what's the point in malloc(0)?

2018-12-31 20:28发布

Just saw this code:

artist = (char *) malloc(0);

and I was wondering why would one do this?

标签: c malloc
16条回答
临风纵饮
2楼-- · 2018-12-31 20:48

malloc(0) will return a valid memory address and whose range will depend on the type of pointer which is being allocated memory. Also you can assign values to the memory area but this should be in range with the type of pointer being used. You can also free the allocated memory. I will explain this with an example:

int *p=NULL;
p=(int *)malloc(0);
free(p);

The above code will work fine in a gcc compiler on Linux machine. If you have a 32 bit compiler then you can provide values in the integer range, i.e. -2147483648 to 2147483647. Same applies for characters also. Please note that if type of pointer declared is changed then range of values will change regardless of malloc typecast, i.e.

unsigned char *p=NULL;
p =(char *)malloc(0);
free(p);

p will take a value from 0 to 255 of char since it is declared an unsigned int.

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荒废的爱情
3楼-- · 2018-12-31 20:50

There are a lot of half true answers around here, so here are the hard facts. The man-page for malloc() says:

If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().

That means, there is absolutely no guarantee that the result of malloc(0) is either unique or not NULL. The only guarantee is provided by the definition of free(), again, here is what the man-page says:

If ptr is NULL, no operation is performed.

So, whatever malloc(0) returns, it can safely be passed to free(). But so can a NULL pointer.

Consequently, writing artist = malloc(0); is in no way better than writing artist = NULL;

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听够珍惜
4楼-- · 2018-12-31 20:51

malloc(0) will return NULL or a valid pointer which can be rightly passed to free. And though it seems like the memory that it points to is useless or it can't be written to or read from, that is not always true. :)

int *i = malloc(0);
*i = 100;
printf("%d", *i);

We expect a segmentation fault here, but surprisingly, this prints 100! It is because malloc actually asks for a huge chunk of memory when we call malloc for the first time. Every call to malloc after that, uses memory from that big chunk. Only after that huge chunk is over, new memory is asked for.

Use of malloc(0): if you are in a situation where you want subsequent malloc calls to be faster, calling malloc(0) should do it for you (except for edge cases).

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素衣白纱
5楼-- · 2018-12-31 20:52

According to the specifications, malloc(0) will return either "a null pointer or a unique pointer that can be successfully passed to free()".

This basically lets you allocate nothing, but still pass the "artist" variable to a call to free() without worry. For practical purposes, it's pretty much the same as doing:

artist = NULL;
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只若初见
6楼-- · 2018-12-31 20:52

There's an answer elsewhere on this page that begins "malloc(0) will return a valid memory address and whose range will depend on the type of pointer which is being allocated memory". This statement is incorrect (I don't have enough reputation to comment on that answer directly, so can't put this comment directly under there).

Doing malloc(0) will not automatically allocate memory of correct size. The malloc function is unaware of what you're casting its result to. The malloc function relies purely on the size number that you give as its argument. You need to do malloc(sizeof(int)) to get enough storage to hold an int, for example, not 0.

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谁念西风独自凉
7楼-- · 2018-12-31 20:52

malloc(0) doesn't make any sense to me, unless the code is relying on behaviour specific to the implementation. If the code is meant to be portable, then it has to account for the fact that a NULL return from malloc(0) isn't a failure. So why not just assign NULL to artist anyway, since that's a valid successful result, and is less code, and won't cause your maintenance programmers to take time figuring it out?

malloc(SOME_CONSTANT_THAT_MIGHT_BE_ZERO) or malloc(some_variable_which_might_be_zero) perhaps could have their uses, although again you have to take extra care not to treat a NULL return as a failure if the value is 0, but a 0 size is supposed to be OK.

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