
r/r+
Set to the size argument.
a/a+
Set to the first NUL byte, or the size argument if one is not found.
Read or write operations advance the buffer, but not to exceed the given
size of the
buffer. Trying to read beyond the
size of the
buffer results in
EOF returned.
NUL bytes are read normally. Trying to write beyond the
size of the
buffer has no effect.
When a stream open for writing is either flushed or closed, a
NUL byte is written at the current position or at the end of the current
size as kept internally, if there is room.