英文:
Why did my array length suddenly change with no apparent reason?
问题
printf("m=%d，M=%d,float_size=%d\n", m, M, floatsize);
a = (float*)malloc(floatsize * m * M);
f = (float*)malloc(floatsize * M);
printf("bytes of a is %d, bytes of f is %d\n", sizeof(a), sizeof(f));
The result is:
m=1，M=3,float_size=4
bytes of a is 8, bytes of f is 8
As screenshot shown below:
Shouldn't it be 3*4=12 instead of 8? How come?
英文:
printf("m=%d，M=%d,float_size=%d\n", m, M, floatsize);
a = (float*)malloc(floatsize * m * M);
f = (float*)malloc(floatsize * M);
printf("bytes of a is %d, bytes of f is %d\n", sizeof(a), sizeof(f));
The result is:
m=1，M=3,float_size=4
bytes of a is 8, bytes of f is 8
As screenshot shown below:
Shouldn't it be 3*4=12 instead of 8? How come?
答案1
得分: 1
a和f不是数组，它们是指向通过malloc(3)分配的内存的指针。
对指针使用sizeof返回的是指针的大小，在您的情况下似乎是8字节，而不是指向的数据的大小。
另外：对malloc(3)系列函数的返回进行强制转换是多余的，只会使代码变得混乱。这些函数返回一个通用的void *，会被隐式转换为任何其他指针类型。（在C语言没有void *之前，需要进行强制转换，因为这些函数返回的是char *）
英文:
a and f are not arrays, they're pointers that are pointing to the memory allocated through malloc(3).
sizeof on a pointer returns the size of the pointer — which in your case seems to be 8 bytes — not the pointed-to data.
Aside: Casting the return of malloc(3) family is redundant and only serves to clutter one's code. These functions return a generic void * that is implicitly converted to any other pointer type. (The cast was required when C didn't have a void *, and these functions returned a char *)
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