serial port - Raspberry Pi UART program in C using termios receives garbage (Rx and Tx are connected directly)

Question

I have a simple program written in C which uses termios to send a basic string to the Raspberry Pi UART and attempts to read and output the response. The Rx and Tx pins on the Raspberry Pi are connected with a jumper so whatever is sent should be immediately received.

Despite the program outputting that it successfully sent and received 5 characters for the chosen string ('Hello'), trying to print the contents of the buffer just produces one or two garbage characters.

The program:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <termios.h>

int main(int argc, char* argv[]) {

    struct termios serial;
    char* str = "Hello";
    char buffer[10];

    if (argc == 1) {
        printf("Usage: %s [device]

", argv[0]);
        return -1;
    }

    printf("Opening %s
", argv[1]);

    int fd = open(argv[1], O_RDWR | O_NOCTTY | O_NDELAY);

    if (fd == -1) {
        perror(argv[1]);
        return -1;
    }

    if (tcgetattr(fd, &serial) < 0) {
        perror("Getting configuration");
        return -1;
    }

    // Set up Serial Configuration
    serial.c_iflag = 0;
    serial.c_oflag = 0;
    serial.c_lflag = 0;
    serial.c_cflag = 0;

    serial.c_cc[VMIN] = 0;
    serial.c_cc[VTIME] = 0;

    serial.c_cflag = B115200 | CS8 | CREAD;

    tcsetattr(fd, TCSANOW, &serial); // Apply configuration

    // Attempt to send and receive
    printf("Sending: %s
", str);

    int wcount = write(fd, &str, strlen(str));
    if (wcount < 0) {
        perror("Write");
        return -1;
    }
    else {
        printf("Sent %d characters
", wcount);
    }

    int rcount = read(fd, &buffer, sizeof(buffer));
    if (rcount < 0) {
        perror("Read");
        return -1;
    }
    else {
        printf("Received %d characters
", rcount);
    }

    buffer[rcount] = '';

    printf("Received: %s
", buffer);

    close(fd);
}

Outputs:

Opening /dev/ttyAMA0
Sending: Hello
Sent 5 characters
Received 5 characters
Received: [garbage]

I can't see any major problem with the code myself, but I might be wrong. I can successfully send and receive characters using PuTTY connected with the same settings, so it can't really be a hardware problem. Although I haven't tried it in PuTTY, trying to connect with anything less than 115200 baud with this program will result in nothing being received.

Where am I going wrong?

Answer 1

int wcount = write(fd, &str, strlen(str));
int rcount = read(fd, &buffer, sizeof(buffer));

In these lines, buffer/str are already pointers. You are passing a pointer to a pointer.

The lines should be:

int wcount = write(fd, str, strlen(str));
int rcount = read(fd, buffer, sizeof(buffer));