The system gives CPU scheduling priority to processes that have not used CPU time recently. This tends to favor interactive processes and processes that execute only for short periods. It is possible to determine the priority currently assigned to a process, process group, or the processes of a specified user, or to alter this priority using the calls: ._d #define PRIO_PROCESS 0 /* process */ #define PRIO_PGRP 1 /* process group */ #define PRIO_USER 2 /* user id */ prio = getpriority(which, who); result int prio; int which, who; setpriority(which, who, prio); int which, who, prio; The value prio is in the range -20 to 20. The default priority is 0; lower priorities cause more favorable execution. 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. Resource utilization

The resources used by a process are returned by a getrusage call, returning information in a structure defined in <sys/resource.h>: ._d #define RUSAGE_SELF 0 /* usage by this process */ #define RUSAGE_CHILDREN -1 /* usage by all children */ getrusage(who, rusage) int who; result struct rusage *rusage; ._f struct rusage { struct timeval ru_utime; /* user time used */ struct timeval ru_stime; /* system time used */ int ru_maxrss; /* maximum core resident set size: kbytes */ int ru_ixrss; /* integral shared memory size (kbytes*sec) */ int ru_idrss; /* unshared data memory size */ int ru_isrss; /* unshared stack memory size */ int ru_minflt; /* page-reclaims */ int ru_majflt; /* page faults */ int ru_nswap; /* swaps */ int ru_inblock; /* block input operations */ int ru_oublock; /* block output operations */ int ru_msgsnd; /* messages sent */ int ru_msgrcv; /* messages received */ int ru_nsignals; /* signals received */ int ru_nvcsw; /* voluntary context switches */ int ru_nivcsw; /* involuntary context switches */ }; The who parameter specifies whose resource usage is to be returned. The resources used by the current process, or by all the terminated children of the current process may be requested. Resource limits

The resources of a process for which limits are controlled by the kernel are defined in <sys/resource.h>, and controlled by the getrlimit and setrlimit calls: ._d #define RLIMIT_CPU 0 /* cpu time in milliseconds */ #define RLIMIT_FSIZE 1 /* maximum file size */ #define RLIMIT_DATA 2 /* maximum data segment size */ #define RLIMIT_STACK 3 /* maximum stack segment size */ #define RLIMIT_CORE 4 /* maximum core file size */ #define RLIMIT_RSS 5 /* maximum resident set size */ #define RLIM_NLIMITS 6 #define RLIM_INFINITY 0x7fffffff ._f struct rlimit { int rlim_cur; /* current (soft) limit */ int rlim_max; /* hard limit */ }; getrlimit(resource, rlp) int resource; result struct rlimit *rlp; setrlimit(resource, rlp) int resource; struct rlimit *rlp;

Only the super-user can raise the maximum limits. Other users may only alter rlim_cur within the range from 0 to rlim_max or (irreversibly) lower rlim_max.