为什么会出现undefined reference to

main.c:(.text+0x41): undefined reference to `set_signal_hander'
main.c:(.text+0x85): undefined reference to `parse_metafile'
main.c:(.text+0xbb): undefined reference to `init_unchoke_peers'
main.c:(.text+0xc0): undefined reference to `create_files'
main.c:(.text+0xf3): undefined reference to `create_bitfield'
main.c:(.text+0x126): undefined reference to `create_btcache'
main.c:(.text+0x159): undefined reference to `download_upload_with_peers'
main.c:(.text+0x15e): undefined reference to `do_clear_work'
collect2: ld returned 1 exit status