Segmentation fault (core dumped)

[lcq170]

下面这段代码， 从fd_r文件的内容接到outputfileFd文件（前面已经打开并写入部分内容）后面，现在是走到memcpy处就段错误，看样子像是p_w p_r指针不可操作，一操作就挂掉了。求指教，没看出代码有啥问题啊？

fd_r = open(infile, O_RDWR);
length_r = lseek(fd_r,0,SEEK_END);
filesizesum += length_r;

off_write = lseek(outputfileFd,0,SEEK_CUR);

lseek(fd_r,0,SEEK_SET);

while(1)
{
p_r = (char*)mmap(NULL,BUF_SIZE,PROT_READ|PROT_WRITE,MAP_SHARED,fd_r,off);
p_w = (char*)mmap(NULL,BUF_SIZE,PROT_WRITE,MAP_SHARED,outputfileFd,off_write);

memcpy(p_w,p_r,BUF_SIZE);
off += BUF_SIZE;
off_write += BUF_SIZE;

if((off+BUF_SIZE) >= length_r)
{
p_r = (char*)mmap(NULL,length_r - off ,PROT_READ|PROT_WRITE,MAP_SHARED,fd_r,off);
p_w = (char*)mmap(NULL,length_r - off,PROT_WRITE,MAP_SHARED,outputfileFd,off_write);
memcpy(p_w,p_r,length_r -off );
munmap(p_r,length_r - off);
munmap(p_w,length_r - off);
break;
}
}