And how to use setuid() ?
thanks

#include <unistd.h>
uid_t myuid(void)
{
    return getuid();
}

setuid() is pretty integral to a safe unix process.
One basic mode is for a root privileged parent to acquire
resources only it can handle (ports < 1024) and then delegate service to setuid(> 0) children/threads, ala OpenSSH and many other pieces of software via IPC.

Given it's not easy to do securely and does pose a considerable security issue: mostly races and various abuses of unsafe programing practices in the privileged process.

thanks jim mcnamara,
i want catch value of getuid() to put into a logfile. How to do it?

one way:

id >> logfile

/* myid.c */
#include <unistd.h>
uid_t myuid(void)
{
    return getuid();
}

int main()
{
     printf("%d\n", myuid() );
     return 0;
}

myid >> logfile