急急急：请教各位大虾关于串口通信的问题！！！！！

这个程序的主要目的是向串口发送AT命令（基于嵌入式Linux），是短信猫发短信，但问题是不能发出短信，已经排除是硬件问题，希望大家帮帮忙，这个程序错在什么地方。
void serial_rw()
{
int i;
char buff[512];
char buff_t[]="AT\n";
char receive[3]="w";
char buff4[] = "AT+CMGD=1\n";
char buff2[] = "AT+CMGS=15045268420\n";
char buff3[] = "this is a test message";
int nread,nwrite;
printf("the num is:%s\n",buff2);
write( serial_fd,buff_t,sizeof(buff_t) );
sleep(5);
read( serial_fd,receive,sizeof(receive) );
printf( "receive: %s\n",receive );
nwrite = write(serial_fd,buff4,sizeof(buff4));
sleep(5);
nwrite = write(serial_fd,buff2,sizeof(buff2));
sleep(5);
printf("nwrite=%d\n",nwrite);
nwrite = write(serial_fd,buff3,sizeof(buff3));
sleep(5);
printf("send nwrite=%d\n",nwrite);
nwrite = write(serial_fd,"\x1a",sizeof("\x1a"));
sleep(5);
printf("end of send message\r\n");
while(1)
{
}
close(serial_fd);
}