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Subject[RFC PATCH v1 0/6] Add dynamic memory allocator support for nolibc

This is the v1 of RFC to add dynamic memory allocator support for

## Background

The need to allocate memory dynamically has become a requirement for
the C programming language. Mainly it happens when the allocation size
is determined at runtime. Many other use cases also do it when the
object's lifetime is long-lived and needs to be recycled at runtime.

Currently, the nolibc header doesn't support such a type of allocation.
This series adds it.

## Implementation

Add basic functions to manage dynamic memory allocation:
- malloc()
- calloc()
- realloc()
- free()

The allocator uses mmap() syscall to allocate the memory and uses
munmap() syscall to free the allocated memory.

The metadata to keep track the length for munmap-ing is simply
defined as a struct below:
struct nolibc_heap {
size_t len;
char user_p[] __attribute__((__aligned__));
malloc(), realloc() and calloc() return a pointer to `user_p`.

## Add my_syscall6() support for x86 32-bit.

mmap() needs 6 arguments to work with. Not all architectures that
nolibc supports have the my_syscall6() wrapper. This series also
adds my_syscall6() wrapper support for i386.


Both Clang and GCC cannot use %ebp in the clobber list and in the "r"
constraint without using -fomit-frame-pointer. To make it always
available for any kind of compilation, the below workaround is

For clang (the Assembly statement can't clobber %ebp):
1) Save the %ebp value to the redzone area -4(%esp).
2) Load the 6-th argument from memory to %ebp.
3) Subtract the %esp by 4.
4) Do the syscall (int $0x80).
5) Pop %ebp.

For GCC, fortunately it has a #pragma that can force a specific function
to be compiled with -fomit-frame-pointer, so it can use "r"(var) where
var is a variable bound to %ebp.

## Limitation

Currently, for mips and arm arch cannot use these dynamic memory allocator
functions because they're missing the my_syscall6() macro.

[ammarfaizi2: I would love to add the support for them too, but I don't
have the hardware to play with MIPS and ARM. ]

## Test

The following simple program can be used to test this series:

## Patchset Summary

1) Patch 1 is a fix for the System V ABI document link.

2) Patch 2 is a fix to support compile with clang.

3) Patch 3 adds my_syscall6() implementation for i386.

4) Patch 4 adds mmap() and munmap() functions.

5) Patch 5 adds malloc(), calloc(), realloc() and free().

6) Patch 6 adds strdup() and strndup().

Signed-off-by: Ammar Faizi <>
Ammar Faizi (6):
tools/nolibc: x86-64: Update System V ABI document link
tools/nolibc: Make the entry point not weak for clang
tools/nolibc: i386: Implement syscall with 6 arguments
tools/nolibc/sys: Implement `mmap()` and `munmap()`
tools/nolibc/stdlib: Implement `malloc()`, `calloc()`, `realloc()` and `free()`
tools/include/string: Implement `strdup()` and `strndup()`

tools/include/nolibc/arch-aarch64.h | 2 +
tools/include/nolibc/arch-arm.h | 2 +
tools/include/nolibc/arch-i386.h | 66 ++++++++++++++++++++++++
tools/include/nolibc/arch-mips.h | 2 +
tools/include/nolibc/arch-riscv.h | 2 +
tools/include/nolibc/arch-x86_64.h | 4 +-
tools/include/nolibc/stdlib.h | 79 +++++++++++++++++++++++++++++
tools/include/nolibc/string.h | 68 +++++++++++++++++++++++++
tools/include/nolibc/sys.h | 62 ++++++++++++++++++++++
9 files changed, 286 insertions(+), 1 deletion(-)

base-commit: fda0d5d1b79d8b7032be3d7720a481a9fde91baf
Ammar Faizi

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