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ucall(2) [bsd man page]

UCALL(2)							System Calls Manual							  UCALL(2)

NAME
ucall - call a kernel subroutine from user mode (2BSD) SYNOPSIS
#include <pdp/psl.h> #include <sys/types.h> ucall(priority, function, arg0, arg1) int priority, arg0, arg1; caddr_t function; DESCRIPTION
Ucall causes the processor priority to be set to priority and the specified kernel function to be called with arguments arg0 and arg1. Priority is one of PSL_BR0, ..., PSL_BR7. Processor priority is reset to PSL_BR0 when function returns. Ucall is allowed only if the user is the superuser. It is obviously extremely dangerous if misused. It's only current use is at system boot time to configure system devices by calling device drivers ... ERRORS
[EPERM] The caller is not the super-user. SEE ALSO
autoconfig(8) BUGS
No address validations are attempted. Ucall is unique to the PDP-11 and 2BSD; its use is discouraged. 3rd Berkeley Distribution January 22, 1987 UCALL(2)

Check Out this Related Man Page

GETPRIORITY(2)						      BSD System Calls Manual						    GETPRIORITY(2)

NAME
getpriority, setpriority -- get/set program scheduling priority SYNOPSIS
#include <sys/time.h> #include <sys/resource.h> int getpriority(int which, int who); int setpriority(int which, int who, int prio); DESCRIPTION
The scheduling priority of the process, process group, or user, as indicated by which and who is obtained with the getpriority() call and set with the setpriority() call. Which is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and who is interpreted relative to which (a process identifier for PRIO_PROCESS, process group identifier for PRIO_PGRP, and a user ID for PRIO_USER). A zero value of who denotes the current process, process group, or user. Prio is a value in the range -20 to 20. The default priority is 0; lower priorities cause more favorable scheduling. The getpriority() call returns the highest priority (lowest numerical value) enjoyed by any of the specified processes. The setpriority() call sets the priorities of all of the specified processes to the specified value. Only the super-user may lower priorities. RETURN VALUES
Since getpriority() can legitimately return the value -1, it is necessary to clear the external variable errno prior to the call, then check it afterward to determine if a -1 is an error or a legitimate value. The setpriority() call returns 0 if there is no error, or -1 if there is. ERRORS
Getpriority() and setpriority() will fail if: [ESRCH] No process was located using the which and who values specified. [EINVAL] Which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER. In addition to the errors indicated above, setpriority() will fail if: [EPERM] A process was located, but neither its effective nor real user ID matched the effective user ID of the caller. [EACCES] A non super-user attempted to lower a process priority. SEE ALSO
nice(1), fork(2), renice(8) HISTORY
The getpriority() function call appeared in 4.2BSD. 4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution
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