The memcpy function in the C programming language is defined in the string.h header file. It is used to copy a block of memory from a source address to a destination address. The function signature is:

void *memcpy(void *dest, const void *src, size_t n);

Here, dest is a pointer to the destination array where the content is to be copied, src is a pointer to the source of data to be copied, and n is the number of bytes to be copied. The function returns a pointer to dest.

The memcpy function is typically used to copy the contents of one array to another or to copy a struct or other complex data type. It is a low-level function and does not perform any bounds checking, so care must be taken to ensure that the memory being copied does not overlap or extend beyond the bounds of the memory allocated for dest.

Here is an example usage of memcpy:

#include <stdio.h>
#include <string.h>

int main() {
    char src[] = "Hello, world!";
    char dest[20];

    // Copy the string from src to dest
    memcpy(dest, src, strlen(src) + 1);

    printf("Source string: %s\n", src);
    printf("Destination string: %s\n", dest);

    return 0;
}

In this example, the memcpy function is used to copy the contents of the src array to the dest array. The strlen(src) + 1 argument specifies the number of bytes to be copied, which is the length of the source string plus one byte for the null terminator. The resulting output will be:

Source string: Hello, world!
Destination string: Hello, world!