Man page 9 : skb_cow_data

skb_cow_data - Check that a socket bufferAqs data buffers are writable

int skb_cow_data(struct sk_buff *  skb , int  tailbits , struct sk_buff **  trailer );

skb
    The socket buffer to check.

tailbits
    Amount of trailing space to be added

trailer
    Returned pointer to the skb where the tailbits space begins

Make sure that the data buffers attached to a socket buffer are writable. If they are not, private copies are made of the data buffers and the socket buffer is set to use these instead.

If tailbits is given, make sure that there is space to write tailbits bytes of data beyond current end of socket buffer. trailer will be set to point to the skb in which this space begins.

The number of scatterlist elements required to completely map the COWAqd and extended socket buffer will be returned.