I386_GET_GSBASE(2) | System Calls Manual (i386) | I386_GET_GSBASE(2)

# NAME

`i386_get_gsbase`, `i386_set_gsbase` — manage i386 per-thread %gs base address

# SYNOPSIS

i386 Architecture Library (libi386, -li386)
`#include <sys/types.h>`
`#include <machine/sysarch.h>`

int
`i386_get_gsbase`(void **base);

int
`i386_set_gsbase`(void *base);

# DESCRIPTION

`i386_get_gsbase`() copies the current base address of the segment that, by default, is referenced by the %gs selector into the memory referenced by base.

`i386_set_gsbase`() sets the base address of the segment that, by default, is referenced by %gs to the address base.

The segment base address is local to each thread. The initial thread of a new process inherits its segment base address from the parent thread. __tfork(3) sets the initial segment base address for threads that it creates.

# RETURN VALUES

Upon successful completion, `i386_get_gsbase`() and `i386_set_gsbase`() return 0. Otherwise, a value of -1 is returned and the global variable errno is set to indicate the error.

# ERRORS

`i386_get_gsbase`() will fail if:

[`EFAULT`]
  base points outside the process's allocated address space.

# SEE ALSO

fork(2), __tfork(3)

Intel, i386 Microprocessor Programmer's Reference Manual.

# WARNING

The ELF Thread-Local Storage ABI reserves %gs for its own use and requires that the dynamic linker and thread library set it to reference data-structures internal to and shared between them. Programs should use the __thread storage class keyword instead of using these calls. To be maximally portable, programs that require per-thread data should use the `pthread_key_create`() interface.

June 5, 2025 | Debian