fwrite() 不写任何东西

Question

I recently learned how to use fwrite on the Web and tried it on my own, but it does nothing on the file The file is created, but it is nothing in it. I tried in line 9 writing as char buffer[]={'r','e','a','d'}; , char buffer[4]={'r','e','a','d'}; , char* buffer="read" . What am I doing wrong? (I also tried fprintf , but result was same.)

#include <stdio.h>

int main()
{
    FILE* fp = NULL;

    fp = fopen_s(&fp, "C:\\Users\\Public\\Downloads\\test.txt", "w");

    int buffer[] = { 'r','e','a','d' };

    fwrite(buffer, sizeof(buffer), 1, &fp);

    fclose(&fp);

    return 0;
}

Answer 1

You have to pass the file pointer fp , not the pointer to the file pointer &fp to fwrite() and fclose() .

Also assigning the return value of fopen_s() to fp is wrong because it is not a file pointer but a value the represents if it was successful.

Try this:

#include <stdio.h>

int main()
{
    FILE* fp = NULL;

    if (fopen_s(&fp, "C:\\Users\\Public\\Downloads\\test.txt", "w") != 0)
    {
        fputs("file open error\n", stderr);
        return 1;
    }

    int buffer[] = { 'r','e','a','d' };

    fwrite(buffer, sizeof(buffer), 1, fp);

    fclose(fp);

    return 0;
}

Answer 2

The primary problem of seeing output in your files is address in the accepted answer. This will address the content that is output to your file.

"I tried in line 9 writing as char buffer[]={'r','e','a','d'};, char buffer[4]={'r','e','a','d'};, char* buffer="read"

But then in your post you have the following:

int buffer[] = { 'r','e','a','d' };
...
fwrite(buffer, sizeof(buffer), 1, fp); //results in file are: 7200 0000 6500 0000 6100 0000 6400 0000  

Important to note, int buffer[] = { 'r','e','a','d' }; does not produce a C string.

All of the following are legal arrays in C , but only the bottom 2 are treatable as a C string, as it is the only one that has the null terminator.

int buffer[] = { 'r','e','a','d' };//int array, ASCII values will be output.
char buffer[]={'r','e','a','d'};//char array, not C string
char buffer[4]={'r','e','a','d'};//char array, not C string
char buffer[]={'r','e','a','d','\0'};//C string (editable)
char* buffer="read";//C string (not editable)

So, if you just happened to be looking for something like "read" to be written to the file rather than 7200 0000 6500 0000 6100 0000 6400 0000 use the following in your code:

char buffer[]={'r','e','a','d','\0'};//note the null terminator to avoid UB
...
fwrite(buffer, sizeof(buffer), 1, fp);  // results are: "read"