libpcap: why can't I get the packet starting at some layer

[timid timber]

When using libpcap, I can write a filter like tcp and src host localhost, but if I call pcap_next_ex or similar, I get the full packet, including the link layer header and such.
As far as I can tell, there is no way to have libpcap remove the outer headers and leave me only with e.g. the tcp header, despite the library clearly having the code required to do so, as the filters are impossible otherwise.
What is the reason for this limitation/what is the magic function I failed to find?

[wild wharf]

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