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Thread-Specific Data

Thread-specific data, also known as thread-private data, is a mechanism for storing and finding data associated with a particular thread. The reason we call the data thread-specific, or thread-private, is that we’d like each thread to access its own separate copy of the data, without worrying about synchronizing access with other threads.

First, sometimes we need to maintain data on a per-thread basis.

The second reason for thread-private data is to provide a mechanism for adapting process-based interfaces to a multithreaded environment.

Before allocating thread-specific data, we need to create a key to associate with the data. The key will be used to gain access to the thread-specific data.

#include 
   
    int pthread_key_create(pthread_key_t *keyp, void (*destroy)(void *));
   

We can break the association of a key with the thread-specific data values for all threads by calling pthread_key_delete.

#include 
   
    int pthread_key_delete(pthread_key_t key);
   

#include 
   
    pthread_once_t initflag=PTHREAD_ONCE_INIT;int pthread_once(pthread_once_t *initflag, void (*initfn)(void));
   

The initflag must be a nonlocal variable (i.e., global or static) and initialized to PTHREAD_ONCE_INIT.

Once a key is created, we can associate thread-specific data with the key by calling pthread_setspecific. We can obtain the address of the thread-specific data with pthread_getspecific.

#include 
   
    void *pthread_getspecific(pthread_key_t key);int pthread_setspecific(pthread_key_t key, const void *value);
   

#include 
   
    #include 
    
     #include 
     
      #include 
      
       #define MAXSTRINGSZ 4096static pthread_key_t key;static pthread_once_t init_done=PTHREAD_ONCE_INIT;pthread_mutex_t env_mutex=PTHREAD_MUTEX_INIALIZER;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(MAXSTRINGSZ);        if(envbuf==NULL)        {            pthread_mutex_unlock(&env_mutex);            return NULL;        }        pthread_setpecific(key,envbuf);    }    len=strlen(name);    for(i=0;environ[i]!=NULL;i++)    {        if((strncmp(name, environ[i],len)&&            (envrion[i][len]=='='))        {            strncpy(envbuf, &environ[i][len+1],MAXSTRINGSZ)            pthread_mutex_unlock(&env_mutex);            return envbuf;        }    }    pthread_mutex_unlock(&env_mutex);    return NULL;}
      
     
    
   

A thread-safe, compatible version of getenv

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