Question

我正在尝试使用c中的unistd.h Linux函数写入MAC OSX上的蓝牙设备。我很好地连接并且成功地写了前几个字节。当我尝试向其写入其他命令时（每15ms有一些字节添加到写缓冲区），即使write（）函数返回1（写入成功），我也看不到任何结果。

如果您开始写入并且在尝试启动另一次写入时没有完成（因为它是非阻塞的），那么这可能会搞砸初始写入吗？（如果是这样，有没有办法检查写入是否已经完成？）这是我唯一能想到的事情，因为写入频繁发生并且前两个成功发送。

qwbyte（）只是在输出数组中添加一个字节并增加其长度

开放端口功能：

BAMid = -1;
struct termios options;
struct termios originalTTYAttrs;

// Open the serial port read/write, nonblocking, with no controlling terminal, and don't wait for a connection.
BAMid = open(strPath, O_RDWR | O_NOCTTY | O_NONBLOCK);
if (BAMid == -1)
{
    printf("Error opening serial port %s - %s(%d).\n",
           strPath, strerror(errno), errno);
    goto error;
}

// Issue TIOCEXCL ioctl to prevent additional opens except by root-owned processes.
if (ioctl(BAMid, TIOCEXCL) == -1)
{
    printf("Error setting TIOCEXCL on %s - %s(%d).\n",
  strPath, strerror(errno), errno);
    goto error;
}

// Get the current options and save them so we can restore the default settings later.
if (tcgetattr(BAMid, &originalTTYAttrs) == -1)
{
    printf("Error getting tty attributes %s - %s(%d).\n",
  strPath, strerror(errno), errno);
    goto error;
}

// The serial port attributes such as timeouts and baud rate are set by modifying the termios
// structure and then calling tcsetattr() to cause the changes to take effect. Note that the
// changes will not become effective without the tcsetattr() call.

options = originalTTYAttrs;

// Set raw input (non-canonical) mode, with reads blocking until either a single character 
// has been received or a one second timeout expires. [should be moot since we are leaving it as nonblocking]

cfmakeraw(&options);
options.c_cc[VMIN] = 1;
options.c_cc[VTIME] = 10;
cfsetspeed(&options, B57600);  // Set 57600 baud    
options.c_cflag |= CS8;    // Use 8 bit words

// Cause the new options to take effect immediately.
if (tcsetattr(BAMid, TCSANOW, &options) == -1)
{
    printf("Error setting tty attributes %s - %s(%d).\n",
  strPath, strerror(errno), errno);
    goto error;
}



//flush old transmissions
if (tcflush(BAMid,TCIOFLUSH) == -1) {
    printf("Error flushing BAM serial port - %s(%d).\n",
    strerror(errno), errno);
}  

oBufLength = 0;

// Ask it to start
if (! qwbyte(CmdStart) ) {
    goto error;
}

if (! qwbyte(CmdFull) ) {
    goto error;
} 
//this transmit works
txbytes();

printf("success opening port!");
return -1;
   // Failure path
error:
 if (BAMid != -1) {
        close(BAMid);
    }
 printf("returning an error--%d",errno);
   return errno;
}

写函数（txbytes）：

int i, bufSize, numBytes;

if(oBufLength != 0) { //if the output array isn't empty

        //duplicating the output array and its size so it can 
        //be overwritten while this write is occuring
        printf("about to transmit: ");
  for(i = 0; i < oBufLength; i++) {
   printf(" %u",oBuf[i]);
   tempBuf[i] = oBuf[i];
  }
  printf("\n");




 bufSize = oBufLength;
 oBufLength = 0; 

 numBytes = write(BAMid, &tempBuf, bufSize);

 printf("bytes written = %d\n",numBytes);

 if (numBytes == -1) {
     printf("Error writing to port - %s(%d).\n", strerror(errno), errno);
 }

 return (numBytes > 0);
}
else {
  return 0;
}

Answer 1

非阻塞写入无法满足您的期望。

如果无法立即完成写入，则write（）会返回到您的代码 - 但不继续尝试在后台发送数据。它只是说“我现在不能写 - 稍后再试”。它通过返回-1来实现，将 errno 设置为EAGAIN。

另外，请记住，成功后，write（）将返回成功写入的字节数。因此，当您要求写入2个字节时，如果获得的返回值为1，则表示它只是部分成功，并且需要在第二个字节的某个时间再次调用write（）。

基本上，如果你正在使用非阻塞IO，你希望你的txbytes（）函数在循环中调用write（），直到你的缓冲区为空，或者它返回-1。如果它返回-1你需要检查errno - 如果它是EAGAIN，你将不得不再次调用write（）;其他任何事情都可能是真正的错误。像这样：

ssize_t written = 0;

while (oBufLength > 0 && written > -1)
{
    size_t i;

    printf("about to transmit %d bytes: ", oBufLength);
    for(i = 0; i < oBufLength; i++) {
            printf(" %u",oBuf[i]);
    }
    printf("\n");

    written = write(BAMid, oBuf, oBufLength);

    printf("Write returned %d\n", written);

    if (written > 0)
    {
        /* The first "written" number of bytes in the buffer have now been sent, so
         * we discard them and move up the remaining bytes (if any) to the start of 
         * the buffer */

        oBufLength -= written;
        memmove(oBuf, oBuf + written, oBufLength);

        printf("Now have %d bytes left to send.\n", oBufLength);
     }
}

if (written > -1 || errno == EAGAIN)
{
    /* No fatal errors... */
    return 0;
} else
    /* error left in errno for caller to inspect */
    return -1;
}

请注意，不需要复制缓冲区 - 因为write（）不会与您的代码并行执行任何操作。如果它说它写了字节，它就不再需要你的缓冲区了。希望有所帮助！

使用c中的unistd.h写入MAC OSX上串行端口的问题

1 个答案: