SDL audio capture callbacks slower than playback - c

Question

SDL capture audio callbacks seem to be called once for every 12 playback callbacks. Am I doing something wrong? This feels like an SDL or PulseAudio bug.
The program below prints "Reading audio..." once every ~12 "Writing audio..." prints. Tested via running the command directly, through gdb, and through Valgrind.
I've tried this in both C and Golang (using github.com/veandco/go-sdl2/sdl), on the same machine.
C code:
// A test program to copy audio in (microphone) to audio out (speaker) via SDL.
//
// Compile: cc inout.c -o inout -lSDL2
// Run: ./inout
#include <SDL2/SDL.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#define BUF_SIZE 1024
// Stereo float32 samples.
static uint8_t saved[BUF_SIZE*2*sizeof(float)];
// Copies audio callback data into the saved buffer.
void audioReader(void* udata, uint8_t* buf, int len) {
fprintf(stderr, "Reading audio: %d -> %ld bytes\n", len, sizeof(saved));
memcpy(saved, buf, len);
}
// Copies saved audio data into the callback buffer.
void audioWriter(void* udata, uint8_t* buf, int len) {
fprintf(stderr, "Writing audio: %ld -> %d bytes\n", sizeof(saved), len);
memcpy(buf, saved, len);
}
// List all devices of the given type, and return the name of the first or NULL.
// Caller must free the returned pointer.
char* ChooseDevice(int is_capture) {
int dev_cnt = SDL_GetNumAudioDevices(is_capture);
if (dev_cnt < 1) {
fprintf(stderr, "No %s devices: %s\n", is_capture ? "capture" : "playback", SDL_GetError());
return NULL;
}
printf("%s devices:\n", is_capture ? "capture" : "playback");
char* dev_name = NULL;
for (int i = 0; i < dev_cnt; i++) {
printf("%c %s\n", !dev_name ? '*' : ' ', SDL_GetAudioDeviceName(i, is_capture));
if (!dev_name) {
const char* tmp = SDL_GetAudioDeviceName(i, is_capture);
dev_name = malloc(strlen(tmp)+1);
strcpy(dev_name, tmp);
}
}
if (!dev_name) {
fprintf(stderr, "No %s devices\n", is_capture ? "capture" : "playback");
}
return dev_name;
}
// Opens and unpauses the first device of the given type, returning its ID, or
// returns 0.
SDL_AudioDeviceID OpenDevice(int is_capture) {
char* dev_name = ChooseDevice(is_capture);
if (!dev_name) return 0;
SDL_AudioSpec spec;
SDL_memset(&spec, 0, sizeof(spec));
spec.freq = 48000;
spec.format = AUDIO_F32;
spec.channels = 2;
spec.samples = BUF_SIZE;
spec.callback = is_capture ? audioReader : audioWriter;
SDL_AudioDeviceID dev_id = SDL_OpenAudioDevice(dev_name, is_capture, &spec, NULL, 0);
if (dev_id == 0) {
fprintf(stderr, "Failed to open %s device %s: %s\n", is_capture ? "input" : "output", dev_name, SDL_GetError());
return 0;
}
free(dev_name);
SDL_PauseAudioDevice(dev_id, SDL_FALSE);
return dev_id;
}
int main(int argc, char** argv) {
SDL_memset(saved, 0, sizeof(saved));
if (SDL_Init(SDL_INIT_AUDIO) < 0) {
fprintf(stderr, "Failed to initialize SDL audio: %s\n", SDL_GetError());
return 1;
}
SDL_AudioDeviceID in_dev_id = OpenDevice(/* is_capture = */ SDL_TRUE);
if (in_dev_id == 0) return 1;
SDL_AudioDeviceID out_dev_id = OpenDevice(/* is_capture = */ SDL_FALSE);
if (out_dev_id == 0) return 1;
SDL_Delay(10000); // 10 seconds
SDL_CloseAudioDevice(in_dev_id);
SDL_CloseAudioDevice(out_dev_id);
SDL_Quit();
return 0;
}