MAN page from RedHat EL 8 fpc-3.2.0-1.el8.x86_64.rpm
Section: Linux console vcsa grabber (1)
Updated: 22 jan 2006Index
grab_vcsa - Linux console vcsa grabber
console allows text modevideo buffer access^
. For each tty device(/dev/tty0..31) there exist two devices (/dev/vcs0..31) and (/dev/vcsa0..31).These devices are two different representations of the content of the console.
When you login on the Linux console, your tty device is changed owner to you,so you can read/write it. When you logout it is takes from you. This does nothappen with the vcs and vcsa devices, i.e. you cannot access the video bufferof your own tty.
Unfortunately there is a lot of reason to use the text mode video buffer. Forinstance, it is not possible to get access to the full 256 characters the VGAadapter can display without using the text mode video buffer (Linux emulatesthe vt100 character set which does not have all VGA characters). Thisannoyance severely limits the porting possibilities to Linux of softwarethat was written with a VGA character set in mind.
grab_vcsais a setuid root program that, when called will change the owner of the vcsand vcsa devices to you, after a series of security checks have been performed.
It is not necessary to change the permissions back again (this would be pointlesssince a user could kill a imaginary release_vcsa program before it can do itswork). The permissions are automatically reverted on logout.
grab_vcsais used by the Free Pascal video unit. The video unit allows programmers todevelop portable programs across Dos, Linux, FreeBSD, Windows, OS/2 andother operating systems while they can still assume being able to use thefull VGA character set.
has the following return codes:
- You are not running on the Linux console
- Error while statting tty.
- Error while executing chown.
- Error while executing chmod.
- You do not own your own tty.
Daniel Mantione <daniel.mantioneAATTfreepascal.org>
- RETURN CODES
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