Why memset() does not work properly when placed inside a loop body?

Yesterday I programmed a small piece code in C++ which contains a loop and an array. In the program I need to reset the array every time the loop starts over. However, if I use

memset(sum,sizeof(sum),0);

Then the array won't be reset to all 0开发者_开发知识库. For this program I used:

for (i=0;i<sizeof(sum)/sizeof(int);i++) sum[i]=0;

instead.

However, I think a loop is not as clear as a function, and it requires an additional variable, not to mention this is a million times uncool than the wizardry of memset() function. Could you guys enlighten me on this?

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You're actually writing the size of sum into the 0 first bytes. You should be doing memset(sum,0,sizeof(sum)) instead.

(In other words, the arguments are target, data and length and you provided data and length in the wrong order.)

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This is C++ so do it the C++ way with fill_n.

std::fill_n(&sum[0], sizeof(sum) / sizeof(sum[0]), 0);

The reason your memcpy didn't work is because, as noted in other answers, you swapped the second and third arguments.

EDIT: fill and fill_n will work on anything that provides or can be treated as an output iterator. For standard containers like vector you can either pre-size the container or use back_inserter while for arrays you can use the form I indicated.

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memset(sum,sizeof(sum),0);

Wrong.

I think you wanted to write:

memset(sum,0, sizeof(sum));

The signature of memset function is this:

void * memset ( void * ptr, int value, size_t num );

And its description is :

Sets the first num bytes of the block of memory pointed by ptr to the specified value (interpreted as an unsigned char).

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I think you have the parameters in the wrong order.

According to cplusplus.com, the size parameter should be last:

void * memset ( void * ptr, int value, size_t num );

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The syntax of function memset is:

void *memset(void *s, int c, size_t n);

The memset() function fills the first n bytes of the memory area pointed to by s with the constant byte c.

So you need:

memset (sum, 0, sizeof(sum));