fread into buffer is blank despite non-empty file

Question

I'm trying to read in 256 bytes into a buffer from a 65536 byte file, treating it as a random-access file by using fopen, fread, fwrite, and fseek. I'm not getting any errors, but the buffer is still empty after the read, even though the file is non-empty and fread reports reading 256 bytes. Here's my code:

    FILE *file = NULL;
    char buffer[255];
    memset(buffer, 0, sizeof(buffer));

    file = fopen("BACKING_STORE.bin","r");

    if(file == NULL) {
            printf("Error: can't open file.\n");
            return NULL;
    } // end if

    if(fread(buffer, 1, 256, file) != 256) {
            printf("Error: reading from file failed.\n");
            return NULL;
    } // end if

    printf("The bytes read are [%s]\n", buffer);

    fclose(file);

And just to confirm, I opened up the BACKING_STORE.bin file in a hex editor just to make sure that it wasn't empty. Here's a screenshot of that:

After running this program, I get the output: "The bytes read are []" but without any errors.

I'm fairly new to C, so I'm sure it's just something simple I'm missing.

Thanks for the help. :)

Undefined behaviour: Your buffer is 255 bytes, but you read 256. — too honest for this site, Nov 25 '16 at 23:45

score 4 · Accepted Answer · answered Nov 25 '16 at 23:39

Because you can't output binary data with "%s" and printf(). If you want to see the contents you can write a loop and print the hex value of each byte, like this

for (size_t i = 0 ; i < 256 ; ++i) {
    fprintf(stdout, "0x%02x ", buffer[i]);
    if ((i + 1) % 8 == 0) {
        fputc('\n', stdout);
    }
}

fread into buffer is blank despite non-empty file

1 Answers1