使用memset初始化三值结构数组

Question

How do I initialize array of structures with values? So, I have structure color with rgb values.

struct color{
    GLfloat r;
    GLfloat g;
    GLfloat b;
}

and trying to initialize it with 1.0f.

color* cArray = (color*) malloc(w*h*sizeof(color));
memset(&cArray, 1.0, sizeof color);

But instead of correct work I get segmentation fault on cArray[0]. What do I miss?

Answer 1

Note: Do not cast void * (result of malloc() ) to other pointers and always check the result of malloc() & friends .

Memset takes an unsigned char (most likely 8 bits) and treats the area passed as an array of char. It has no idea of structs, arrays etc. Just of a 1-dimensional array of char. (as @chqrlie noted, memset() actually takes an int ; this is, however, internally converted to unsigned char ).

The segfault is because you pass the address of cArray which is a pointer to the actual array, not its value which would be the array itself.

For float, memset() most likely only makes sense setting it to all- 0 :

memset(cArray, 0, sizeof(struct color) * w * h);

Note: memset has no idea on data types. It just takes a pointer to a block of memory and a count and stores the value to that area. It is your responsibility all arguments are valid!

Note that writing the memory area with 0 is actually float zero ( 0.0 , actually +0.0 ), as that has all bits cleared. That was a briliant intention by the guys who dsigned the encoding for floats.

As you allocate right before, you can combine this using calloc() . That will allocate and clear the memory area of the array. However, if you intend to set all values explicitly anyway, you better stick with malloc() .

If you want to set other float values, you have to write your own loop. However, you can threat the array as 1-dimensional for that.

for ( size_t i = 0 ; i < w * h ; i++ )
    cArray[i] = (struct color){ .r = 1.0, .g = 1.0, .b = 1.0 };

That uses a compound literal . Alternatively you can also set the fields seperately.

If you are upt for speed, the compount-literal approach might be the fastest. This way ,the compiler might very well load al values into three registers and store them directly into memory using store-multiple. However it might do, I would bet the compiler will recognize that pattern and optimize as hell, as it is commonly used for very large loops.

Answer 2

You can't use memset() to set float values.

The memset() function is used for bytes, so using it for float s is not allowed, you need to explicitly initialize each member

This is memset() 's signature

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

although it expects int ¹ for it's second parameter, the standard says

7.24.6 Miscellaneous functions

7.24.6.1 The memset function

2. The memset function copies the value of c (converted to an unsigned char ) into each of the first n characters of the object pointed to by s .

" converted to an unsigned char " so it sets bytes.

Also take in consideration the following:

1. You don't need to cast the return value of malloc() , and it's better if you don't do it.

2. Always check that malloc() returned non- NULL , specially when allocating a lot of memory.

---

¹ _{As you can see you can't pass float.}

Answer 3

There are multiple problems with your code:

You allocate a matrix w x h of color structures:

color *cArray = (color*) malloc(w * h * sizeof(color));

The cast is not necessary in C, but opinions differ about the recommended alternatives. A safer version would be:

color *cArray = malloc(w * h * sizeof(*cArray));

More importantly, our call to memset is incorrect in multiple ways:

memset(&cArray, 1.0, sizeof color);

1. You pass the address of the pointer instead of the value of the pointer cArray .
2. You only pass the size of a single color structure. This size exceeds the size of the pointer, hence the crash. If you intended to set the whole array you should have store the size into a temporary variable.
3. You pass a double instead of an int as the value to set. You cannot initialize an array of double values with memset : this function initializes a block of memory by setting all bytes to the same value. Passing 0 would initialize double values correctly if your system uses IEEE-754 representation for floating point values, otherwise it might invoke undefined behaviour.

You must initialize the matrix with a loop:

for (size_t i = 0, n = w * h; i < n; i++) {
    cArray[i].r = cArray[i].g = cArray[i].b = 1.0;
}

This loop can be optimized by modern compilers. I suggest you play with this very interesting tool: http://gcc.godbolt.org/# .

You can also keep a copy of a pre-initialized array and use memcpy to initialize the allocated array.

Answer 4

First, colors are (in your case) 3 bytes, not 3 floats.

then this code:

struct color{
    GLfloat r;
    GLfloat g;
    GLfloat b;
}

and trying to initialize it with 1.0f.

color* cArray = (color*) malloc(w*h*sizeof(color));
memset(&cArray, 1.0, sizeof color);

will cause a compiler to raise several warnings:

due to the compiler will not knowing what a 'color' is due to:

syntax errors in the struct definition
and references to the struct are missing the 'struct' modifier.

when calling malloc() (and family of functions) always check (!=NULL) the returned value to assure the operation was successful.

In C, do not cast the returned value from malloc() (and family of functions)

notice that a struct definition ends with a ';'

suggest using:

struct color
{
    char r;
    char g;
    char b;
};

struct color* cArray = NULL;
if( NULL == (cArray =malloc(w*h*sizeof(struct color))))
{ // then malloc failed
    perror( "malloc for struct color array failed" );
    exit( EXIT_FAILURE )
}

// implied else, malloc successful

memset(&cArray, 1, w*h*(sizeof (struct color) );