pthread_getspecific and mutex lock

Question

I want to make thread-local buffer for strerror_r call and write my own thread-safe char * my_strerror(int) that will use thread local buffer and call strerror_r.

While reading example regarding pthread_getspecific() in Advanced Programming in Unix Environment by R.Stevens i feel discrepancy - why mutex is used in example below?

Example from book:

#include <limits.h>
#include <string.h>
#include <pthread.h>
#include <stdlib.h>

static pthread_key_t key;
static pthread_once_t init_done = PTHREAD_ONCE_INIT;
pthread_mutex_t env_mutex = PTHREAD_MUTEX_INITIALIZER;

extern char **environ;

static void
thread_init(void)
{
    pthread_key_create(&key, free);
}

char *
getenv(const char *name)
{
    int     i, len;
    char    *envbuf;

    pthread_once(&init_done, thread_init);
    pthread_mutex_lock(&env_mutex);
    envbuf = (char *)pthread_getspecific(key);
    if (envbuf == NULL) {
        envbuf = malloc(ARG_MAX);
        if (envbuf == NULL) {
            pthread_mutex_unlock(&env_mutex);
            return(NULL);
        }
        pthread_setspecific(key, envbuf);
    }
    len = strlen(name);
    for (i = 0; environ[i] != NULL; i++) {
        if ((strncmp(name, environ[i], len) == 0) &&
          (environ[i][len] == '=')) {
            strcpy(envbuf, &environ[i][len+1]);
            pthread_mutex_unlock(&env_mutex);
            return(envbuf);
        }
    }
    pthread_mutex_unlock(&env_mutex);
    return(NULL);
}

Answer 1

The mutex is needed for the protection of the environ variable, for example, from putenv. The lock call is badly placed, though, it's better to be immediately after the strlen.