su command example for learning setuid & setgid in c

Makefile

CC = cc
SRC = $(wildcard *.c)
OBJ = $(SRC:.c=.o)
TARGET = su

all: $(TARGET)

$(TARGET): $(OBJ)
	$(CC) $(OBJ) -o $(TARGET)

%.o: %.c
	$(CC) -c $< -o $@

install: $(TARGET)
	chown root:root $(TARGET)
	chmod 4755 $(TARGET)

clean:
	rm -rf $(TARGET) $(OBJ)

su.c

#include <unistd.h>
#include <pwd.h>

int main(int argc, char const *argv[])
{
    struct passwd *pw = getpwnam("root");

    setgid(0);
    setuid(0);
    execlp(pw->pw_shell, pw->pw_shell, NULL);

    return 0;
}