This is procpri/t_setpriority.c (Listing 35-1, page 736), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* t_setpriority.c

   Demonstrate the use of setpriority(2) and getpriority(2) to change and
   retrieve a process's nice value.

   Usage: t_setpriority {p|g|u} id priority
#include <sys/time.h>
#include <sys/resource.h>
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    int which, prio;
    id_t who;

    if (argc != 4 || strchr("pgu", argv[1][0]) == NULL)
        usageErr("%s {p|g|u} who priority\n"
                "    set priority of: p=process; g=process group; "
                "u=processes for user\n", argv[0]);

    /* Set nice value according to command-line arguments */

    which = (argv[1][0] == 'p') ? PRIO_PROCESS :
                (argv[1][0] == 'g') ? PRIO_PGRP : PRIO_USER;
    who = getLong(argv[2], 0, "who");
    prio = getInt(argv[3], 0, "prio");

    if (setpriority(which, who, prio) == -1)

    /* Retrieve nice value to check the change */

    errno = 0;                  /* Because successful call may return -1 */
    prio = getpriority(which, who);
    if (prio == -1 && errno != 0)

    printf("Nice value = %d\n", prio);



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