copy

#include <stdio.h>

#define BUFF_SIZE 4096

int main(int argc, char **argv)
{
    if (argc != 4)
    {
        fprintf(stderr, "usage: %s source_file1 source_file2 destenation_file3\n", argv[0]);
        return 1;
    }

    FILE *fsrc1, *fsrc2, *fdst3;

    if ((fsrc1 = fopen(argv[1], "rb")) == NULL)
    {
        perror("error source_file1");
        return 2;
    }

    if ((fsrc2 = fopen(argv[2], "rb")) == NULL)
    {
        perror("error source_file2");
        fclose(fsrc1);
        return 3;
    }

    if ((fdst3 = fopen(argv[3], "wb")) == NULL)
    {
        fclose(fsrc1);
        fclose(fsrc2);
        perror("error destenation_file3");
        return 4;
    }

    size_t byte_read = 0;
    char buff[BUFF_SIZE];

    while ((byte_read = fread(buff, sizeof(char), BUFF_SIZE, fsrc1)))
        fwrite(buff, sizeof(char), byte_read, fdst3);


    while ((byte_read = fread(buff, sizeof(char), BUFF_SIZE, fsrc2)))
        fwrite(buff, sizeof(char), byte_read, fdst3);

    fclose(fsrc1);
    fclose(fsrc2);
    fclose(fdst3);

    return 0;
}