Member Avatar for xenanovich

hi,
(on a linux system) i wrote the following code snippet to
1. write to a serial port
2. read the data that i have just written to the serial port.
the write is functioning, but the read is not.
if i use just the read (without "write", but by connecting a serial device), it is working fine.

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h> 
#include <string.h>  
#include <unistd.h>  
#include <fcntl.h>   
#include <errno.h>   
#include <termios.h>

int main(int argc, char *argv[])
{
char line[1024];
int chkin;
char input[1024];
int file= open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
 if(file == 0)
            {
                  perror("open_port: Unable to open       /dev/ttyS0 - ");
            }
            else
                  fcntl(file, F_SETFL, 0);
while(1)
{

printf("enter input data:\n");
scanf("%s",&input);

chkin=write(file,input,sizeof input);
if(chkin<0)
{
printf("cannot write to port\n");
}
fcntl(file, F_SETFL, FNDELAY);

chkin=read(file,line,sizeof line);
if(chkin<0)
{
printf("cannot read from port\n");
}
printf("line=%s\n",line);
/*CODE TO EXIT THE LOOP GOES HERE
*/
}
close(file);
return 0;
}

sample output:

enter input data:
fjhfgfg
cannot read from port
line=

i am using http://www.easysw.com/~mike/serial/serial.html#2_5_4 for reference.
where am i goofing up?
thanks!

Edited by xenanovich because: n/a
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Dani AI

Generated

Good catch by — serial ports must be configured before you can reliably write/read. A few concrete points that dig into the original snippet and will help others who find this thread later:

  • Check open() correctly: open() returns -1 on failure (not 0). Print errno with perror when it fails ( pointed this out).
  • Writing: do not use sizeof input when writing a C string; use strlen(input) (or include the exact length you want).
  • Blocking/non‑blocking: O_NDELAY/FNDELAY make the fd non‑blocking; reads will return -1 with errno==EAGAIN if no data is available. Either remove non‑blocking or use select()/poll() to wait.
  • Termios: set baud, parity, disable canonical/echo if needed, and set c_cc[VMIN]/c_cc[VTIME] so read() behaves as you expect. Also use tcdrain() if you need to wait for transmit completion and tcflush() to clear input/output queues.
  • Physical layer: writing to the port does not automatically give you the same bytes back — you need a loopback (TX->RX) or a device that echoes.

Small example pattern (conceptual):

int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY);
if (fd < 0) { perror("open"); return; }

ssize_t nw = write(fd, buf, strlen(buf));
if (nw < 0) perror("write");
tcdrain(fd);

/* wait with select() then read() */

Troubleshooting checklist: verify open() return, remove/handle non‑blocking mode, configure termios (baud, raw vs canonical, VMIN/VTIME), use proper write length and safe input functions (e.g. fgets() or scanf("%1023s", input)), and confirm physical wiring or device echo. For termios details see the POSIX/Linux manual (termios). (See termios documentation: https://man7.org/linux/man-pages/man3/termios.3.html)

Recommended Answers

All 4 Replies

Member Avatar for abhimanipal

Print the value of the file descriptor. When I printed the value I got -1.
One reason could be, as the article mentioned that I am unable to open the file due to lack of permissions.
Have you taken care of this issue ?

Member Avatar for xenanovich

i have. i am the root user, so i had the required permissions.
in the other examples i came across, there are mentions of "configuring the port".
is this essential? reading or writing works fine separately. when i do both it isn't working.

Member Avatar for Tellalca

Does the function ( read/write ) you first call in the program works and the following does not?

If so maybe you should do some procedure before attempting to do another work. I never programmed serial port before , it is just an idea :)

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