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I am connected to my Raspberry PI 3 B+ running Raspbian through an SSH terminal, and I am trying to make an application that uses the UART through the GPIO.

Whenever I try to open the port "/dev/serial0" I get in the case where I should not be getting, according to the tutorial I am following 1. I do not know exactly what means the the output of the function open(), but I am trusting the tutorial :

#include <stdio.h>
#include <unistd.h>         //Used for UART
#include <fcntl.h>          //Used for UART
#include <termios.h>        //Used for UART

void setup_uart(){
    int uart0_filestream = -1;
    uart0_filestream = open("/dev/serial0", O_RDWR | O_NOCTTY | O_NDELAY);      //Open in non blocking read/write mode
    if (uart0_filestream == -1)
    {
        //ERROR - CAN'T OPEN SERIAL PORT
        printf("Error - Unable to open UART.  Ensure it is not in use by another application\n");
    }
    ...
}

void hello_uart(){
//----- TX BYTES -----
    unsigned char tx_buffer[20];
    unsigned char *p_tx_buffer;

    p_tx_buffer = &tx_buffer[0];
    *p_tx_buffer++ = 'H';
    *p_tx_buffer++ = 'e';
    *p_tx_buffer++ = 'l';
    *p_tx_buffer++ = 'l';
    *p_tx_buffer++ = 'o';

    if (uart0_filestream != -1)
    {
        int count = write(uart0_filestream, &tx_buffer[0], (p_tx_buffer - &tx_buffer[0]));      //Filestream, bytes to write, number of bytes to write
        if (count < 0)
        {
            printf("UART TX error\n");
        }
    }
}


int main(){
    setup_uart();
    hello_uart();
}

I have not used the port ttyS0 before (at least not consciously in the program I am working on), and I suspect that the SSH may be the problem, if it is using the miniUART (in serial0). Moreover, when I try to use the port serial1, I do not it compiles and runs without problems, but I can not test if it is working or not.

How can I investigate what is causing it and, even better, do you have any suggestions for how could I use UART through the GPIO?

Would it be a good idea do try to remap the UART PL011 to the GPIO ports 8 and 10, taking the miniUART out from there, or is there anything more straightforward?

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I bet the problem is that you have enabled serial console. Your tutorial said to turn that off, but it wasn't very clear. Try

sudo raspi-config

and select no for the serial console on boot, but yes for serial hardware. Then /dev/ttyS0 will be set with permissions

crw-rw---- 1 root dialout 4, 64 Feb 24 19:04 ttyS0

Which is what you want. With serial console enabled /dev/ttyS0 is created with permissions

crw--w---- 1 root tty 4, 64 Jan 25 15:21 ttyS0

Which won't work because the default user pi isn't in the group tty and doesn't have group permissions to read ttyS0 anyway.

Also, to learn about open(), open a terminal and read

man 2 open
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Your code is using /dev/serial1 which is usually connected to Bluetooth.

There is no need to fiddle with port mappings. This should be done through raspi-config and/or Device Tree.

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