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/* Build with gcc -Wall -D_POSIX_PTHREAD_SEMANTICS */
#include <stdio.h>>
#include <pwd.h>
#include <errno.h>
#include <string.h>

#ifdef _SC_GETPW_R_SIZE_MAX
#define USER_BUF_SIZE (sysconf(_SC_GETPW_R_SIZE_MAX))
#else
#define USER_BUF_SIZE 1024
#endif

int main(){
   char buf[USER_BUF_SIZE];
   struct passwd pwd;
   struct passwd* pwdbuf;
   uid_t uid = 99999999; /* bogus */

   if(getpwuid_r(uid, &pwd, buf, sizeof(buf), &pwdbuf) != 0){
      printf("Error: %s\n", strerror(errno));
      return -1;
   }

   printf("Name is: %s\n",pwdbuf->pw_name);
   return 0;
}