Article: The mem… Functions!

C learners are generally pretty comfortable with the str… series of functions:

1. strlen()
2. strcpy()
3. strcat()
4. strcmp()
5. strlwr()
6. strupr()

However, I generally find a very poor awareness of the mem… series of functions:

1. memcpy()
2. memmove()
3. memset()
4. memchr()

These functions are extremely useful when dealing with data in memory. So first let us differentiate between “strings” and just “data”. A string in C is a sequence of characters terminated by the null character. Any arbitrary data in memory, therefore, cannot be considered to be a string. The strcpy() function, thus, copies a source string to a destination memory location, and while doing so, performs the following:

1. Stops copying the moment it encounters the terminating null character in the source string
2. Ensures that the destination is also null terminated.

The mem… series of functions do not assume that they are dealing with strings and hence do not care about the presence or absence of the null character. A brief explanation of the mem… functions is given below:

1. void* memcpy(void *destination, const void *source, size_t num);
   This function copies num number of bytes from source to destination and returns a pointer to the destination block.
2. void* memmove(void *destination, const void *source, size_t num);
   This function also copies num number of bytes from source to destination and returns a pointer to the destination block, but can handle cases where the source and destination regions overlap. This is a safer form of copying data. Do not go by it’s deceptive name: this function copies data!
3. void* memset(void *ptr, int value, size_t num);
   This function is used to fill a block of memory with a particular value. The block is pointed to by ptr and the size of the block is passed via num. The most practical use of this function perhaps is initializing a block of memory to zeros.
4. void* memchr(const void *ptr, int value, size_t n);
   This function searches for the first occurrence of the given value within the block of memory of size n bytes, starting at the location pointed to by ptr. If the value was found, it returns a pointer to that byte within the memory block, and if the value was not found, it returns NULL.

Given below is a sample implementation of the calloc() function, using the malloc() and memset() functions:

void* calloc(size_t num, size_t size)
{
    void *ptr;

    if ((ptr = malloc(num*size)) memset(ptr,0,num*size);
    return ptr;
}