文件流缓冲区是如何工作的?

我理解 fopen() 打开文件并为该文件的读写操作创建缓冲区。 fopen() 返回该缓冲区的指针。

所以我的问题是,在下面的代码中 _copy 功能体有一个 temp 要在 fread() 和 fwrite() . 为什么我不能直接从一个缓冲区转移到另一个缓冲区?

/* example: copyfile.exe xxxxx.txt   zzzzzz.txt */
#include <stdio.h>
#include <stdlib.h>

#define BUFF 8192
void _copy(FILE *source, FILE *destination);

int main(int argc, char *argv[])
{
    FILE *fp1, *fp2;  // fp1 source file pointer// fp2 copied file pointer 

    if (argc !=3 )           //command line must have 3 arguments
    {
        fprintf(stderr, "Usage: %s (source file) (copy file)\n", argv[0][0]);
        exit(EXIT_FAILURE);
    }

    if ((fp1 = fopen(argv[1], "rb")) == NULL)    //Opening source file
    {
        fprintf(stderr, "Could not open %s\n",argv[1]);
        exit(EXIT_FAILURE);
    }
    if((fp2 = fopen(argv[2], "ab+")) == NULL)  //Opening destination file
    {
        fprintf(stderr, "could not create %s \n",argv[2]);
        exit(EXIT_FAILURE);
    }
    if( setvbuf(fp1,NULL, _IOFBF, BUFF) != 0) //Setting buffer for source file
    {
        fputs("Can't create output buffer\n", stderr);
        exit(EXIT_FAILURE);
    }
    if( setvbuf(fp2,NULL, _IOFBF, BUFF) != 0) //Setting buffer for destination file
    {
        fputs("Can't create input buffer\n", stderr);
        exit(EXIT_FAILURE);
    }


    _copy(fp1, fp2);  
    if (ferror(fp1)!=0)
        fprintf(stderr, "Error reading file %s\n", argv[1]);
    if(ferror(fp2)!=0)
        fprintf(stderr, "Error writing file %s\n",argv[2]);
    printf("Done coping %s (source) to %s (destination) \n",argv[1], argv[2]);
    fclose(fp1);
    fclose(fp2);
    return (0);
}

void _copy(FILE  *source, FILE *destination)
{
    size_t bytes;
    static char temp[BUFF];

    while((bytes = fread(temp,sizeof(char),BUFF,source))>0)
        fwrite(temp,sizeof(char),bytes,destination);
}