FFMPEG - Encode a .wav file to G711 - c

Question

My question is the following: how to encode a .wav file to G.711/PCM A-law?
I tried to edit this example file, but I got an EINVAL error when tried to call av_frame_get_buffer(frame, 0) with frame->nb_samples = c->frame_size which is 0 (I think this is the problem) because of pcm.c:37 (at least I think because of it).
So with which parameters allocate an audio data buffer by myself if av_frame_get_buffer() doesn't do this for me and how to use it?
Thanks for reply!
EDIT:
My code:
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <libavcodec/avcodec.h>
#include <libavutil/channel_layout.h>
#include <libavutil/common.h>
#include <libavutil/frame.h>
#include <libavutil/samplefmt.h>
/* check that a given sample format is supported by the encoder */
static int check_sample_fmt(const AVCodec *codec, enum AVSampleFormat sample_fmt)
{
const enum AVSampleFormat *p = codec->sample_fmts;
while (*p != AV_SAMPLE_FMT_NONE) {
if (*p == sample_fmt)
return 1;
p++;
}
return 0;
}
/* just pick the highest supported samplerate */
static int select_sample_rate(const AVCodec *codec)
{
const int *p;
int best_samplerate = 0;
if (!codec->supported_samplerates)
return 44100;
p = codec->supported_samplerates;
while (*p) {
if (!best_samplerate || abs(44100 - *p) < abs(44100 - best_samplerate))
best_samplerate = *p;
p++;
}
return best_samplerate;
}
/* select layout with the highest channel count */
static int select_channel_layout(const AVCodec *codec)
{
const uint64_t *p;
uint64_t best_ch_layout = 0;
int best_nb_channels = 0;
if (!codec->channel_layouts)
return AV_CH_LAYOUT_STEREO;
p = codec->channel_layouts;
while (*p) {
int nb_channels = av_get_channel_layout_nb_channels(*p);
if (nb_channels > best_nb_channels) {
best_ch_layout = *p;
best_nb_channels = nb_channels;
}
p++;
}
return best_ch_layout;
}
static void encode(AVCodecContext *ctx, AVFrame *frame, AVPacket *pkt,
FILE *output)
{
int ret;
/* send the frame for encoding */
ret = avcodec_send_frame(ctx, frame);
if (ret < 0) {
fprintf(stderr, "Error sending the frame to the encoder\n");
exit(1);
}
/* read all the available output packets (in general there may be any
* number of them */
while (ret >= 0) {
ret = avcodec_receive_packet(ctx, pkt);
if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
return;
else if (ret < 0) {
fprintf(stderr, "Error encoding audio frame\n");
exit(1);
}
fwrite(pkt->data, 1, pkt->size, output);
av_packet_unref(pkt);
}
}
int main(int argc, char **argv)
{
const char *filename;
const AVCodec *codec;
AVCodecContext *c = NULL;
AVFrame *frame;
AVPacket *pkt;
int i, j, k, ret;
FILE *f;
uint16_t *samples;
float t, tincr;
if (argc <= 1)
{
fprintf(stderr, "Usage: %s <output file>\n", argv[0]);
return 0;
}
filename = argv[1];
codec = avcodec_find_encoder(AV_CODEC_ID_PCM_ALAW);
if (!codec)
{
fprintf(stderr, "Codec not found\n");
exit(1);
}
c = avcodec_alloc_context3(codec);
if (!c)
{
fprintf(stderr, "Could not allocate audio codec context\n");
exit(1);
}
c->bit_rate = 128000;
c->sample_fmt = AV_SAMPLE_FMT_S16;
if (!check_sample_fmt(codec, c->sample_fmt))
{
fprintf(stderr, "Encoder does not support sample format %s",
av_get_sample_fmt_name(c->sample_fmt));
exit(1);
}
c->sample_rate = select_sample_rate(codec);
c->channel_layout = select_channel_layout(codec);
c->channels = av_get_channel_layout_nb_channels(c->channel_layout);
if (avcodec_open2(c, codec, NULL) < 0)
{
fprintf(stderr, "Could not open codec\n");
exit(1);
}
f = fopen(filename, "wb");
if (!f)
{
fprintf(stderr, "Could not open %s\n", filename);
exit(1);
}
pkt = av_packet_alloc();
if (!pkt)
{
fprintf(stderr, "Could not allocate the packet\n");
exit(1);
}
frame = av_frame_alloc();
if (!frame)
{
fprintf(stderr, "Could not allocate audio frame\n");
exit(1);
}
frame->nb_samples = c->frame_size;
frame->format = c->sample_fmt;
frame->channel_layout = c->channel_layout;
// ERROR Here
ret = av_frame_get_buffer(frame, 0);
if (ret < 0)
{
fprintf(stderr, "Could not allocate audio data buffers\n");
exit(1);
}
t = 0;
tincr = 2 * M_PI * 440.0 / c->sample_rate;
for (i = 0; i < 200; i++)
{
ret = av_frame_make_writable(frame);
if (ret < 0)
{
exit(1);
}
samples = (uint16_t*)frame->data[0];
for (j = 0; j < c->frame_size; j++)
{
samples[2*j] = (int)(sin(t)*10000);
for (k = 1; k < c->channels; k++)
{
samples[2*j + k] = samples[2*j];
}
t += tincr;
}
encode(c, frame, pkt, f);
}
encode(c, NULL, pkt, f);
fclose(f);
av_frame_free(&frame);
av_packet_free(&pkt);
avcodec_free_context(&c);
return 0;
}