C中的fwrite（）函数未编写预期的答案

Question

Hello, everyone!

I found a programming exercise into an old book:

"The content of the a.txt file is: 11 2 13 4 15 6 17 8 19 .We have the following program in C that writes something in the b.txt file:

#include <stdio.h>

int main()
{
    FILE *f, *g;
    short int v[10];

    f = fopen("a.txt", "r");
    g = fopen("b.txt", "w");

    fread(v, 8, 1, f);
    fwrite(v, 6, 1, g);

    fclose(f); fclose(g);

    return 0;
}

I expected my program to write these numbers: 11 2 13 .But instead, it wrote: 11 2 1

I tried to see what happens with the v array with the Debugger, but the values showed there does not correspond to my calculations.

Before the fread() is called, v is uninitialized like this: 6448 64 -108 96 6542 64 6448 64 68 0

After the fread() function completes, the v in Debug mode is: 12593 12832 12576 8243 6542 64 6448 64 68 0

I don't study C anymore, but I'm curious how fread() reads the numbers from a.txt and how fwrite() writes them in b.txt. I know the functions prototipes and how they generally work, because I wrote some little programs using them:

int fread(void *p, int dim, int n, FILE *f);
//reads n elements from f, each one having dim bytes and stores them in p

int fwrite(void *p, int dim, int n, FILE *f);
//writes n elements in f from p, each one having dim bytes

But in this particular case, I'm confused. I even tried to change the type of my array from short int to int , but the result is the same. I checked the book and the int type has 4 bytes and "short int" has 2 bytes if that helps.

Answer 1

Well fwrite works as follow (from tutorialspoint ) :

size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream)

• ptr − This is the pointer to the array of elements to be written.
• size − This is the size in bytes of each element to be written.
• nmemb − This is the number of elements, each one with a size of size bytes.
• stream − This is the pointer to a FILE object that specifies an output stream.

So this line

fwrite(v, 6, 1, g);

Writes 6 bytes from "11 2 13 4 15 6 17 8 19". Do not forget, there are spaces in this line. So the function writes the very 6 first char (since the size, in almost every cases, of a char is 1) wich are : '1', '1', ' ', '2', ' ', '1'.