thr_keycreate(3C) Standard C Library Functions thr_keycreate(3C)NAME
thr_keycreate, thr_setspecific, thr_getspecific - thread-specific data
functions
SYNOPSIS
cc -mt [ flag... ] file...[ library... ]
#include <thread.h>
int thr_keycreate(thread_key_t *keyp, void (*destructor)(void *));
int thr_setspecific(thread_key_t key, void *value);
int thr_getspecific(thread_key_t key, void **valuep);
DESCRIPTION
Create Key
In general, thread key creation allocates a key that locates data spe‐
cific to each thread in the process. The key is global to all threads
in the process, which allows each thread to bind a value to the key
once the key has been created. The key independently maintains specific
values for each binding thread. The thr_keycreate() function allocates
a global key namespace, pointed to by keyp, that is visible to all
threads in the process. Each thread is initially bound to a private
element of this key, which allows access to its thread-specific data.
Upon key creation, a new key is assigned the value NULL for all active
threads. Additionally, upon thread creation, all previously created
keys in the new thread are assigned the value NULL.
Optionally, a destructor function destructor can be associated with
each key. Upon thread exit, if a key has a non-null destructor function
and the thread has a non-null value associated with that key, the
destructor function is called with the current associated value. If
more than one destructor exists for a thread when it exits, the order
of destructor calls is unspecified.
An exiting thread runs with all signals blocked. All thread termination
functions, including thread-specific data destructor functions, are
called with all signals blocked.
Set Value
Once a key has been created, each thread can bind a new value to the
key using thr_setspecific(). The values are unique to the binding
thread and are individually maintained. These values continue for the
life of the calling thread.
Proper synchronization of key storage and access must be ensured by
the caller. The value argument to thr_setspecific() is generally a
pointer to a block of dynamically allocated memory reserved by the
calling thread for its own use. See EXAMPLES below.
At thread exit, the destructor function, which is associated at time of
creation, is called and it uses the specific key value as its sole
argument.
Get Value
thr_getspecific() stores the current value bound to key for the calling
thread into the location pointed to by valuep.
RETURN VALUES
If successful, thr_keycreate(), thr_setspecific() and thr_getspecific()
return 0. Otherwise, an error number is returned to indicate the error.
ERRORS
If the following conditions occur, thr_keycreate() returns the corre‐
sponding error number:
EAGAIN The system lacked the necessary resources to create another
thread-specific data key.
ENOMEM Insufficient memory exists to create the key.
If the following conditions occur, thr_keycreate() and thr_setspe‐
cific() return the corresponding error number:
ENOMEM Insufficient memory exists to associate the value with the
key.
The thr_setspecific() function returns the corresponding error number:
EINVAL The key value is invalid.
EXAMPLES
Example 1 Call the thread-specific data from more than one thread with‐
out special initialization.
In this example, the thread-specific data in this function can be
called from more than one thread without special initialization. For
each argument passed to the executable, a thread is created and pri‐
vately bound to the string-value of that argument.
/* cc -mt thisfile.c */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <thread.h>
void *thread_specific_data(void *);
void cleanup(void*);
#define MAX_ARGC 20
thread_t tid[MAX_ARGC];
int num_threads;
int
main(int argc, char *argv[]) {
int i;
num_threads = argc - 1;
for (i = 0; i < num_threads; i++)
thr_create(NULL, 0, thread_specific_data, argv[i+1], 0, &tid[i]);
for (i = 0; i < num_threads; i++)
thr_join(tid[i], NULL, NULL);
return (0);
} /* end main */
void *
thread_specific_data(void *arg) {
static mutex_t keylock; /* static ensures only one copy of keylock */
static thread_key_t key;
static int once_per_keyname = 0;
char *private_data = arg;
void *tsd = NULL;
void *data;
if (!once_per_keyname) {
mutex_lock(&keylock);
if (!once_per_keyname) {
thr_keycreate(&key, cleanup);
once_per_keyname++;
}
mutex_unlock(&keylock);
}
thr_getspecific(key, &tsd);
if (tsd == NULL) {
data = malloc(strlen(private_data) + 1);
strcpy(data, private_data);
thr_setspecific(key, data);
thr_getspecific(key, &tsd);
}
printf("tsd for %d = %s\n", thr_self(), (char *)tsd);
thr_getspecific(key, &tsd);
printf("tsd for %d remains %s\n", thr_self(), (char *)tsd);
return (NULL);
} /* end thread_specific_data */
void
cleanup(void *v) {
/* application-specific clean-up function */
free(v);
}
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Stable │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOthr_exit(3C), attributes(5), standards(5)WARNINGS
The thr_getspecific() and thr_getspecific() functions can be called
either explicitly or implicitly from a thread-specific data destructor
function. Calling thr_setspecific() from a destructor can result in
lost storage or infinite loops.
SunOS 5.10 2 Nov 2007 thr_keycreate(3C)
