getenv() fails

Giorgos Keramidas keramida at
Mon Jul 12 23:43:53 PDT 2004

On 2004-07-13 01:33, Miguel Cardenas <mfcardenas at> wrote:
> Hello...
> I have a problem getting the hostname from the HOSTNAME var...
> #include <stdlib.h>
> ...
> char* host = getenv("HOSTNAME");
> returns always NULL... why? if I do 'echo $HOSTNAME' it is visible,
> but inside my C program returns NULL... what is wrong? is it a bug?

No, it's not a bug.  I don't think there's something wrong either.

What you see is most likely a result of the fact that HOSTNAME is not an
'environment variable' of your shell but just a plain shell variable.

The following small program that uses getenv() can be used to test this.
The commands provided by your shell can be used too.  See below:

     1	#include <stdio.h>
     2	#include <stdlib.h>
     4	int
     5	main(void)
     6	{
     7	        char *s;
     9	        s = getenv("HOSTNAME");
    10	        if (s == NULL) {
    11	                fprintf(stderr, "getenv error\n");
    12	                exit(EXIT_FAILURE);
    13	        }
    14	        printf("HOSTNAME=%s\n", s);
    15	        return EXIT_SUCCESS;
    16	}

By running 'env' you can see what variables are exported to the child
processes of your shell.

: keramida at orion:~$ env | grep HOSTNAME
: keramida at orion:~$ echo $HOSTNAME
: orion.daedalusnetworks.priv

Clearly HOSTNAME isn't one of them.  Using the program shown above and
env(1) you can verify this:

: keramida at orion:~$ gcc -O -Wall -o lala lala.c
: keramida at orion:~$ ./lala
: getenv error
: keramida at orion:~$ env HOSTNAME="testhost" ./lala
: HOSTNAME=testhost
: keramida at orion:~$

Obiously, HOSTNAME is set in the parent shell, but not exported to
`lala' when it runs.  Explicitly setting it with env(1) works as
expected though.


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