device write() system calls

[hanku]

Hi All,

How does the write() system call work if it's writing it to a device port and how would I know if the data is being sent immediately and not to a buffer? For example,

int fd;
fd = open( "/dev/ttyAM0", 0_RDWR );
write( fd, "abc\r", 4 )

I would imagine that after invoking the above system call, "abc\r" is being immediately sent to the device port at the path "/dev/ttyAM0". Is that true or is "abc\r" sent to somewhere else?

Thanks in advance for the help.

[fernape]

It depends on the device: block devices (hard disks, cdroms, etc...) are buffered while character devices are not.

Since you are dealing with a character device, your string will be sent inmediately. But if you need an "inmediate" writing for block devices, use fflush(filedescriptor).

And remember: you are not working with write() system call, but with write() form libc that performs a lot of tasks before calling the actual write system call.

Best regards

[hanku]

Hi fernape,

Thanks for the reply. One follow-up question, is there a way that I can buffer the characters sent to a character device? So the device that the program talks to can receive "abc\r" at once rather than 'a', 'b', 'c' '\r'.

Cheers!

[fernape]

I think not. You can delay the delivery using a temporary variable, but then, the device will receive the characters one by one.

Best regards

[hanku]

Hi fernape,

When the device receives the character one by one, is this process the same as transmitting a line of text with character delay?

I am trying to mimic the process of Windows' hyperterminal transfering a 3-line text file with 1000 ms line delay and 0 ms char. delay by using the low-level write() system call. Any idea of how I can achieve that? Thanks.

Cheers!

[fernape]

I think yes, but it is possible that if you delay characteres for a long time, the device "closes conection" or performs some action (I'm thinking about a serial modem dealing with AT command.)

I think you can assume that when you send something like "abcdefgh" the delay between characters is 0 unless problems with the line or similar. So you need a way to hold the emission of each complete line (use a string).

Best regards

[vkv.raju]

Hi All,

This is in continuation with the original question asked by the OP. So, what is the maximum buffer size write() uses for the block devices?

Is this buffer a configurable thing?

Thanks in advance,

Regards,

[vkv.raju]

Hi All,

How does the write() system call work if it's writing it to a device port and how would I know if the data is being sent immediately and not to a buffer? For example,

int fd;
fd = open( "/dev/ttyAM0", 0_RDWR );
write( fd, "abc\r", 4 )

I would imagine that after invoking the above system call, "abc\r" is being immediately sent to the device port at the path "/dev/ttyAM0". Is that true or is "abc\r" sent to somewhere else?

Thanks in advance for the help.

If you want this to be written into a file immediately, use the O_FSYNC option too while opening the file. Or, alternatively you can use fsync() after the write. Hope, this helps,