I modify the test program, the line used "open" to : int f=open("./bb",O_WRONLY | O_CREAT,0666); Then tested it on AIX5300-05-05,It works ok,the bug exists now. open("./bb",O_WRONLY | O_CREAT,0666); -bash-3.00$ id uid=202(cloud) gid=1(staff) -bash-3.00$ oslevel -s 5300-05-05 -bash-3.00$ cat tt.c #include<fcntl.h> int main() { int f=open("./bb",O_WRONLY | O_CREAT,0666); printf("euid=%i\n",geteuid()); printf("f=%i\n",f); write(2,"hello\n",6); } -bash-3.00$ cat k.c int main() { close(2); //close 2 before call tt execl("./tt","./tt",0); } -bash-3.00$ ls -l k -rwsr-xr-x 1 root staff 58287 Jan 1 09:55 k -bash-3.00$ ls -l tt -rwxrwxrwx 1 cloud staff 59457 Jan 1 10:24 tt -bash-3.00$ ls -l bb bb not found -bash-3.00$ ./k euid=0 f=2 -bash-3.00$ ls -l bb -rw-rw-rw- 1 root staff 6 Jan 1 10:27 bb -bash-3.00$ cat bb hello -bash-3.00$