UART reply includes previous command?

Question

I am trying to read from a UART device in a Linux environment using a C program, but I experience different results respect to communicating to the UART using screen.

The C code I use to test the UART communication is the following:

#include <stdio.h>
#include <stdlib.h>
#include <libgen.h>
#include <unistd.h>
#include <string.h>
#include <strings.h>
#include <getopt.h>
#include <stdbool.h>
#include <sys/stat.h>
#include <termios.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <sys/file.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/select.h>
#include <signal.h>
#include <ctype.h>

bool loop;

void sigHandler(int32_t sig)
{   
    if(sig == SIGINT)
    {
        printf("Catched SIGINT");
        loop = false;
    }
}

int main(int argc, char *argv[])
{
    char *devname = argv[1];
    int fd = -1;

    int nread = -1;
    int nwrite = -1;

    int ret;

    struct termios t_new = {0};
    struct termios t_old = {0};


    signal(SIGINT, sigHandler);


    fd = open(devname, O_RDWR | O_NOCTTY |O_NONBLOCK);
    if(fd > 0)
    {   
        printf("TTY open ! Configuring TTY"); 
    }
    else
    {
        fd = -1;
        return 1;
    }

    ret = tcgetattr(fd, &t_old);
    if(ret < 0)
    {
        perror("tcgetattr ");
        close(fd);
        fd = -1;
        return 1;
    }

    t_new = t_old;
    t_new.c_cflag = (B9600 | CS8 | CREAD );
    t_new.c_oflag = 0;
    t_new.c_iflag = 0;
    t_new.c_lflag = 0;

    ret = tcsetattr(fd, TCSANOW, &t_new);

    loop = true;
    while(loop)
    {
        char s[] = "at+gmi=?\r\n";
        nwrite = write(fd, s, strlen(s));
        if(nwrite == strlen(s))
        {
            fd_set rfd;
            struct timeval tm = {.tv_sec = 0, .tv_usec = 500000};
            FD_ZERO(&rfd);
            FD_SET(fd, &rfd);

            char buffer[64] = {0};

            if(select(fd + 1, &rfd, NULL, NULL, &tm) > 0)
                nread = read(fd, buffer, sizeof(buffer));

            if(nread > 0)
                printf("Reply is: %s\n", buffer);
        }

        usleep(500000);
    }
}

But when I read the reply, it always includes the string I have sent. I don't experience this problem using screen.

What is the best way to read from an UART in C using Linux ? Could the multiplexed way (using select) causing the problems ?

EDIT For completeness, the output is:

Reply is: at+gmi=?

OK

Also, sometimes I don't read anything.

Answer 1

But when I read the reply, it always includes the string I have sent.

Since your termios configuration obliterated the local echo attributes and you're sending an AT modem command, you should try sending an ATE0 command to disable echoing by the modem.

I don't experience this problem using screen.

This observation confirms that the connected modem has its echoing enabled.

The AT command is echoed (by the modem) as you type, but you don't object to this received data in this situation (because you want to see what you type).
If the modem did not have echoing enabled, then you would be complaining that what you type in screen was not visible.

IOW echo is desired when using a terminal emulator program (such as screen), but echoing needs to be disabled when sending data by a program.

What is the best way to read from an UART in C using Linux ?

(Technically you are not reading from a "UART", but rather from a serial terminal that fully buffers all input and output.)
Code that conforms to POSIX standard as described in Setting Terminal Modes Properly and Serial Programming Guide for POSIX Operating Systems would be far better that what you have now.
I'm surprised that it works at all (e.g. CREAD is not enabled).

Could the multiplexed way (using select) causing the problems ?

Not the echo "problem".
Your program does not do anything that requires using select() and nonblocking mode.

Also, sometimes I don't read anything.

When you write code that is not POSIX compliant, you should not expect reliable program behavior.