Trying to retrieve data from a serial port but program is stuck at getchar

Question

I am using an embedded system to send data from 25 sensors to a putty terminal on my computer. Works great.

I wanted to add a read from terminal functionality to the embedded system (so I can send commands). So I tried using getchar() to read whatever I would write on my putty terminal. First I just wanted to getchar and print the character back on putty. It kinda works, but my sensor data, which is supposed to print every 500ms, does not print until I type a char in putty. It is as if my code was stuck on getchar() and stuck in a while loop until getchar() reads something.

Here is my forever loop in my int main(). I am not sharing the rest as it is not really needed and too bulky (its just initializing modules). In this loop I am reading a sensor, trying to read from putty, writing to putty, and starting my next scan:

for(;;)
{

    CapSense_ProcessAllWidgets(); // Process all widgets 
    CapSense_RunTuner();    // To sync with Tuner application           

    read_sensor(curr_elem);  //read curr_elem

    (curr_elem < RX4_TX4)?(curr_elem++):(curr_elem = 0, touchpad_readings_flag++);

// Here is the part to read I added which blocks until I type in something.
// If I remove this if and all of what's in it, I print to putty every 500ms
    if(touchpad_readings_flag) 
        {
                    char received_char = getchar();
                if (received_char) //if something was returned, received_char != 0
                    {
                        printf("%c", received_char);
                    }
        }

//Here I write to putty. works fine when I remove getchar()    
    if (print_counter_flag && touchpad_readings_flag) 
    {
        print_counter_flag = 0;
        touchpad_readings_flag = 0;
        for (int i = 0; i < 25; i++)
        {
            printf("\n");
            printf("%c", 97 + i);
            printf("%c", val[i] >> 8);
            printf("%c", val[i] & 0x00ff);  // For raw counts
            printf("\r");
        }  
    }       


    /* Start next scan */
    CapSense_UpdateAllBaselines();
    CapSense_ScanAllWidgets();
}

score 1 · Accepted Answer · answered Apr 10 '20 at 10:51

Apparently, your getchar() call is blocking unless there is input data to retrieve. One solution to change this behaviour has been given by another article on different SE board.

Please also note that getchar() is a wrapper for getc() that is acting on stdin as this site¹ describes. For getc() you find further discussions. In one of those, it is pointed out that some important implementations even wait for a newline character until input is delivered to your function. I think this depends on standard libraries/kind of embedded system you actually use - please check the documentation of your toolchain vendor.²

¹ I didn't look up a normative source, this is just my first google hit.

² The question doesn't specify the kind of embedded system, so a generic answer is wanted instead of a discussion of particular target/toolchain combinations, IMO.

You were right. The device used was the PSoC 6 development board by Cypress semiconductors. the solution was to add an if statement to check if the buffer was empty or not before calling getchar(). In case it was empty, calling getchar() was what was blocking my code! — JCSB, Apr 10 '20 at 21:33
Fine. I added the arch. information explicitly to your question because I overlooked the `psoc` tag alone when reading for the first time. — HelpingHand, Apr 11 '20 at 12:36

Trying to retrieve data from a serial port but program is stuck at getchar

1 Answers1