c - 将不同类型的指针分配给现有内存

Question

我尝试编写一个与类型无关的mem_take。
它将需要预先分配的内存并将其 block 分配给多个指针。这些指针可以指向不同的类型:浮点型、 double 型等。

unsigned int mem_take( void** mem_input, void** mem_pos, const int size_bytes, const int alignment )
{
    // get address
    unsigned int addr = (unsigned int)*mem_pos;

    // align
    unsigned int adjustment_bytes = 0;
    unsigned int misalignment = addr % alignment;
    if(misalignment != 0)
    {
        adjustment_bytes = alignment - misalignment;
        addr += adjustment_bytes;
    }

    // take aligned address
    *mem_input = (void*)addr;

    // move current position to next free location
    addr += size_bytes;
    *mem_pos = (void*)addr;

    // return bytes taken
    return (size_bytes + adjustment_bytes);
}

示例:

main()
{
    char mem[SOME_SIZE];
    void* mem_pos = mem;

    float*  f;
    double* d;
    int bytes_taken_f = mem_take((void**)&f , &mem_pos, 2 * sizeof(float) , 8); // 2 floats
    int bytes_taken_d = mem_take((void**)&d , &mem_pos, 3 * sizeof(double), 8); // 3 doubles
    // etc.

    // now free to use the memory via arrays
    f[0] = 1.0f;
    f[1] = 2.0f;

    d[0] = 1.0f;
    d[1] = 2.0f;
    d[2] = 3.0f;
}

这样做的原因是平台 - DSP 处理器，其内存非常有限。 (说来话长。)

这个解决方案有效吗？ mem_take 写得正确吗？

附:请注意，实际用例是定点 DSP 处理器，因此上面的示例是“简化”的，类型不会是 float、double，而是特定于处理器的类型。

Answer 1

正在做:

 void f(void **f);
 float a;
 f((void**)&a);

不是一个好主意，因为void**不可移植。您需要获取 void* 变量的地址:

 void f(void **f);
 float a;
 void *tmp = &a;
 f(&tmp);

，但你可以像 malloc 一样返回 void*!
这就像编写简单的 malloc 函数，在自定义堆栈上分配内存。

#include <stdio.h>
#include <stdint.h>
#include <assert.h>

void *mem_take(void **mem, size_t *memsize, size_t nmemb, size_t size, size_t alignment, size_t *bytes_to_take)
{
    assert(mem != NULL);
    assert(nmemb != 0); // we can't "free"
    assert(size != 0);
    assert(alignment != 0);

    // get array size
    const size_t s = nmemb * size;
    if (nmemb != 0 && s / nmemb != size) {
        // overflow!
        return NULL;
    }

    // align
    const size_t rest = (size_t)((uintptr_t)*mem % alignment);
    const size_t alignmentadd = rest == 0 ? 0 : alignment - rest;
    // fprintf(stderr, "alignment %d %d %d %d\n", alignmentadd, ((uintptr_t)*mem % alignment), *memsize, s);

    // inform of bytes we need to take
    if (bytes_to_take != NULL) {
        *bytes_to_take = alignmentadd + s;
    }

    // check free memory
    if (*memsize < alignmentadd + s) {
        //fprintf(stderr, "ENOMEM %d %d %d \n", *memsize, alignmentadd, s);
        // ENOMEM!
        return NULL;
    }

    // update state, get s + alignmentadd bytes, return ret
    char *memc = *mem;
    memc += alignmentadd;
    void * const ret = memc;
    memc += s;
    *mem = memc;
    *memsize -= alignmentadd + s;

    return ret;
}

int main()
{
    #define SOME_SIZE  (sizeof(float) * 2 + 3 * sizeof(double) + 8)
    char mem[SOME_SIZE];
    void *mem_pos = mem;
    size_t memsize = sizeof(mem);

    float  *f = mem_take(&mem_pos, &memsize, 2, sizeof(float), 8, NULL); // 2 floats
    assert(f != NULL);
    double *d = mem_take(&mem_pos, &memsize, 3, sizeof(double), 8, NULL); // 3 doubles
    assert(d != NULL);
    // etc.

    // now free to use the memory via arrays
    f[0] = 1.0f;
    f[1] = 2.0f;

    d[0] = 1.0f;
    d[1] = 2.0f;
    d[2] = 3.0f;

    printf("%f %f %lf %lf %lf\n", f[0], f[1], d[0], d[1], d[2]);
}