多線程實現文件拷貝

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <pthread.h>
#define N 10
void* ptr_fun(void* arg);
int size;
int rd,wd;
void main(int argc,char** argv)
{
    if(argc!=3)
    {
        printf("參數不足\n");
        return;
    }
    struct stat t;
    stat(argv[1],&t);
    size=t.st_size;

    rd=open(argv[1],O_RDONLY);
    wd=open(argv[2],O_WRONLY);
    if(-1==wd)
    {
            printf("文件不存在，創建文件\n");
            wd=open(argv[2],O_CREAT,0666);
            close(wd);
            wd=open(argv[2],O_WRONLY);
    }

    int i;
    pthread_t tid[N];
    for(i=0;i<N;i++)
    {
        if(0!=pthread_create(&tid[i],NULL,ptr_fun,(void*)i))
        {
            perror("pthread_create");
        }
    }

    char buf[100];
    int num;
    lseek(rd,size/(N+1)*N,SEEK_SET);
    lseek(wd,size/(N+1)*N,SEEK_SET);
    while(num=read(rd,buf,sizeof(buf)))
    {
        write(wd,buf,num);
    }
    for(i=0;i<N;i++)
    {
        pthread_join(tid[i],NULL);
    }

    close(rd);
    close(wd);
}
void* ptr_fun(void* arg)
{
    int p=(int)arg;
    char buf[size/(N+1)];

    lseek(rd,size/(N+1)*p,SEEK_SET);
    lseek(wd,size/(N+1)*p,SEEK_SET);

    read(rd,buf,size/(N+1));
    write(wd,buf,size/(N+1));

    return NULL;
}