在做ffmpeg+Qt+SDL的项目的时候，出现这个错误，
/usr/local/lib/libavdevice.a(alsa-audio-dec.o): In function `audio_read_packet':
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:139: undefined reference to `snd_pcm_readi'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:146: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:154: undefined reference to `snd_pcm_htimestamp'
/usr/local/lib/libavdevice.a(alsa-audio-dec.o): In function `audio_read_header':
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:90: undefined reference to `snd_pcm_type'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:95: undefined reference to `snd_pcm_sw_params_malloc'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:97: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:102: undefined reference to `snd_pcm_sw_params_current'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:103: undefined reference to `snd_pcm_sw_params_set_tstamp_mode'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:105: undefined reference to `snd_pcm_sw_params'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:106: undefined reference to `snd_pcm_sw_params_free'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:108: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-dec.c:123: undefined reference to `snd_pcm_close'
/usr/local/lib/libavdevice.a(alsa-audio-enc.o): In function `audio_write_packet':
/opt/libav-0.6.2/libavdevice/alsa-audio-enc.c:79: undefined reference to `snd_pcm_writei'
/opt/libav-0.6.2/libavdevice/alsa-audio-enc.c:86: undefined reference to `snd_strerror'
/usr/local/lib/libavdevice.a(alsa-audio-enc.o): In function `audio_write_header':
/opt/libav-0.6.2/libavdevice/alsa-audio-enc.c:68: undefined reference to `snd_pcm_close'
/usr/local/lib/libavdevice.a(alsa-audio-common.o): In function `ff_alsa_xrun_recover':
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:174: undefined reference to `snd_pcm_prepare'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:176: undefined reference to `snd_strerror'
/usr/local/lib/libavdevice.a(alsa-audio-common.o): In function `ff_alsa_open':
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:73: undefined reference to `snd_pcm_open'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:75: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:80: undefined reference to `snd_pcm_hw_params_malloc'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:82: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:87: undefined reference to `snd_pcm_hw_params_any'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:89: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:94: undefined reference to `snd_pcm_hw_params_set_access'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:96: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:101: undefined reference to `snd_pcm_hw_params_set_format'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:103: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:108: undefined reference to `snd_pcm_hw_params_set_rate_near'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:110: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:115: undefined reference to `snd_pcm_hw_params_set_channels'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:117: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:122: undefined reference to `snd_pcm_hw_params_get_buffer_size_max'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:124: undefined reference to `snd_pcm_hw_params_set_buffer_size_near'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:126: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:131: undefined reference to `snd_pcm_hw_params_get_period_size_min'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:132: undefined reference to `snd_pcm_hw_params_set_period_size_near'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:134: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:140: undefined reference to `snd_pcm_hw_params'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:142: undefined reference to `snd_strerror'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:147: undefined reference to `snd_pcm_hw_params_free'
make: Leaving directory `/home/jacky/workspace/Roarsoft/EasyPlayer-build-desktop'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:153: undefined reference to `snd_pcm_hw_params_free'
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:155: undefined reference to `snd_pcm_close'
/usr/local/lib/libavdevice.a(alsa-audio-common.o): In function `ff_alsa_close':
/opt/libav-0.6.2/libavdevice/alsa-audio-common.c:163: undefined reference to `snd_pcm_close'
这个是alsa的库，因此只要再加上－lasound即可。