How can I implement timeout for read() when reading from a serial port (C/C++)

c, timeout, tty, unix

Solution

Yes, use `select(2)`. Pass in a file descriptor set containing just your fd in the read set and empty write/exception sets, and pass in an appropriate timeout. For example:

int fd = open(...);

// Initialize file descriptor sets
fd_set read_fds, write_fds, except_fds;
FD_ZERO(&read_fds);
FD_ZERO(&write_fds);
FD_ZERO(&except_fds);
FD_SET(fd, &read_fds);

// Set timeout to 1.0 seconds
struct timeval timeout;
timeout.tv_sec = 1;
timeout.tv_usec = 0;

// Wait for input to become ready or until the time out; the first parameter is
// 1 more than the largest file descriptor in any of the sets
if (select(fd + 1, &read_fds, &write_fds, &except_fds, &timeout) == 1)
{
    // fd is ready for reading
}
else
{
    // timeout or error
}

Problem

I am reading bytes from a serial port in C++ using a file descriptor and the posix/unix read() function. In this example, I am reading 1 byte from the serial port (baud rate settings and similiar are omitted for clarity): ``` #include <termios.h> #include <fcntl.h> #include <unistd.h> int main(void) { int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY); char buf[1]; int bytesRead = read(fd, buf, 1); close(fd); return 0; } ``` If the device connected to /dev/ttyS0 does not send any information, the program will hang. How can I set a timeout? I have tried setting a time out like this: ``` struct termios options; tcgetattr(fd, &options); options.c_cc[VMIN] = 0; options.c_cc[VTIME] = 10; tcsetattr(fd, TCSANOW, &options); ``` I thought it was supposed to give 1 second timeout, but it makes no difference. I think I have misunderstood VMIN and VTIME. What is VMIN and VTIME used for? Then I searched the web and found somebody talking about the select() function. Is that the solution and if so, how would one apply that to the program above to make 1 second timeout? Any help is appreciated. Thanks in advance :-)

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