Question

i need to write a function

void reverse(void *base, int nel, int width) 
{ 
    // ... 
}

where base is the pointer to the start of the array, nel – number of elements in the array, and width is the size of each element in bytes.

For example, how can I swap first two elements of the array?

Answer 1

You can simply use memcpy (since it is a builtin on many compilers), with the help of width value. You also need a temporary variable.

/* C99 (use `malloc` rather than VLAs in C89) */
#include <string.h>

void reverse(void *base, size_t nel, size_t width) 
{
    if (nel >= 2) {
        char *el1 = base;
        char *el2 = (char *)base + width;
        char tmp[width];

        memcpy(tmp, el1, width);
        memcpy(el1, el2, width);
        memcpy(el2, tmp, width);
    }
}