Hands-On System Programming with Linux

Code example

We write a simple C program (ch7/query_creds.c); when run, it prints to stdout its process credentials (we show the relevant code):

#define SHOW_CREDS() do {        \  printf("RUID=%d EUID=%d\n"    \         "RGID=%d EGID=%d\n",    \        getuid(), geteuid(),    \        getgid(), getegid());   \} while (0)int main(int argc, char **argv){    SHOW_CREDS();    if (geteuid() == 0) {        printf("%s now effectively running as root! ...\n", argv[0]);        sleep(1);    }    exit (EXIT_SUCCESS);}

Build it and try it out:

$ ./query_credsRUID=1000 EUID=1000RGID=1000 EGID=1000$ sudo ./query_creds[sudo] password for seawolf: xxx RUID=0 EUID=0RGID=0 EGID=0./query_creds now effectively running as root! ...$ 

Note the following:

• On the first run, the four process credential values are the usual ...