Correct Usage Of memset()

[Frederick2]

The memset function declared in memory.h is declared this way...

void* memset(void* dest, int c, size_t count);

and the MSDN example of its use I have shows this for setting the 1st four bytes of a buffer to a '*'...

memset(buffer, '*', 4);

The 2nd parameter confuses me a bit because its typed as an int which is a 32 bit quantity in 32 bit Windows, yet the example above shows a literal char being used. I frequently use this function and that always confuses me. I frequently use it to zero out a buffer, and I do this...

memset(szBuffer, '\0', iSomeCount);

Is that correct?

What about just this...

memset(szBuffer, 0, iSomeCount);

Are these equivalent? Is one form preferable to the other? Is one or the other incorrect?

[mike_2000_17]

They are both correct.

Both just make sure that you use the sizeof() function to determine the size of you buffer, like:
char buffer[1024];
size_t bufferSize = 1024*sizeof(char); //NOT just 1024.

From www.cplusplus.com:

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

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

[Frederick2]

Thanks Mike. Just wanted to make sure. Can't be too careful, it seems!

[Ancient Dragon]

>>The 2nd parameter confuses me a bit because its typed as an int which is a 32 bit quantity in 32 bit Windows, yet the example above shows a literal char being used

character is just a one-byte signed integer. The compiler will promote it to an integer of the required size.

In c and c++ languages there really is no such thing as a character -- they are all represented as integers. If you look at any standard ascii chart you will see what the decimal/hex value is for all 255 possible characters in the English alphabet. So, the letter 'A' for example has a decimal value of 65. You can even do mathimatical operations of characters, such as int n = 'C' - 'A' + 'B' / 'E' >>size_t bufferSize = 1024*sizeof(char); //NOT just 1024.

sizeof(char) is guaraneed to be 1, so that statement is like saying 1024*1, which makes little sense.

[mike_2000_17]

>> sizeof(char) is guaraneed to be 1, so that statement is like saying 1024*1, which makes little sense.

It may be so for char but never assume a type "in general" has a particular size in bytes, most types do not have a cross-all-platforms guarantee of that. (program on micro-controllers for a while and you will start to put sizeof() everywhere as an instinctive reflex.. you know sometimes, in some obscure platforms or hardware, there are weird things, like char of size 7 bits and such)

[arkoenig]

If you're serious about writing C++ programs, rather than writing C programs and using a C++ compiler to compile them, std::fill is a better choice than memset because it's type-safe and works even for class types.

[Frederick2]

Here's a twist then I thought of after my original post. Lately I've been switching a lot between ansi and unicode, and going through replacing wchar_t type stuff with the t macros in tchar.h. When I come to lines such as this...

memset(pBuffer, '\0', iSomeCount);

I was tempted to do this...

memset(pBuffer, _T('\0'), iSomeCount);

...simply because I was going through a lot of code changing "some text" type stuff to _T("some text"). But using AncientDragon's line of reasoning I'm assumming a double NULL, i.e., "00" would be interpreted as just one numeric zero. Right?