Cannot use alloc_system allocator for binary

[mssun]

According to the the book, we can override default allocator of a binary from jemelloc to system allocator by:

#![feature(alloc_system)]

extern crate alloc_system;

fn main() {
    let a = Box::new(4); // Allocates from the system allocator.
    println!("{}", a);
}

This feature works in previous nightly (back to Rust 1.9.0 and 1.11.0-nightly a967611 2016-05-30) according to this this blog, but it was broken in recent nightly versions.

I try to compile this code with cargo build --release, and here is the result:

$ ls -alh target/release/hello
-rwxrwxr-x 2 mssun mssun 3.9M Sep 11 22:44 target/release/hello

$ strings target/release/hello | grep je_malloc
je_malloc_mutex_prefork
je_malloc_printf
je_malloc_message malloc_message
je_malloc malloc
je_malloc_mutex_postfork_parent
je_malloc_conf malloc_conf
je_malloc_stats_print malloc_stats_print
je_malloc_mutex_unlock
je_mallocx mallocx
je_malloc_mutex_boot
je_malloc_mutex_postfork_child
je_malloc_mutex_init
je_malloc_mutex_lock
je_malloc_usable_size malloc_usable_size
je_malloc_tsd_boot0
je_malloc_tsd_boot1
je_malloc_strtoumax
je_malloc_write
je_malloc_mutex_assert_owner
je_malloc_mutex_assert_not_owner
je_malloc_snprintf
je_malloc_cprintf
je_malloc_tsd_cleanup_register
je_malloc_tsd_malloc
je_malloc_tsd_dalloc
je_malloc_tsd_no_cleanup
je_malloc_vsnprintf
je_malloc_vcprintf
je_malloc_mutex_init
je_malloc_tsd_boot0
je_malloc_printf
je_malloc_mutex_prefork
je_malloc_mutex_postfork_child
je_malloc_mutex_postfork_parent
je_malloc_mutex_boot
je_malloc_cprintf
je_malloc_tsd_boot1
je_malloc_snprintf
je_malloc_write
je_malloc_strtoumax
je_malloc_vsnprintf
je_malloc_tsd_no_cleanup
je_malloc_vcprintf

The binary should not this large, the strings results indicate that je_malloc is still used as the allocator.

Meta

rustc --version --verbose:
rustc 1.22.0-nightly (ca94c75 2017-09-10)
binary: rustc
commit-hash: ca94c75
commit-date: 2017-09-10
host: x86_64-unknown-linux-gnu
release: 1.22.0-nightly
LLVM version: 4.0

[sfackler]

It looks like the documentation there is unfortunately out of date - this is the way the allocator is selected now: https://github.com/rust-lang/rust/blob/master/src/doc/unstable-book/src/language-features/global-allocator.md.

[mssun]

Thanks, @sfackler. Indeed, the documentation is outdated.

[steveklabnik]

is #![feature(alloc_system)] gone entirely?

[sfackler]

It's still present for the alloc_system library - it's just that the method of selecting the global allocator changed.

https://github.com/rust-lang/rust/blob/master/src/liballoc_system/lib.rs#L13-L16

[lilith]

Can we please add an informative warning for #![feature(alloc_system)] use? This really bit me.

[steveklabnik]

tagging p-high and assigning myself; i think @SimonSapin or someone sent in a PR to address this?

[SimonSapin]

The book’s example became incomplete when #[global_allocator] was introduced. (extern crate became no longer sufficient to select an allocator.) I fixed the example in #45217, I think this can be closed.

The way to select the system allocator is now:

#![feature(alloc_system, global_allocator, allocator_api)]

extern crate alloc_system;

#[global_allocator]
static A: alloc_system::System = alloc_system::System;