source: azure_iot_hub/trunk/musl-1.1.18/src/env/putenv.c@ 388

#include <stdlib.h>
#include <string.h>
#include "libc.h"

char *__strchrnul(const char *, int);

#ifndef __c2__
static void dummy(char *old, char *new) {}
weak_alias(dummy, __env_rm_add);
#else
extern void __env_rm_add(char *old, char *new);
#endif

int __putenv(char *s, size_t l, char *r)
{
        size_t i=0;
        if (__environ) {
                for (char **e = __environ; *e; e++, i++)
                        if (!strncmp(s, *e, l+1)) {
                                char *tmp = *e;
                                *e = s;
                                __env_rm_add(tmp, r);
                                return 0;
                        }
        }
        static char **oldenv;
        char **newenv;
        if (__environ == oldenv) {
                newenv = realloc(oldenv, sizeof *newenv * (i+2));
                if (!newenv) goto oom;
        } else {
                newenv = malloc(sizeof *newenv * (i+2));
                if (!newenv) goto oom;
                if (i) memcpy(newenv, __environ, sizeof *newenv * i);
                free(oldenv);
        }
        newenv[i] = s;
        newenv[i+1] = 0;
        __environ = oldenv = newenv;
        if (r) __env_rm_add(0, r);
        return 0;
oom:
        free(r);
        return -1;
}

int putenv(char *s)
{
        size_t l = __strchrnul(s, '=') - s;
        if (!l || !s[l]) return unsetenv(s);
        return __putenv(s, l, 0);
}