Serial I/O in C with termios: unreliable output capitalization - c

Question

I have a very small C program which sends and receives newline-terminated ASCII strings to and from a serial device. It's plugged into my computer with a USB adapter, on /dev/ttyUSB0.
Most of the time it sends the commands just find, but occasionally it will capitalize all the lower-case letters to upper-case. It leaves all special characters alone.
The string I am sending is /home\n. About 1 out of every five times I run the program (by simply running ./a.out without recompiling), the sent message understood by the device is /HOME\n.
Here is my source code:
#include <stdio.h>
#include <stdlib.h>
#include "zserial.h"
int main() {
char buf[256];
int fd = connect("/dev/ttyUSB0");
char *cmd = "/home\n";
send(fd, cmd);
receive(fd, buf, 256);
puts(buf);
exit(0);
}
And zserial.c:
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "zserial.h"
int send(int fd, char *buf) {
int len = strlen(buf);
int nb = write(fd, buf, len);
if (len != nb || nb < 1)
perror("Error: wrote no bytes!");
tcdrain(fd);
return nb;
}
int receive(int fd, char *dst, int nbytes) {
int i;
char c;
for(i = 0; i < nbytes;) {
int r = read(fd, &c, 1);
/* printf("Read %d bytes\n", r); */
if (r > 0) {
dst[i++] = c;
if (c == '\n') break;
}
}
dst[i] = 0; /* null-terminate the string */
return i;
}
int connect(char *portname) {
int fd;
struct termios tio;
fd = open(portname, O_RDWR | O_NOCTTY | O_NONBLOCK);
tio.c_cflag = CS8|CREAD|CLOCAL;
if ((cfsetospeed(&tio, B115200) & cfsetispeed(&tio, B115200)) < 0) {
perror("invalid baud rate");
exit(-1);
}
tcsetattr(fd, TCSANOW, &tio);
return fd;
}
What am I doing wrong? Is there some termios flag which modifies the output on a serial port?

Answer 1

c_oflag & OLCUC turns on the mapping of lowercase to uppercase on output. Since you never initialized tio, it's not surprising you got some random flags set.
You have two choices:
tcgetattr the current settings into a termios struct to initialize it, then modify the ones you're interested in, then write them back with tcsetattr
initialize all the termios fields to known values, not just c_cflag and the speed fields.