[재질문 소스포함] 뮤텍스 extern으로 선언시... 에러가..

main.c 파일 내용

#include <stdlib.h>
#include <pthread.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h> 
#include <stdio.h> 

extern int abcd;

void cdef(void *arg);
void abcde(void *arg);

extern pthread_mutex_t mymutex=PTHREAD_MUTEX_INITIALIZER;

int main()
{
	pthread_t tid;
	pid_t pid;

	if ( pthread_create(&tid, NULL, (void*) &abcde, NULL )) {
			printf("error creating thread.");
	}

	if ( pthread_create(&tid, NULL, (void*) &cdef, NULL )) {
			printf("error creating thread.");
	}

	sleep(5);

}

void abcde(void *arg) {
	//int abcd;

	pthread_detach(pthread_self());

	pthread_mutex_lock(&mymutex);
	abcd++;
	printf("=>%d\n", abcd);
	fflush(stdout);
	pthread_mutex_unlock(&mymutex);
}

send_file.c 파일 내용

#include <pthread.h>
#include <stdio.h>

int abcd;
pthread_mutex_t mymutex=PTHREAD_MUTEX_INITIALIZER;

void cdef(void *arg) {
	int abcd;

	pthread_detach(pthread_self());
	pthread_mutex_lock(&mymutex);
	abcd++;
	printf("=>%d\n", abcd);
	fflush(stdout);
	pthread_mutex_unlock(&mymutex);
}

컴파일

#  gcc -o main main.c send_file.c -lpthread
main.c:14: warning: `mymutex' initialized and declared `extern'
send_file.c:16:2: warning: no newline at end of file
/tmp/cc3C3NOu.o(.data+0x0): multiple definition of `mymutex'
/tmp/ccKBnSkV.o(.data+0x0): first defined here
collect2: ld returned 1 exit status

왜 이러는지 ㅠ.ㅠ 알려주세욥 ㅠ.ㅠ

흐흐 답변 부탁드립니다 ㅠ.ㅠ