pbuf_alloc() function

Allocates a pbuf of the given type (possibly a chain for PBUF_POOL type). The actual memory allocated for the pbuf is determined by the layer at which the pbuf is allocated and the requested size (from the size parameter). - PBUF_RAM: buffer memory for pbuf is allocated as one large chunk. This includes protocol headers as well. - PBUF_ROM: no buffer memory is allocated for the pbuf, even for protocol headers. Additional headers must be prepended by allocating another pbuf and chain in to the front of the ROM pbuf. It is assumed that the memory used is really similar to ROM in that it is immutable and will not be changed. Memory which is dynamic should generally not be attached to PBUF_ROM pbufs. Use PBUF_REF instead. - PBUF_REF: no buffer memory is allocated for the pbuf, even for protocol headers. It is assumed that the pbuf is only being used in a single thread. If the pbuf gets queued, then pbuf_take should be called to copy the buffer. - PBUF_POOL: the pbuf is allocated as a pbuf chain, with pbufs from the pbuf pool that is allocated during pbuf_init().