Usage: find /path/to/flac/folder -name "*.flac" | xargs -d '\n' /path/to/fflthread

fflthread.c

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>

/**
 * Structure de données partagée pour limiter le nombre de threads.
 */

struct mutex {
	pthread_mutex_t mutex;
	pthread_cond_t cond;
	size_t nbWorkers;
	size_t maxThreads;
};

struct mutex jobs;

/**
 * Fonction d'appel à ffmpeg, passée aux threads.
 * @param data Nom du fichier à transcoder.
 */

void * exec_ffmpeg(void * data) {
	char * flacName;
	char * fileName;
	size_t nbChar = strlen(data);

	// should I be working?
	if (pthread_mutex_lock(&jobs.mutex) == EINVAL) return NULL;
	while (jobs.nbWorkers > jobs.maxThreads) {
		pthread_cond_wait(&jobs.cond, &jobs.mutex);
	}

	// yes I should
	jobs.nbWorkers++;
	if (pthread_mutex_unlock(&jobs.mutex) == EINVAL) return NULL;

	// cleaned up flac name
	flacName = (char *) malloc((nbChar + 1) * sizeof(char));
	strncpy(flacName, data, nbChar);
	flacName[nbChar] = '\0';
	// nb chars in flac == mp3 + \0
	fileName = (char *) malloc(nbChar * sizeof(char));
	// remove "flac" + '\0'
	strncpy(fileName, data, nbChar - 4);
	// concatenate "mp3" and null terminate
	strcat(fileName, "mp3");

	// ffmpeg options
	char ffmpeg[] = "ffmpeg -i \"";
	char ffargs[] = "\" -y -acodec libmp3lame -ab 320k \"";
	char ffsend[] = "\"";

	// build command
	char cmd[strlen(ffmpeg) + strlen(flacName) + strlen(ffargs) + strlen(fileName) + strlen(ffsend) + 1];
	strncpy(cmd, ffmpeg, strlen(ffmpeg) + 1);
	strncat(cmd, flacName, strlen(data) + 1);
	strncat(cmd, ffargs, strlen(ffargs) + 1);
	strncat(cmd, fileName, strlen(fileName) + 1);
	strncat(cmd, ffsend, strlen(ffsend) + 1);

	// exec command
	if (system(cmd) == -1) {
		perror("system");
		exit(EXIT_FAILURE);
	}
	
	// remove flac file
	if (remove(flacName) != 0) {
		perror("remove");
		exit(EXIT_FAILURE);
	}

	// end of job
	if (pthread_mutex_lock(&jobs.mutex) == EINVAL) return NULL;
	jobs.nbWorkers--;
	pthread_cond_broadcast(&jobs.cond);
	if (pthread_mutex_unlock(&jobs.mutex) == EINVAL) return NULL;

	return(data);
}

/**
 * Enjoy ffmpeg lame transcodes from flac to mp3 in full throttle mode
 */

int main(int argc, char * argv[]) {
	int nbThreads = argc - 1;
	pthread_t tid[nbThreads];
	void * status[nbThreads];
	int i;

	// init nb threads
	jobs.nbWorkers = 0;
	jobs.maxThreads = 4;

	// init mutex & cond
	if (pthread_mutex_init(&jobs.mutex, NULL) == -1) {
		perror("pthread_mutex_init");
		exit(EXIT_FAILURE);
	}

	if (pthread_cond_init(&jobs.cond, NULL) == -1) {
		perror("pthread_cond_init");
		exit(EXIT_FAILURE);
	}

	// one thread per flac file
	for (i = 0; i < nbThreads; i++) {
		if (pthread_create(&tid[i], NULL, exec_ffmpeg, argv[i + 1]) != 0) {
			perror("pthread_create");
			exit(EXIT_FAILURE);
		}
	}

	// wait for every transcode
	for (i = 0; i < nbThreads; i++) {
		if (pthread_join(tid[i], &status[i]) != 0) {
			perror("pthread_join");
			exit(EXIT_FAILURE);
		}
	}

	return EXIT_SUCCESS;
}