#include <unistd.h>

       int nice(int inc);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


       nice()  adds  inc to the nice value for the calling process.  (A higher
       nice value means a low priority.)  Only the  superuser  may  specify  a
       negative increment, or priority increase.  The range for nice values is
       described in getpriority(2).

       On success, the new nice value is returned (but see NOTES  below).   On
       error, -1 is returned, and errno is set appropriately.

       EPERM  The  calling  process attempted to increase its priority by sup-
              plying a negative inc but has  insufficient  privileges.   Under
              Linux  the  CAP_SYS_NICE  capability  is required.  (But see the
              discussion of the RLIMIT_NICE resource limit in setrlimit(2).)

       SVr4, 4.3BSD, POSIX.1-2001.  However, the Linux  and  (g)libc  (earlier
       than  glibc  2.2.4) return value is nonstandard, see below.  SVr4 docu-
       ments an additional EINVAL error code.

       SUSv2 and POSIX.1-2001 specify that nice() should return the  new  nice
       value.  However, the Linux syscall and the nice() library function pro-
       vided in older versions of (g)libc (earlier than glibc 2.2.4) return  0
       on success.  The new nice value can be found using getpriority(2).

       Since  glibc  2.2.4,  nice()  is implemented as a library function that
       calls getpriority(2) to obtain the new nice value to be returned to the
       caller.   With  this implementation, a successful call can legitimately
       return -1.  To reliably detect an error, set  errno  to  0  before  the
       call, and check its value when nice() returns -1.

       nice(1),  renice(1), fork(2), getpriority(2), setpriority(2), capabili-

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