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.Dd February 21, 2005
.Dt TLS 2
.Os
.Sh NAME
.Nm set_tls_area ,
.Nm get_tls_area
.Nd kernel TLS (thread local storage) support
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/tls.h
.Ft int
.Fn set_tls_area "int which" "struct tls_info *info" "size_t infosize"
.Ft int
.Fn get_tls_area "int which" "struct tls_info *info" "size_t infosize"
.Sh DESCRIPTION
The
.Fn set_tls_area
system call creates an entry for the TLS facility
.Fa which
representing thread local storage as specified by the
.Fa info
structure.
A descriptor representing the facility is returned, or -1 if an error occurred.
The facility may be cleared by specifying a NULL pointer and an infosize of 0.
The
.Fn get_tls_area
system call retrieves the requested TLS facility.
A descriptor representing the facility is returned, or -1 if an error occurred.
If you simply want the
descriptor you may specify a NULL pointer and an infosize of 0.
.Pp
The returned descriptor and the TLS mechanism is machine-dependent.
On IA32
three global segment descriptors are supported  (0, 1, and 2) and the %gs
load value is returned.
.Pp
The
.Fa tls_info
structure passed to
.Fn set_tls_area
should first be zerod (to remain compatible with future extensions)
and then initialized.
.Bd -literal
struct tls_info {
        void	*base;		/* base address of TLS area */
        int	size;		/* size of TLS area in bytes */
};
.Ed
.Pp
The actual implementation of the area is machine-dependent.
If the kernel
is unable to accommodate the supplied size it may create a larger area.
If the kernel is unable to accommodate the supplied base address an error
will be returned.
.Sh RETURN VALUES
A return value of 0 is returned on success, -1 on error.
.Sh EXAMPLES
.Bd -literal -compact

/*
 * Pseudo example showing how the TLS system calls work on IA32.
 */
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/tls.h>

int X;

static int getdata(int offset);

int
main(int ac, char **av)
{
    int i;
    int gs;
    struct tls_info info;

    info.base = &X;
    info.size = sizeof(X);
    if ((gs = set_tls_area(0, &info, sizeof(info))) < 0) {
        perror("setarea");
        exit(1);
    }
    printf("gs = %04x\en", gs);
    __asm __volatile("mov %0,%%gs" : : "g" (gs) );

    if (get_tls_area(0, &info, sizeof(info)) < 0) {
        perror("getarea");
        exit(1);
    }
    printf("%p/%d\en", info.base, info.size);

    X = 1;
    printf("should be 1: %d\en", getdata(0));
    X = 2;
    printf("should be 2: %d\en", getdata(0));

    printf("this should fault:\en");
    fflush(stdout);
    getdata(4);

    return(0);
}

static int
getdata(int offset)
{
    int rv;
    __asm __volatile("movl %%gs:(%0),%%eax; movl %%eax,%1" : "+r" (offset) : "m"
 (rv) : "ax");
    return (rv);
}

.Ed
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er ERANGE
The specified facility index,
.Fa which ,
is not supported.
.It Bq Er EINVAL
An invalid parameter has been specified.
.It Bq Er ENOENT
(get_tls_area) The specified facility has not been initialized with
.Fn sys_set_tls_area .
.El
.Sh SEE ALSO
.Xr umtx 2
.Sh HISTORY
The
.Fn set_tls_area ,
and
.Fn get_tls_area
function calls first appeared in
.Dx 1.1 .