基于ALSA的放音小程序

介绍一个小型的基于ALSA的播放wav音乐的小程序，对于wav文件，其头部一般有44字节的头部信息，里面标示了音频文件的采样率，量化位数，通道数，以及音频文件的数据大小（文件大小+44字节的头）。
1、playback.c

/*

This example reads from the default PCM device
and writes to standard output for 5 seconds of data.

*/

/* Use the newer ALSA API */
#define ALSA_PCM_NEW_HW_PARAMS_API

#include <alsa/asoundlib.h>

/**************************************************************/
#define ID_RIFF 0x46464952
#define ID_WAVE 0x45564157
#define ID_FMT  0x20746d66
#define ID_DATA 0x61746164

typedef unsigned long uint32_t;
typedef unsigned short uint16_t;

#define FORMAT_PCM 1

static uint32_t totle_size = 0;

struct wav_header {
    /* RIFF WAVE Chunk */
    uint32_t riff_id;
    uint32_t riff_sz;
    uint32_t riff_fmt;
    /* Format Chunk */
    uint32_t fmt_id;
    uint32_t fmt_sz;
    uint16_t audio_format;
    uint16_t num_channels;
    uint32_t sample_rate;
    uint32_t byte_rate;       /* sample_rate * num_channels * bps / 8 */
    uint16_t block_align;     /* num_channels * bps / 8 */
    uint16_t bits_per_sample;
    /* Data Chunk */
    uint32_t data_id;
    uint32_t data_sz;
}__attribute__((packed));

static struct wav_header hdr;

/**************************************************************/
int playback_file(unsigned rate, uint16_t channels, int fd, uint32_t total_count)
{
    int rc;
    int size;
    uint32_t left_size;
    snd_pcm_t *handle;
    snd_pcm_hw_params_t *params;
    int dir;
    snd_pcm_uframes_t frames;
    char *buffer;                    /* TODO */

    /* Open PCM device for playbacking. */
    rc = snd_pcm_open(&handle, "default", SND_PCM_STREAM_PLAYBACK, 0);
    if (rc < 0) {
        fprintf(stderr, "unable to open pcm device: %s\n", snd_strerror(rc));
        exit(1);
    }

    /* Allocate a hardware parameters object. */
    snd_pcm_hw_params_alloca(&params);

    /* Fill it in with default values. */
    snd_pcm_hw_params_any(handle, params);

    /* Set the desired hardware parameters. */

    /* Interleaved mode */
    snd_pcm_hw_params_set_access(handle, params, SND_PCM_ACCESS_RW_INTERLEAVED);

    /* Signed 16-bit little-endian format */
    snd_pcm_hw_params_set_format(handle, params, SND_PCM_FORMAT_S16_LE);

    /* Two channels (stereo) */
    snd_pcm_hw_params_set_channels(handle, params, channels);

    /* 44100 bits/second sampling rate (CD quality) */
    snd_pcm_hw_params_set_rate_near(handle, params, &rate, &dir);

    /* Set period size to 32 frames. */
    frames = 32;
    snd_pcm_hw_params_set_period_size_near(handle, params, &frames, &dir);

    /* Write the parameters to the driver */
    rc = snd_pcm_hw_params(handle, params);
    if (rc < 0) {
        fprintf(stderr, "unable to set hw parameters: %s\n", snd_strerror(rc));
        exit(1);
    }

    /* Use a buffer large enough to hold one period */
    snd_pcm_hw_params_get_period_size(params, &frames, &dir);

    size = frames * 2 * channels; /* 2 bytes/sample(16bit), 2 channels */
    buffer = (char *) malloc(size);

    while (left_size > 0) {
        rc = read(fd, buffer, size);
        totle_size += rc;                        /* totle data size */
        left_size = total_count - totle_size;
        if (rc != size)
          fprintf(stderr, "short read: read %d bytes\n", rc);

        rc = snd_pcm_writei(handle, buffer, frames);
        if (rc == -EPIPE) {
          /* EPIPE means overrun */
          fprintf(stderr, "overrun occurred\n");
          snd_pcm_prepare(handle);
        } else if (rc < 0) {
          fprintf(stderr, "error from write: %s\n", snd_strerror(rc));
        } else if (rc != (int)frames) {
          fprintf(stderr, "short write, write %d frames\n", rc);
        }

    }

    snd_pcm_drain(handle);
    snd_pcm_close(handle);
    free(buffer);

    return 0;
}

/**************************************************************/
int play_wav(const char *fn)
{
    unsigned rate, channels;
    int fd;

    fd = open(fn, O_RDONLY, 0777);
    if (fd < 0) {
        fprintf(stderr, "playback: cannot open '%s'\n", fn);
        return -1;
    }

    if (read(fd, &hdr, sizeof(hdr)) != sizeof(hdr)) {
        fprintf(stderr, "playback: cannot read header\n");
        return -1;
    }
    fprintf(stderr,"playback: %d ch, %d hz, %d bit, %s, file_size %ld\n",
            hdr.num_channels, hdr.sample_rate, hdr.bits_per_sample,
            hdr.audio_format == FORMAT_PCM ? "PCM" : "unknown", hdr.data_sz);

    return playback_file(hdr.sample_rate, hdr.num_channels, fd, hdr.data_sz);
}

int main(int argc, char **argv)
{
    if (argc != 2) {
        fprintf(stderr,"usage: playback \n");
        return -1;
    }

    return play_wav(argv[1]);
}

程序默认为16bit放音，可以支持不同采样率和通道数。

2、Makefile

exe = playback
src = playback.c
CC = unicore32-linux-gcc
INC = -I/nfs/usr/local/mplayer/include
LDFLAGS = -L/nfs/usr/local/mplayer/lib -lasound

$(exe) : $(src) FORCE
    $(CC) -o $@ $(src) $(LDFLAGS) $(INC)


FORCE:

clean:
    rm -f ./*.o $(exe)

我是基于嵌入式的编译环境，我的alsa-lib库在我的nfs下，故链接库指定在那里。