ENV31-C. Do not rely on an environment pointer following an operation that may invalidate it

Under many hosted environments it is possible to access the environment through a modified form of main:

main(int argc, char **argv, char **envp)

According to C99 [[ISO/IEC 9899-1999:TC2]]:

In a hosted environment, the main function receives a third argument, char *envp[], that points to a null-terminated array of pointers to char, each of which points to a string that provides information about the environment for this execution of the program.

However, using the setenv() or putenv() functions might cause environment memory to be reallocated, leaving envp pointing to the wrong location. To illustrate:

extern char **environ;

int main(int argc, char **argv, char **envp) { 
   printf("environ:  %p\n", environ); 
   printf("envp:     %p\n", envp); 
   putenv("MY_NEW_VAR=new_value", 1);
   printf("--Added MY_NEW_VAR--\n");
   printf("environ:  %p\n", environ); 
   printf("envp:     %p\n", envp); 
}

Yields:

% ./envp-environ
environ: 0xbf8656ec
envp:    0xbf8656ec
--Added MY_NEW_VAR--
environ: 0x804a008
envp:    0xbf8656ec

If you need to directly access or manipulate the environment, it is safer to use environ.

Non-Compliant Coding Example

After a call to setenv(), envp may not have the proper environment values.

int main(int argc, char **argv, char **envp) {
   setenv("MY_NEW_VAR", "new_value", 1);

   if (envp != NULL) {
      for (i = 0; envp[i] != NULL; i++) {
         printf("%s\n", envp[i]); 
      }
   }

   return 0;
}

envp no longer points to the current environment, so MY_NEW_VAR will not be found in envp.

Compliant Solution (POSIX)

Use environ in place of envp.

extern char **environ;

int main(int argc, char **argv) {
   setenv("MY_NEW_VAR", "new_value", 1);

   if (environ != NULL) { 
      for (i = 0; environ[i] != NULL; i++) {
         printf("%s\n", environ[i]); 
      }
   }

   return 0;
}

Note: if you have a great deal of unsafe envp code, you could save time in your remediation by aliasing. Change:

main(int argc, char **argv, char **envp)

To:

extern char **environ;
#define envp environ
 
main(int argc, char **argv)

Risk Assessment

The program would not be using current environment values, causing unexpected results.

References

[[ISO/IEC 9899-1999:TC2]] Section J.5.1, "Environment Arguments"