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The meaning of "appropriate privileges" varies from platform to platform. For example, on Solaris appropriate privileges for setuid() means that the PRIV_PROC_SETID privilege is in the effective privilege set of the process. On BSD, it means that the effective user ID (EUID) is zero (that is, the process is running as root) or that uid=geteuid(). On Linux, it means that the process has CAP_SETUID capability and that setuid(geteuid()) will fail if the effective user ID (EUID) is not equal to 0, the real user ID (RUID), or the saved set-user-ID (SSUID).
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