usb_bulk_write failed: Resource temporarily unavailable

I am learning embed linux now. The develop board I use is mini2440, which contains s3c2440 board and arm920t cpu inside. The common develop platform is windows OS , but I love operating on linux. Minicom can contract to serial port of board and dnw2 can download binary program to board.
When I download program by using dnw2 on linux , I often get the following message:

85%      524288 bytes      usb_bulk_write failed: Resource temporarily unavailable

But often dnw2 can download small binary successfully. Error comes with big binary!!
The source code of dnw2 is :
dnw2.c

/* dnw2 linux main file. This depends on libusb.
 *
 * Author: 	Fox <hulifox008@163.com>
 * License:	GPL
 *
 */



#include <stdio.h>
#include <usb.h>
#include <errno.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define	 	QQ2440_SECBULK_IDVENDOR		0x5345
#define		QQ2440_SECBULK_IDPRODUCT	0x1234


struct usb_dev_handle * open_port()
{
	struct usb_bus *busses, *bus;

	usb_init();
	usb_find_busses();
	usb_find_devices();

	busses = usb_get_busses();
	for(bus=busses;bus;bus=bus->next)
	{
	 	struct usb_device *dev;
		for(dev=bus->devices;dev;dev=dev->next)
		{
			if( QQ2440_SECBULK_IDVENDOR==dev->descriptor.idVendor
			&&  QQ2440_SECBULK_IDPRODUCT==dev->descriptor.idProduct)
			{
				printf("Target usb device found!\n");
				struct usb_dev_handle *hdev = usb_open(dev);
				if(!hdev) 
				{
					perror("Cannot open device");	
				}
				else
				{
					if(0!=usb_claim_interface(hdev, 0))
					{
						perror("Cannot claim interface");
						usb_close(hdev);
						hdev = NULL;
					}
				}
				return hdev;
			}
		}
	}
	
	printf("Target usb device not found!\n");

	return NULL;
}

void usage()
{
	printf("Usage: dnw2 <file>\n\n");
}

unsigned char* prepare_write_buf(char *filename, unsigned int *len)
{
	unsigned char *write_buf = NULL;
	struct stat fs;

	int fd = open(filename, O_RDONLY);
	if(-1==fd)
	{
		perror("Cannot open file");
		return NULL;
	}
	if(-1==fstat(fd, &fs))
	{
		perror("Cannot get file size");
		goto error;
	}
	write_buf = (unsigned char*)malloc(fs.st_size+10);
	if(NULL==write_buf)
	{
		perror("malloc failed");
		goto error;
	}

	if(fs.st_size != read(fd, write_buf+8, fs.st_size))
	{
		perror("Reading file failed");
		goto error;
	}

	printf("Filename : %s\n", filename);
	printf("Filesize : %d bytes\n", fs.st_size);

	*((u_int32_t*)write_buf) = 0x30000000;		//download address
	*((u_int32_t*)write_buf+1) = fs.st_size + 10;	//download size;

	*len = fs.st_size + 10;
	return write_buf;

error:
	if(fd!=-1) close(fd);
	if(NULL!=write_buf) free(write_buf);
	fs.st_size = 0;
	return NULL;
	
}

int main(int argc, char *argv[])
{
	if(2!=argc)
	{
		usage();
		return 1;
	}

	struct usb_dev_handle *hdev = open_port();
	if(!hdev)
	{
		return 1;
	}

	unsigned int len = 0;
	unsigned char* write_buf = prepare_write_buf(argv[1], &len);
	if(NULL==write_buf) return 1;

	unsigned int remain = len;
	unsigned int towrite;
	printf("Writing data ...\n");
	while(remain)
	{
		towrite = remain>512 ? 512 : remain; 
		if(towrite != usb_bulk_write(hdev, 0x03, write_buf+(len-remain), towrite, 3000))
		{
			perror("usb_bulk_write failed");
			break;
		}
		remain-=towrite;
		printf("\r%d%\t %d bytes     ", (len-remain)*100/len, len-remain);
		fflush(stdout);
	}
	if(0==remain) printf("Done!\n");
	return 0;
}

Is there anything wrong with dnw2.c or the libusb it called.