我正在通过this article学习ALSA编程，我试图编译以下示例：
/*
This example opens the default PCM device, sets
some parameters, and then displays the value
of most of the hardware parameters. It does not
perform any sound playback or recording.
*/
/* Use the newer ALSA API */
#define ALSA_PCM_NEW_HW_PARAMS_API
/* All of the ALSA library API is defined
* in this header */
#include <alsa/asoundlib.h>
int main() {
int rc;
snd_pcm_t *handle;
snd_pcm_hw_params_t *params;
unsigned int val, val2;
int dir;
snd_pcm_uframes_t frames;
/* Open PCM device for playback. */
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(¶ms);
/* 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, 2);
/* 44100 bits/second sampling rate (CD quality) */
val = 44100;
snd_pcm_hw_params_set_rate_near(handle,
params, &val, &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);
}
/* Display information about the PCM interface */
printf("PCM handle name = '%s'\n",
snd_pcm_name(handle));
printf("PCM state = %s\n",
snd_pcm_state_name(snd_pcm_state(handle)));
snd_pcm_hw_params_get_access(params,
(snd_pcm_access_t *) &val);
printf("access type = %s\n",
snd_pcm_access_name((snd_pcm_access_t)val));
snd_pcm_hw_params_get_format(params, (snd_pcm_format_t *) &val);
printf("format = '%s' (%s)\n",
snd_pcm_format_name((snd_pcm_format_t)val),
snd_pcm_format_description(
(snd_pcm_format_t)val));
snd_pcm_hw_params_get_subformat(params,
(snd_pcm_subformat_t *)&val);
printf("subformat = '%s' (%s)\n",
snd_pcm_subformat_name((snd_pcm_subformat_t)val),
snd_pcm_subformat_description(
(snd_pcm_subformat_t)val));
snd_pcm_hw_params_get_channels(params, &val);
printf("channels = %d\n", val);
snd_pcm_hw_params_get_rate(params, &val, &dir);
printf("rate = %d bps\n", val);
snd_pcm_hw_params_get_period_time(params,
&val, &dir);
printf("period time = %d us\n", val);
snd_pcm_hw_params_get_period_size(params,
&frames, &dir);
printf("period size = %d frames\n", (int)frames);
snd_pcm_hw_params_get_buffer_time(params,
&val, &dir);
printf("buffer time = %d us\n", val);
snd_pcm_hw_params_get_buffer_size(params,
(snd_pcm_uframes_t *) &val);
printf("buffer size = %d frames\n", val);
snd_pcm_hw_params_get_periods(params, &val, &dir);
printf("periods per buffer = %d frames\n", val);
snd_pcm_hw_params_get_rate_numden(params,
&val, &val2);
printf("exact rate = %d/%d bps\n", val, val2);
val = snd_pcm_hw_params_get_sbits(params);
printf("significant bits = %d\n", val);
snd_pcm_hw_params_get_tick_time(params,
&val, &dir);
printf("tick time = %d us\n", val);
val = snd_pcm_hw_params_is_batch(params);
printf("is batch = %d\n", val);
val = snd_pcm_hw_params_is_block_transfer(params);
printf("is block transfer = %d\n", val);
val = snd_pcm_hw_params_is_double(params);
printf("is double = %d\n", val);
val = snd_pcm_hw_params_is_half_duplex(params);
printf("is half duplex = %d\n", val);
val = snd_pcm_hw_params_is_joint_duplex(params);
printf("is joint duplex = %d\n", val);
val = snd_pcm_hw_params_can_overrange(params);
printf("can overrange = %d\n", val);
val = snd_pcm_hw_params_can_mmap_sample_resolution(params);
printf("can mmap = %d\n", val);
val = snd_pcm_hw_params_can_pause(params);
printf("can pause = %d\n", val);
val = snd_pcm_hw_params_can_resume(params);
printf("can resume = %d\n", val);
val = snd_pcm_hw_params_can_sync_start(params);
printf("can sync start = %d\n", val);
snd_pcm_close(handle);
return 0;
}
问题是如果我用
编译它gcc test.c -std = c99 -lasound
我得到以下内容：
test.c: In function ‘main’:
test.c:36:3: warning: implicit declaration of function ‘alloca’ [-Wimplicit-function-declaration]
snd_pcm_hw_params_alloca(¶ms);
^
但如果我不使用
，则不会出现此警告-std = C99
答案 0 :(得分：5)
根据man alloca，您应该在使用alloca.h时向您的附加内容添加alloca()：
#include <alloca.h>
这不是通常的函数，因为它应该直接修改调用它的函数的堆栈。它在编译器中专门处理，例如在gcc中（如所描述的in the man）
关于GNU版本的说明
Normally, gcc(1) translates calls to alloca() with inlined code. This is not done when either the -ansi, -std=c89, -std=c99, or the -std=c11 option is given and the header <alloca.h> is not included. Otherwise, (without an -ansi or -std=c* option) the glibc version of <stdlib.h> includes <alloca.h> and that contains the lines...
您正在使用alloca因为snd_pcm_info_alloca被定义为alloca的宏（谢谢，Matt McNabb）
#define snd_pcm_info_alloca(ptr) do { assert(ptr); *ptr = (snd_pcm_info_t *) alloca(snd_pcm_info_sizeof()); memset(*ptr, 0, snd_pcm_info_sizeof()); } while (0)
不完整ALSA doc about snd_pcm_info_alloca说
snd_pcm_hw_params_alloca（PTR） 使用标准alloca
分配无效的snd_pcm_hw_params_t
但是alloca不是标准的;当你使用它时，你应该包括alloca.h
我认为http://www.linuxjournal.com/article/6735?page=0,1中的例子不正确;像http://webpages.lss.supelec.fr/perso/kowalski/?download=playback.c或https://gitorious.org/alsa/alsa-tools/source/f768eb0ff4124ac7e795cee09acb797fd01b63f1:ac3dec/output.c这样的好例子有#include <alloca.h>。
应谨慎使用此snd_pcm_hw_params_alloca宏，不正确的使用可能导致免费错误后使用：http://mailman.alsa-project.org/pipermail/alsa-devel/2008-March/006700.html