I am trying to set the baud-rate of "/dev/ttyS4" and return the Text(EditText) using C code with the help of JNI in android studio

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <termios.h>
#define DEBUG 1

#if DEBUG
#include <android/log.h>
#  define  D(x...)  __android_log_print(ANDROID_LOG_INFO,"hello-jni",x)
#else
#  define  D(...)  do {} while (0)
#endif

char*  str;
char buffer[512];

extern "C" JNIEXPORT jstring JNICALL
Java_com_android_uart_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */, jstring text)
{
    {
        asprintf(&str, "Hi\n");
        strlcpy(buffer, str, sizeof buffer);
        free(str);

        int ttyFD = 0;
        struct termios tty;
        const char *stringInC = env->GetStringUTFChars(text, NULL);
        auto SampleCmd = stringInC;
        int i_WriteBytes = 0;

        memset (&tty, 0, sizeof tty);

        ttyFD = open("/dev/ttyS4", O_RDWR| O_NONBLOCK | O_NDELAY );

        if(ttyFD < 0)
        {
            asprintf(&str, "Error in opening /dev/ttyS4\n");
            strlcat(buffer, str, sizeof buffer);
            free(str);


        }
        else
        {
            asprintf(&str, "ttyS4 port opened successfully\n");
            strlcat(buffer, str, sizeof buffer);
            free(str);
        }
        if (tcgetattr ( ttyFD, &tty ) != 0 )
        {
            asprintf(&str, "Error from tcgetattr\n");
            strlcat(buffer, str, sizeof buffer);
            free(str);

        }
        cfsetspeed(&tty, B0);


        tty.c_cflag &= ~PARENB;                            /* parity */
        tty.c_cflag &= ~CSTOPB;                            /* stop bits */
        //tty.c_cflag   &=  ~CSIZE;                             /* */
        tty.c_cflag |= CS8;                                /* data bits */
        tty.c_cflag &= ~CRTSCTS;                           /* no hardware flow control */
        tty.c_iflag &= ~(IXON | IXOFF | IXANY);            /* no s/w flow ctrl */
        tty.c_lflag = 0;                                  /* non canonical */
        tty.c_oflag = 0;                                  /* no remapping, no delays */
        tty.c_cc[VMIN] = 0;                                  /* read doesn't block */
        tty.c_cc[VTIME] = 0;                                  /* read timeout */
        tty.c_cflag |=
                CREAD | CLOCAL;                     /* turn on READ & ignore ctrl lines */
        tty.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);    /* */
        //tty.c_oflag     &=  ~OPOST;                           /* */

        tcflush(ttyFD, TCIFLUSH);

        if (tcsetattr(ttyFD, TCSANOW, &tty) != 0) {
            asprintf(&str, "Error from tcsetattr\n");
            strlcat(buffer, str, sizeof buffer);
            free(str);
        }

        i_WriteBytes = write(ttyFD, SampleCmd, sizeof(SampleCmd) - 1);
        asprintf(&str, "written bytes = %d\n", i_WriteBytes);
        strlcat(buffer, str, sizeof buffer);
        free(str);

        if (i_WriteBytes < 0) {
            asprintf(&str, "failed to write value on port\n");
            strlcat(buffer, str, sizeof buffer);
            free(str);
        }
        sleep(1);
        char read_buf[1024];
        memset(&read_buf, '\0', sizeof(read_buf));
        int num_bytes = read(ttyFD, &read_buf, sizeof(read_buf));

        if (num_bytes < 0) {
            asprintf(&str, "Error reading: %s", strerror(errno));
            strlcat(buffer, str, sizeof buffer);
            free(str);
        }
        asprintf(&str, "Read bytes = %i \n Received message: %s", num_bytes, read_buf);
        strlcat(buffer, str, sizeof buffer);
        free(str);

        close(ttyFD);

        return env->NewStringUTF(buffer);
}

My problem is that WriteBytes and Read bytes only take 7 bytes of the edittext"Text" and it only display 7 characters of it. What i want is to display the entire "Text" from the EditText.

This is the result iam getting:

**Input:**
EditText(baud Rate)=0
EditText(message) =this is a test app

**Output:**
TextView(Details)
hi
ttyS4 port opened successfully
written bytes = 7 
Read bytes =7
Received message:this is 

CodePudding user response：

The line

        i_WriteBytes = write(ttyFD, SampleCmd, sizeof(SampleCmd) - 1);

is wrong.

Variable SampleCmd, defined as

        const char *stringInC = env->GetStringUTFChars(text, NULL);
        auto SampleCmd = stringInC;

is of type int in C or type const char * in C . (Note that auto is an old keyword in C which defines a storage class is the default for variables inside functions and forbidden at file scope. As you don't specify a type, it defaults to int.)

The value sizeof(SampleCmd) is not related to the length of the string pointed to by stringInC. It is the size of a variable of type int (in C, or type char* in C ). Apparently this size is 8 on your platform, so you will always write 7 bytes.

You could use

        i_WriteBytes = write(ttyFD, SampleCmd, strlen(SampleCmd));

or use fdopen on ttyFD and use output functions from stdio.h.