#include <nng/nng.h>
int nng_msg_realloc(nng_msg *msg, size_t size);

Tip

nng_msg_realloc is suitable for creating space for direct writing of data.
When appending many small pieces of data to a message using nng_msg_append(), allocations may be reduced by first using nng_msg_reserve() to create sufficient space. In any case, reallocating or appending to a message is guaranteed to succeed if the resulting body length is less than nng_msg_capacity().

Note

Pointers to message body and header content obtained prior to this
function must not be in use, as the underlying memory used for the message may have changed, particularly if the message size is increasing.

Insufficient free memory exists to reallocate a message.