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man pages section 2: System Calls

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Updated: Wednesday, July 27, 2022

issetugid (2)


issetugid - determine if current executable is running setuid or setgid


#include <unistd.h>

int issetugid(void);


The issetugid() function enables library functions (in libtermlib, libc, or other libraries) to guarantee safe behavior when used in setuid or setgid programs or programs that run with more privileges after a successful exec(2). Some library functions might be passed insufficient information and not know whether the current program was started setuid or setgid because a higher level calling code might have made changes to the uid, euid, gid, or egid. These low-level library functions are therefore unable to determine if they are being run with elevated or normal privileges.

The issetugid() function should be used to determine if a path name returned from a getenv(3C) call can be used safely to open the specified file. It is often not safe to open such a file because the status of the effective uid is not known.

The result of a call to issetugid() is unaffected by calls to setuid(), setgid(), or other such calls. In case of a call to fork(2), the child process inherits the same status.

The status of issetugid() is affected only by execve() (see exec(2)). If a child process executes a new executable file, a new issetugid() status will be 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, or if the permitted set before the call to the exec function is not a superset of the inheritable set at that time, issetugid () returns 1 in the new process.

Return Values

The issetugid() function returns 1 if the process was made setuid or setgid as the result of the last or a previous call to execve(). Otherwise it returns 0.


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


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

exec(2), fork(2), setuid(2), getenv(3C), attributes(7), privileges(7)