What is the maximum size of an environment variabl

问题:

Is there a limit to the amount of data that can be stored in an environment variable on Linux, and if so: what is it?

For Windows, I've found following KB article which summarizes to: Windows XP or later: 8191 characters Windows 2000/NT 4.0: 2047 characters

回答1:

I don't think there is a per-environment variable limit on Linux. The total size of all the environment variables put together is limited at execve() time. See "Limits on size of arguments and environment" here for more information.

A process may use setenv() or putenv() to grow the environment beyond the initial space allocated by exec.

Here's a quick and dirty program that creates a 256 MB environment variable.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(void)
{
    size_t size = 1 << 28; /* 256 MB */
    char *var;

    var = malloc(size);
    if (var == NULL) {
        perror("malloc");
        return 1;
    }

    memset(var, 'X', size);
    var[size - 1] = '\0';
    var[0] = 'A';
    var[1] = '=';

    if (putenv(var) != 0) {
        perror("putenv");
        return 1;
    }

/*  Demonstrate E2BIG failure explained by paxdiablo */
    execl("/bin/true", "true", (char *)NULL);
    perror("execl");   


    printf("A=%s\n", getenv("A"));

    return 0;
}

回答2:

Well, it's at least 4M on my box. At that point, I got bored and wandered off. Hopefully the terminal output will be finished before I'm back at work on Monday :-)

export b1=A
export b2=$b1$b1
export b4=$b2$b2
export b8=$b4$b4
export b16=$b8$b8
export b32=$b16$b16
export b64=$b32$b32
export b128=$b64$b64
export b256=$b128$b128
export b512=$b256$b256
export b1k=$b512$b512
export b2k=$b1k$b1k
export b4k=$b2k$b2k
export b8k=$b4k$b4k
export b16k=$b8k$b8k
export b32k=$b16k$b16k
export b64k=$b32k$b32k
export b128k=$b64k$b64k
export b256k=$b128k$b128k
export b512k=$b256k$b256k
export b1m=$b512k$b512k
export b2m=$b1m$b1m
export b4m=$b2m$b2m
echo $b4m
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
:    :    :    :    :    :    :    :    :    :    :    :
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

If you're worried that 4M may not be enough for your environment variable, you may want to rethink how you're doing things.

Perhaps it would be a better idea to put the information into a file and then use an environment variable to reference that file. I've seen cases where, if the variable is of the form @/path/to/any/fspec, it gets the actual information from the file path/to/any/fspec. If it doesn't begin with @, it uses the value of the environment variable itself.

---

Interestingly enough, with all those variables set, every single command starts complaining that the argument list is too long so, even though it lets you set them, it may not be able to start programs after you've done it (since it has to pass the environment to those programs).

回答3:

I did a quick test on my Linux box with the following snippet:

a="1"
while true
do
    a=$a$a
    echo "$(date) $(numfmt --to=iec-i --suffix=B --padding=7 ${#a})" 
done

On my box (Gentoo 3.17.8-gentoo-r1) this results in (last lines of output):

Wed Jan  3 12:16:10 CET 2018   16MiB
Wed Jan  3 12:16:11 CET 2018   32MiB
Wed Jan  3 12:16:12 CET 2018   64MiB
Wed Jan  3 12:16:15 CET 2018  128MiB
Wed Jan  3 12:16:21 CET 2018  256MiB
Wed Jan  3 12:16:33 CET 2018  512MiB
xrealloc: cannot allocate 18446744071562068096 bytes

So: the limit is quite high!

回答4:

The command line (with all argument) plus the environment variable should be less then 128k.

回答5:

I used this very quick and dirty php code (below), modifying it for different values, and found that it works for variable lengths up to 128k. After that, for whatever reason, it doesn't work; no exception is raised, no error is reported, but the value does not show up in the subshell.

Maybe this is a php-specific limit? Maybe there are php.ini settings that might affect it? Or maybe there's a limit on the size of vars that a subshell will inherit? Maybe there are relevant kernel or shell config settings..

Anyway, by default, in CentOS, the limit for setting a var in the environment via putenv in php seems to be about 128k.

<?php

  $s = 'abcdefghijklmnop';
  $s2 = "";
  for ($i = 0; $i < 8100; $i++) $s2 .= $s;
  $result = putenv('FOO='.$s2);
  print shell_exec('echo \'FOO: \'${FOO}');
  print "length of s2: ".strlen($s2)."\n";
  print "result = $result\n";
?>

Version info -

[root@localhost scratch]# php --version
PHP 5.2.6 (cli) (built: Dec  2 2008 16:32:08) 
<..snip..>

[root@localhost scratch]# uname -a
Linux localhost.localdomain 2.6.18-128.2.1.el5 #1 SMP Tue Jul 14 06:36:37 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux

[root@localhost scratch]# cat /etc/redhat-release 
CentOS release 5.3 (Final)

回答6:

Don't know exactly but a quick experiment shows that no error occurs e.g. with 64kB of value:

% perl -e 'print "#include <stdlib.h>\nint main() { return setenv(\"FOO\", \"", "x"x65536, "\", 1); }\n";'\
| gcc -x c -o envtest - && ./envtest && echo $?
0