在Linux操作系统中编程声音设备实例-红联Linux系统门户

　　Linux下的声音设备编程比大多数人想象的要简单得多。一般说来，我们常用的声音设备是内部扬声器和声卡，它们都对应/dev目录下的一个或多个设备文件，我们象打开普通文件一样打开它们，用ioctl（）函数设置一些参数，然后对这些打开的特殊文件进写操作。

　　由于这些文件不是普通的文件，所以我们不能用ANSI C（标准C）的fopen、fclose等来操作文件，而应该使用系统文件I/O处理函数（open、read、write、lseek和close）来处理这些设备文件。ioctl（）或许是Linux下最庞杂的函数，它可以控制各种文件的属性，在Linux声音设备编程中，最重要的就是使用此函数正确设置必要的参数。

　　下面我们举两个实际的例子来说明如何实现Linux下的声音编程。由于此类编程涉及到系统设备的读写，所以，很多时候需要你有root权限，如果你将下面的例子编译后不能正确执行，那么，首先请你检查是否是因为没有操纵某个设备的权限。

　　对内部扬声器编程内部扬声器是控制台的一部分，所以它对应的设备文件为/dev/console。变量KIOCSOUND在头文件 /usr /include /linux /kd.h中声明，ioctl函数使用它可以来控制扬声器的发声，使用规则为：

锟斤拷锟斤拷:
　　ioctl ( fd, KIOCSOUND, (int) tone);

　　fd为文件设备号，tone 是音频值。当tone为0时，终止发声。必须一提的是它所理解的音频和我们平常以为的音频是不同的，由于计算机主板定时器的时钟频率为1.19MHZ，所以要进行正确的发声，必须进行如下的转换：扬声器音频值=1190000/我们期望的音频值。

　　扬声器发声时间的长短我们通过函数usleep（unsigned long usec）来控制。它是在头文件/usr /include /unistd.h中定义的，让程序睡眠usec微秒。下面即是让扬声器按指定的长度和音频发声的程序的完整清单：

锟斤拷锟斤拷:
#include < fcntl.h >
#include < stdio.h >
#include < stdlib.h >
#include < string.h >
#include < unistd.h >
#include < sys/ioctl.h >
#include < sys/types.h >
#include < linux/kd.h >

/* 设定默认值 */
#define DEFAULT_FREQ 440 /* 设定一个合适的频率 */
#define DEFAULT_LENGTH 200 /* 200 微秒，发声的长度是以微秒为单位的*/
#define DEFAULT_REPS 1 /* 默认不重复发声 */
#define DEFAULT_DELAY 100 /* 同样以微秒为单位*/

/* 定义一个结构，存储所需的数据*/
typedef struct {
int freq; /* 我们期望输出的频率，单位为Hz */
int length; /* 发声长度，以微秒为单位*/
int reps; /* 重复的次数*/
int delay; /* 两次发声间隔，以微秒为单位*/
} beep_parms_t;

/* 打印帮助信息并退出*/
void usage_bail ( const char *executable_name ) {
printf ( "Usage: \n \t%s [-f frequency] [-l length] [-r reps] [-d delay] \n ",
executable_name );
exit(1);
}

/ * 分析运行参数，各项意义如下：
* "-f <以HZ为单位的频率值 >"
* "-l <以毫秒为单位的发声时长 >"
* "-r <重复次数 >"
* "-d <以毫秒为单位的间歇时长 >"
*/
void parse_command_line(char **argv, beep_parms_t *result) {
char *arg0 = *(argv++);
while ( *argv ) {
if ( !strcmp( *argv,"-f" )) { /*频率*/
int freq = atoi ( *( ++argv ) );
if ( ( freq <= 0 ) | | ( freq > 10000 ) ) {
fprintf ( stderr, "Bad parameter: frequency must be from 1..10000\n" );
exit (1) ;
} else {
result->freq = freq;
argv++;
}
} else if ( ! strcmp ( *argv, "-l" ) ) { /*时长*/
int length = atoi ( *(++argv ) );
if (length < 0) {
fprintf(stderr, "Bad parameter: length must be >= 0\n");
exit(1);
} else {
result->length = length;
argv++;
}
} else if (!strcmp(*argv, "-r")) { /*重复次数*/
int reps = atoi(*(++argv));
if (reps < 0) {
fprintf(stderr, "Bad parameter: reps must be >= 0\n");
exit(1);
} else {
result->reps = reps;
argv++;
}
} else if (!strcmp(*argv, "-d")) { /* 延时 */
int delay = atoi(*(++argv));
if (delay < 0) {
fprintf(stderr, "Bad parameter: delay must be >= 0\n");
exit(1);
} else {
result->delay = delay;
argv++;
}
} else {
fprintf(stderr, "Bad parameter: %s\n", *argv);
usage_bail(arg0);
}
}
}

int main(int argc, char **argv) {
int console_fd;
int i; /* 循环计数器 */
/* 设发声参数为默认值*/
beep_parms_t parms = {DEFAULT_FREQ, DEFAULT_LENGTH, DEFAULT_REPS,
DEFAULT_DELAY};
/* 分析参数，可能的话更新发声参数*/
parse_command_line(argv, &parms);

/* 打开控制台，失败则结束程序*/
if ( ( console_fd = open ( "/dev/console", O_WRONLY ) ) == -1 ) {
fprintf(stderr, "Failed to open console.\n");
perror("open");
exit(1);
}

/* 真正开始让扬声器发声*/
for (i = 0; i < parms.reps; i++) {
/* 数字1190000从何而来，不得而知*/
int magical_fairy_number = 1190000/parms.freq;

ioctl(console_fd, KIOCSOUND, magical_fairy_number); /* 开始发声 */
usleep(1000*parms.length); /*等待... */
ioctl(console_fd, KIOCSOUND, 0); /* 停止发声*/
usleep(1000*parms.delay); /* 等待... */
} /* 重复播放*/
return EXIT_SUCCESS;
}

　　将上面的例子稍作扩展，用户即可以让扬声器唱歌。只要找到五线谱或简谱的音阶、音长、节拍和频率、发声时长、间隔的对应关系就可以了。我现在还记得以前在DOS下编写出《世上只有妈妈好》时的兴奋。最后，说一些提外话，这其实是一个很简单的程序，但是我们却用了很长的篇幅，希望读者从以上的代码里能体会到写好的程序的一些方法，或许最重要的是添加注释吧。一个程序的注释永远不会嫌多，即便你写的时候觉得它根本是多余，但相信我，相信曾这样告诉我们的许多优秀的程序员：养成写很多注释的习惯。

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