Can't pass 2nd environment variable with execve

Question

I would like to display 2 environment variables that are passed as argument to another process using execve() function:

Main.c:

int main(){


    char USERNAME[10];
    strcpy(USERNAME, "USERNAME=");
    for (int i=1;i<10;i++){
        strcpy(USERNAME+i, "1");
    }

    char PATH[169];
    strcpy(PATH, "PATH=");
    for (int i=5;i<169;i++){
        strcpy(PATH+i, "A");
    }


    char * newargv[] = {"./get","", (char*)0};
    char * newenviron[] = {PATH,USERNAME};
    execve("./get", newargv, newenviron);
    return 0;
}

get.c:

int main()
{
    const char* s = getenv("PATH");
    printf("PATH :%s\n",s);
    const char* s2 = getenv("USERNAME");
    printf("USERNAME :%s\n",s2);
}

So I compile Main.c to Main and get.c to get, and execute Main, I get this output:

PATH :AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA USERNAME :(null)

I don't understand why USERNAME is NULL here.

You could try doing `printf("<%s>\n", USERNAME);` inside the loop of your first program. You will see that it does not contain what you think. Or even better, learn how to use a debugger. — rodrigo, Mar 05 '17 at 23:03
@JosephSible mybad, but still, the last `strcpy` in the loop will overrun the array, because of the terminating `'\0'`. — Weather Vane, Mar 05 '17 at 23:10
Closely related to [Can't pass properly environment with `execve()` function](http://stackoverflow.com/questions/42635818/cant-pass-properly-environment-with-execve-function) by user [juRioqs75](http://stackoverflow.com/users/7457216/jurioqs75). — Jonathan Leffler, Mar 06 '17 at 22:45

score 2 · Accepted Answer · answered Mar 05 '17 at 23:09

2

You're missing a null terminator on the end of your newenviron array. Also, you're writing one more byte to each string than you've allocated space for (the ending \0 counts as a character).

answered Mar 05 '17 at 23:09

Joseph Sible-Reinstate Monica

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You mean like `char * newenviron[] = {PATH,USERNAME,NULL};` ? It does not work... – SdFRO0QS78 Mar 05 '17 at 23:13
Yes, you need to do that. What does "does not work" mean? What exactly happened when you did it? And did you fix your string sizes? – Joseph Sible-Reinstate Monica Mar 05 '17 at 23:14

score 0 · Answer 2 · answered Mar 05 '17 at 23:19

0

Oups my bad, I erased the "USERNAME=" part of the USERNAME tab, because loop started from 1 instead of 9...

answered Mar 05 '17 at 23:19

SdFRO0QS78

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arf ! the second printf display USERNAME but **also** PATH, how to prevent this ? – SdFRO0QS78 Mar 05 '17 at 23:30
As already been told to you, you are missing to tailing zero `\0` – t.elazari Mar 05 '17 at 23:46

Can't pass 2nd environment variable with execve

2 Answers2