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ISSETUGID(2) System Calls Manual ISSETUGID(2)

issetugid
is current executable running setuid or setgid

#include <unistd.h>
int
issetugid(void);

The issetugid() function returns 1 if the process was made setuid or setgid as the result of the last or other previous execve() system calls. Otherwise it returns 0.
This system call exists so that library routines (inside libtermlib, libc, or other libraries) can guarantee safe behavior when used inside setuid or setgid programs. Some library routines may be passed insufficient information and hence not know whether the current program was started setuid or setgid because higher level calling code may have made changes to the uid, euid, gid, or egid. Hence these low-level library routines are unable to determine if they are being run with elevated or normal privileges.
In particular, it is wise to use this call to determine if a pathname returned from a getenv() call may safely be used to open() the specified file. Quite often this is not wise because the status of the effective uid is not known.
The issetugid() system call's result is unaffected by calls to setuid(), setgid(), or other such calls. In case of a fork(), the child process inherits the same status.
The status of issetugid() is only affected by execve(). If a child process executes a new executable file, a new issetugid status will be determined. This status is based on the existing process's uid, euid, gid, and egid permissions and on the modes of the executable file. If the new executable file modes are setuid or setgid, or if the existing process is executing the new image with uid != euid or gid != egid, the new process will be considered issetugid.

The issetugid() function is always successful, and no return value is reserved to indicate an error.

execve(2), setegid(2), seteuid(2), setgid(2), setuid(2), getenv(3)

The issetugid() function call first appeared in OpenBSD 2.0.
September 10, 2015 OpenBSD-5.9