What is the maximum size of a Linux environment variable value?

environment-variables, linux, shell

Solution

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;
}

Problem

Original source