从二进制文件读取到int

Question

i'm a little puzzled about binary files and how to read from them, so if someone could help that would be great. i have a file that contains the following bits:

00000000000000000000000000000111 

(32 bits. i counted)

now i have written this code:

int main()
{
    FILE * f1;
    f1 = fopen("file2", "rb");
    int i = 0;
    fread(&i, sizeof(int), 1, f1);
    printf("%d", i);
    fclose(f1);
    return 0;

}

that prints me 808464432. why? shouldnt it print 7? thank you for reading.

Answer 1

That is not a binary file, it's a text file, try this instead

#include <stdio.h>
#include <stdlib.h>

int main()
{
    FILE *file;
    int   value;
    char  text[33];
    char *endptr;

    file = fopen("file2", "r");
    if (file == NULL) /* check that the file was actually opened */
        return -1;
    if (fread(text, 1, sizeof(text) - 1, f1) != sizeof(text) - 1)
    {
        fclose(file);
        return -1;
    }
    text[32] = '\0';
    value    = strtol(text, &endptr, 2);
    if (*endptr == '\0')
        printf("%d", value);
    fclose(file);
    return 0;
}

To write a binary file you need this

#include <stdio.h>

void hexdump(const char *const filename)
{
    FILE         *file;
    unsigned char byte;


    file = fopen(filename, "rb");
    if (file == NULL)
        return;
    fprintf(stdout, "hexdump of `%s': ", filename);
    while (fread(&byte, 1, 1, file) == 1)
        fprintf(stdout, "0x%02x ", byte);
    fprintf(stdout, "\n");
}

int main()
{
    const char *filename;
    FILE       *file;
    int         value;

    filename = "file.bin";
    file     = fopen(filename, "wb");
    if (file == NULL)
        return -1;
    value = 7;
    if (fwrite(&value, sizeof(value), 1, file) != 1)
        fprintf(stderr, "error writing to binary file\n");
    fclose(file);

    /* check that the content of the file is not printable, i.e. not text */
    hexdump(filename);

    file = fopen(filename, "rb");
    if (file == NULL)
        return -1;
    value = 0;
    if (fread(&value, sizeof(value), 1, file) != 1)
        fprintf(stderr, "error writing to binary file\n");
    else
        fprintf(stdout, "The value read was %d\n", value);
    fclose(file);

    return 0;

}

as you will see from the example above, the stored data in file.bin is not in text format, you cannot inspect it with a text editor because the bytes 0x00 and 0x07 are not printable, in fact 0x00 is the nul byte which is used in c to mark the end of a string.

Answer 2

This is one way you can write and read to a binary file. you should know the difference between a binary and ASCII file, before reading from them. Read this once https://stackoverflow.com/a/28301127/3095460 and understand what is type of file you are reading using your code.

if you open a file in an editor and see 00000000000000000000000000000111 it does not mean its a binary file, most of the editor process files as ascii text file only. you need a binary editor to open a binary file and read meaningful data from them.

#include <stdio.h>

int main()
{
    FILE *Read_fptr  = NULL;
    FILE *Write_fptr = NULL;
    int   data       = 20;
    size_t nElement  = 1;

    if ( (Write_fptr = fopen("data.bin", "wb")) != NULL ) {
        if ( fwrite(data, nElement, sizeof data, Write_fptr) != sizeof data ) {
            fprintf(stderr,"Error: Writing to file\n");
            fclose(Write_fptr);
            return -1;
        }
    } else {
        fprintf(stderr,"Error: opening file for writing\n");
        return -1;
    }
    fclose(Write_fptr);
    data = 0;
    if ( (Read_fptr = fopen("data.bin", "rb")) != NULL ) {
        if ( fread(data, nElement, sizeof data, Read_fptr) != sizeof data) {
            if( !feof(Read_fptr) ) {
                fprintf(stderr,"Error: Reading from file\n");
                fclose(Read_fptr);
                return -1;
            }
        }
    } else {
        fprintf(stderr,"Error: opening file for reading\n");
        return -1;
    }
    fclose(Read_fptr);

    printf("%d\n",data);
    return 0;

}