将其写入文件后如何读回8字节数字？

Question

I am able to write an 8 byte representation of a number to a file. However when I go to read it back, I do not get the number I am expecting. In my code below, I am attempting to write and read back the number 5000 to testfile.txt .

#include <stdio.h>

int main()
{
    // Open file
    FILE *fp;
    if ((fp = fopen("testfile.txt","w+")) == NULL) 
    {
        // Handle error
    }

    // Write 8 byte number to file
    long long n = 5000;
    fwrite(&n, 8, 1, fp);

    // Seek to EOF and check that the file is 8 bytes
    fseek(fp, 0, SEEK_END);
    long locend = ftell(fp);
    printf("Endbyte: %ld\n",locend);

    // Seek back to start of file and print out location
    fseek(fp, -8, SEEK_END);
    long loc = ftell(fp);
    printf("Location: %ld\n",loc);

    // Read and print out number
    long long *out;
    fread(out, 8, 1, fp);
    long long num = (long long) out;
    printf("Number: %lld\n", num); 

    /* Cleanup */
    close(fp); 
    return(0);
}

Doing a hexdump of testfile.txt gives me the following:

00000000  88 13 00 00 00 00 00 00                   |........|                 
00000008

The binary representation of the hex values of 13 and 88 make 5000 , which confirms that it is being written correctly (I believe).

Unfortunately the output of my program does not agree:

Endbyte: 8                                                                    
Location: 0                                                             
Number: 140734934060848

As you can see, the number read back does not match the number written. I am assuming it is a problem with the way I am reading it back.

Answer 1

I'm surprised that's even running without crashing! fread is essentially the exact same thing as fwrite , just in the other direction. It expects a pointer to a block of memory, but you're passing it an uninitialized pointer.

long long *out; //This is a pointer that is pointing to an undefined area of memory.
fread(out, 8, 1, fp); //fread is now writing the number to that undefined area of memory

What you want to do is create a plain old long long and pass a reference to it just like you did with fwrite .

long long out; //This is a location in memory that will hold the value
fread(&out, 8, 1, fp); //fread is now writing the number to the area of memory defined by the 'out' variable

Answer 2

out是一个指针，需要先取消引用才能将其分配给num。

Answer 3

out is a pointer so it must point to the valid address before you can assign it a value and for get it's value you must use & not casting.
and this a correct code for this :

long long num;
fread(&num, 8, 1, fp);
printf("Number: %lld\n", num);

and one more thing, please correct your close function as follow

fclose(fp);

please note that close using file descriptor and fclose using FILE *