目录
一、概述
二、newlibc heap
2.1 stm32cubeide .ld heap
2.2 e2studio .ld heap
三、glibc源码
3.1 Ubuntu c heap
四、总结
一、概述
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结论：在嵌入式c语言中，heap通常8字节对齐
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本文主要分析这个问题的分析过程
二、newlibc heap
newlibc:
https://github.com/blueskycoco/newlibc/tree/master/newlib/libc
2.1 stm32cubeide .ld heap
stm32 .ld
/* User_heap_stack section, used to check that there is enough "RAM" Ram type memory left */._user_heap_stack :{. = ALIGN(8);PROVIDE ( end = . );PROVIDE ( _end = . );. = . + _Min_Heap_Size;. = . + _Min_Stack_Size;. = ALIGN(8);} >RAM
2.2 e2studio .ld heap
ra6m3
.heap (NOLOAD):{. = ALIGN(8);__HeapBase = .;/* Place the STD heap here. */KEEP(*(.heap))__HeapLimit = .;} > RAM
/* real_size is the size we actually have to allocate, allowing foroverhead and alignment. */
#define REAL_SIZE(sz) \((sz) < sizeof (struct freelist_entry) - sizeof (size_t) \? sizeof (struct freelist_entry) \: sz + sizeof (size_t) + M_ALIGN(sz, sizeof (size_t)))
三、glibc源码
glibc:
https://github.com/bminor/glibc/tree/master
/* MALLOC_ALIGNMENT is the minimum alignment for malloc'ed chunks. Itmust be a power of two at least 2 * SIZE_SZ, even on machines forwhich smaller alignments would suffice. It may be defined as largerthan this though. Note however that code and data structures areoptimized for the case of 8-byte alignment. */
#define MALLOC_ALIGNMENT (2 * SIZE_SZ < __alignof__ (long double) \? __alignof__ (long double) : 2 * SIZE_SZ)3.1 Ubuntu c heap
Ubuntu：
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h> int main() { // 不同大小的请求，包括不是8的倍数 size_t sizes[] = {15, 16, 23, 24, 31, 32, 64, 127, 128, 255, 256, 1023, 1024}; size_t num_sizes = sizeof(sizes) / sizeof(sizes[0]); for (size_t i = 0; i < num_sizes; i++) { void *ptr = malloc(sizes[i]); // 检查 malloc 是否成功 if (ptr == NULL) { perror("malloc failed"); return EXIT_FAILURE; } // 获取指针的地址 uintptr_t addr = (uintptr_t)ptr; // 检查地址是否是8字节对齐 if (addr % 8 == 0) { printf("Pointer for size %zu is 8-byte aligned: %p\n", sizes[i], ptr); } else { printf("Pointer for size %zu is NOT 8-byte aligned: %p\n", sizes[i], ptr); } // 释放分配的内存 free(ptr); } return EXIT_SUCCESS;
}
jerry@jerry-virtual-machine:~/桌面$ ./check
Pointer for size 15 is 8-byte aligned: 0x5615aab2d2a0
Pointer for size 16 is 8-byte aligned: 0x5615aab2d2a0
Pointer for size 23 is 8-byte aligned: 0x5615aab2d2a0
Pointer for size 24 is 8-byte aligned: 0x5615aab2d2a0
Pointer for size 31 is 8-byte aligned: 0x5615aab2d6d0
Pointer for size 32 is 8-byte aligned: 0x5615aab2d6d0
Pointer for size 64 is 8-byte aligned: 0x5615aab2d700
Pointer for size 127 is 8-byte aligned: 0x5615aab2d750
Pointer for size 128 is 8-byte aligned: 0x5615aab2d750
Pointer for size 255 is 8-byte aligned: 0x5615aab2d7e0
Pointer for size 256 is 8-byte aligned: 0x5615aab2d7e0
Pointer for size 1023 is 8-byte aligned: 0x5615aab2d8f0
Pointer for size 1024 is 8-byte aligned: 0x5615aab2d8f0
jerry@jerry-virtual-machine:~/桌面$ uname -m
x86_64cat /usr/lib/x86_64-linux-gnu/ldscripts/elf32_x86_64.x
四、总结
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不论是在STM32的32位微控制器上还是在64位的Ubuntu系统中，heap 8字节对齐都因其带来的性能优化、硬件兼容性和代码可移植性而成为一种广泛接受的标准。这种做法帮助开发者构建既高效又可靠的应用程序，符合现代计算环境的需求。
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