RUID & EUID after exec() - linux

Question

With fork() operation child process inherits attribute real and effective user Id's from parent process, how it behaves when exec() is performed ?

Answer 1

Exec does not change any of them. From the linux manual :
The exec() family of functions replaces the current process image with a new process image.
The exec changes the process image (the code and data segment in memory), but it does not change the process descriptor of the new process created with fork. The process descriptor contains the real and effective id, because this is not changed by an exec call, the effective and real id are not changed neither.
I hope that I have been clear explaining the concept.
The real and effective UID and GID of the child process are equal to the real and effective UID and GID of the parent process. Therefore, when the child process calls exec those values are not modified.
In order to prove this I wrote a small application that creates a child process which calls exec. The exec system call runs an application that prints out the value of the GID and UID of the current process. In addition the GID and UID of the parent process are showen as well, so that we can compare them.
main.c
#include <stdio.h>
#include <unistd.h>
void print_info () {
printf(" UID GID \n"
"Real %d Real %d \n"
"Effective %d Effective %d \n",
getuid (), getgid (),
geteuid(), getegid()
);
return;
}
int main () {
pid_t pid;
int status;
pid = fork();
if (!pid) {
puts("Childe process\n");
execv("./uid.out", NULL);
return;
}
wait(status);
printf("Father %d -------------------\n", getpid());
print_info();
puts("--------------------------------");
return 0;
}
uid.c
#include <stdio.h>
#include <unistd.h>
int main () {
printf("CHILD %d -------------------\n", getpid());
printf(" UID GID \n"
"Real %d Real %d \n"
"Effective %d Effective %d \n",
getuid (), getgid (),
geteuid(), getegid()
);
puts("---------------------------------");
return 0;
}
Output :
CHILD 17436 -------------------
UID GID
Real 1000 Real 1000
Effective 1000 Effective 1000
---------------------------------
Father 17435 -------------------
UID GID
Real 1000 Real 1000
Effective 1000 Effective 1000
--------------------------------
Let me know if you need more info.