C program keeps sending same data on serial port - c

Question

I'm working on a SOM board running Linux embedded for ARM, and I'm developing a C program to communicate with an external device through a serial (RS232) port. I am experiencing a strange behavior though. I'm also using another serial port of the board to communicate with the linux running on the board.
The software has a simple structure: is a text-only console-like program, with this as main menu:
Possible commands:
1 - 4: Select serial device (pump should be on 1)
m - pump op. mode configuration
r - reads from the serial device
w - writes to the serial device
>>>>>>>>>>>>>>Current device is /dev/ttymxc1
>>>>>>>>>>>>>>Enter input (q quits):
and a secondary menu (opened by the "m" option above)
SPEED:
r - rpm (sends 1M<CR>) //<CR> stands for carriage return
f - flow rate (sends 1N<CR>)
QUANTITY:
v - volume (sends 1H<CR>)
t - time (sends 1O<CR>)
DIRECTION:
c - clockwise (sends 1T<CR>)
a - c-clockwise (sends 1K<CR>)
>>>>>>>>>>>>>>Enter input (q quits):
Communication using the main menu options "r" and "w" works fine (thus removing any doubt I may have regarding serial settings like baud rate): "w" invokes a routine ("serial_write" below) that sends a single character input by the user, while "r" returns the data read as soon it arrives (using "serial_read" below). The character I send arrives correctly, and the answer is shown correctly on the console, no matter the times I repeat the "w" and "r" cycle.
The options in the secondary menu should behave in the same way: they simply invoke a routine ("sendSimpleCmd" below) that invokes "serial_write" with a costant char as argument (different for each option), and then it invokes "serial_read".
The problem is that this only works for the first option selected: after that, the program keeps sending data linked to the first option selected, no matter the option I choose. It keeps doing this until I go back to the main menu, then choose again "m": at this point the data sent is the one I expect, but the subsequent choices will be ignored until I go back to the main menu (or close the software, if that matters).
The strangest thing is that I receive the expected data on the same serial I'm using to communicate with board while on the "right" serial port I keep getting the first message. This is the text pasted from the console when I choose "a" as second option, after having chosen "f" as first option (comments added by me):
SPEED:
r - rpm (sends 1M<CR>)
f - flow rate (sends 1N<CR>)
QUANTITY:
v - volume (sends 1H<CR>)
t - time (sends 1O<CR>)
DIRECTION:
c - clockwise (sends 1T<CR>)
a - c-clockwise (sends 1K<CR>)
>>>>>>>>>>>>>>Enter input (q quits):
a //second option
1Knding 1M //mixup of data
wrote 4 characters on fs 4
serial_read: *
The mixup is made by the software output ("Sending data 1M") and the data that should be sent after choosing option "a" (1K). Since on the "right" port I get the same message over and over, while on the "wrong" port I get the right message, it seems that somehow the software autonomously changes port.
The question is:
Could this behavior be caused by my coding, or is bound to something else, like some kernel configuration? If more information is needed, just ask.
Thank you in advance
Serial_write
void serial_write(char text[], int length){
if (selectedDevice == 0){
printf("Select device first!\r\n");
return;
}
int n;
length = length +1 + 2;
char toBeSent[length];
strcat(toBeSent, PUMP_CMD_MSG_START); //header, "1"
strcat(toBeSent, text);
strcat(toBeSent, PUMP_CMD_MSG_END); //footer, "<CR>"
printf("Sending %s\r\n", toBeSent);
n = write (fd, toBeSent, length);
if (n<0){
printf("writing failed on /dev/ttymxc%i\r\n", selectedDevice);
} else {
printf("wrote %i characters on fs %i\r\n", n, fd);
}
}
Serial_read
int serial_read(char *buffer, int size){
int bytes = 0;
int n;
int i = 0;
char tmp_buffer[size];
while(1){
ioctl(fd, FIONREAD, &bytes);
if (bytes > 0){
break;
}
i++;
if(i> 1000){
printf("FIONREAD tries exceeded 1000, aborting read\r\n");
return;
}
usleep(1000);
}
n=read(fd, tmp_buffer, sizeof(tmp_buffer));
for(i=0;i<n;i++) {
buffer[i]=tmp_buffer[i];
}
printf("serial_read: %s\r\n", buffer);
return 0;
}
sendSimpleCmd
void sendSimpleCmd(char text[]){
int bufSize= 20;
char answer[bufSize];
serial_write(text,1);
if (serial_read(answer, bufSize) == 0) {
printf("Ricevuto da pompa \"%s\":", answer);
//handling of possible answers, doesn't do anything relevant since it always receives "*" as answer
if (strcmp(answer, PUMP_ANS_OK) == 0){ //PUMP_ANS_OK is "*"
printf("ok!\r\n");
} else if (strcmp(answer, PUMP_ANS_NOK) == 0){
printf("errore!\r\n");
} else {
printf("sconosciuto!\r\n");
}
} else {
// printf("read failed\r\n");
}
}

Answer 1

You should be initializing toBeSent's contents before using strcat.
Your compiler might be saving you by initializing the array with 0s rather than garbage, but if not, it could be causing buffer overflows. Theres no code protecting against such, so this could be the cause of unexpected program behavior. Without seeing the rest of your code and knowing some other details, it will be difficult to know what exactly the issue is. If this serves as an example of the rest of the code, then the solution is likely to revise the code to fix these issues.
Consider using safe string functions to help prevent buffer overflows.