Thread: Working with binary files

It's better to post actual code than saying "I tried x and something happened".

This is checking if file exists, if not, creating and writing 1 (as in 1st member) at the beginning (or so I thought).

Code:

int x = 1;
    FILE* open_F = NULL;
    open_F = fopen("myfile.bin", "rb");
    if (open_F == NULL) {
        open_F = fopen("myfile.bin", "wb");
        if (open_F == NULL) {
            perror("File error");
        }
        MEMBER* member = NULL;
        member = (MEMBER*)calloc(1, sizeof(MEMBER));
        if (member == NULL) {
            perror("Struct error");
        }
        /*entering members in struct*/
        rewind(open_F);
        fwrite(&x, sizeof(int), 1, open_F);
        fwrite(&member, sizeof(MEMBER), 1, open_F);
        fclose(open_F);
        free(member);
}

This is for reading / printing members:

Code:

FILE* open_F = NULL;
    open_F = fopen("myfile.bin", "rb");
    if (open_F == NULL) {
        perror("File error");
    }
    else{
        int x;
        rewind(open_F);
        fread(&x, sizeof(int), 1, open_F);
        MEMBER* member = NULL;
        member = (MEMBER*)calloc(x, sizeof(MEMBER));
        if (member == NULL) {
            perror("Struct error");
        }
        fread(&member, sizeof(MEMBER), x, open_F);
        for(int i = 0; i < x; i++){
            /*printing out members/*
        }    
        fclose(open_F);
        free(member);
}

Member is a pointer, so

fwrite(member, sizeof(MEMBER), 1, open_F);

fread(member, sizeof(MEMBER), x, open_F);

To read it

Code:

        rewind(open_F);
        fread(&x, sizeof(int), 1, open_F);

To write it, and append a new record to the end of the file

Code:

        rewind(open_F);
        fwrite(&x, sizeof(int), 1, open_F);
        fseek(open_F, 0, SEEK_END);
        fwrite(member, sizeof(MEMBER), 1, open_F);
        fflush(open_F);

You only need the fflush() if you're keeping the file open for further reading.

Originally Posted by Salem

To read it

Code:

        rewind(open_F);
        fread(&x, sizeof(int), 1, open_F);

To write it, and append a new record to the end of the file

Code:

        rewind(open_F);
        fwrite(&x, sizeof(int), 1, open_F);
        fseek(open_F, 0, SEEK_END);
        fwrite(member, sizeof(MEMBER), 1, open_F);
        fflush(open_F);

You only need the fflush() if you're keeping the file open for further reading.

Thank you for your help! I've done that and now I get error "stream.valid()" when trying to read. Read code:

Code:

FILE* open_F = NULL;open_F = fopen("myfile.bin", "rb");
if (open_F == NULL) {
    perror("File error");
}
else {
    int x, i;
    rewind(open_F);
    fread(&x, sizeof(int), 1, open_F);
    MEMBER* member = NULL;
    member = (MEMBER*)calloc(x, sizeof(MEMBER));
    if (member == NULL) {
        perror("Struct error");
    }
    fread(&member, sizeof(MEMBER), x, open_F);
    for (i = 0; i < x; i++) {
        /*printing members
        e.g. fprintf(stdout, "Name: %s", (member + i)->name);*/
    }
    fclose(open_F);
    free(member);
}

EDIT: Plus, "counter" is not working, no matter how many members I enter, always shows "1", even tho when I open my *.bin file in VS it shows proper values.

It seems to me that you could do with more stringent error checking, e.g.,

Code:

FILE *open_F = fopen("myfile.bin", "rb");
if (!open_F) {
    perror("fopen() failed to open the file");
    return;
}

int num_records;
if (fread(&num_records, sizeof(num_records), 1, open_F) != 1) {
    perror("fread() failed to read the number of records");
    fclose(open_F);
    return;
}

MEMBER *members = calloc(num_records, sizeof(*members));
if (!members) {
    perror("calloc() failed to allocate space for the records");
    fclose(open_F);
    return;
}

if (fread(&members, sizeof(*members), num_records, open_F) != num_records) {
    perror("fread() failed to read the records");
    free(members);
    fclose(open_F);
    return;
}
fclose(open_F);

for (int i = 0; i < num_records; ++i) {
    printf("Name: %s\n", members[i].name);
}
free(members);

One thing that you have not shown: what is the layout of the MEMBER struct? In particular, does it contain a pointer member or a member of struct type that does contain a pointer member?

> Thank you for your help! I've done that and now I get error "stream.valid()" when trying to read. Read code:
...
> fread(&member, sizeof(MEMBER), x, open_F);
Yes, and you're still passing the addresses of pointers.

I thought you understood that point from previous.