Homework & Coursework Questions

Hi, i'm trying to copy a struct into a binary file using the unix instruction write, so i declare and fill the struct "superbloque" in one function "initSB" and then i pass the pointer to another function called bwrite (for block write) which calls write. The problem is that i call the function with a pointer to my struct and write only creates the file and doesn't write anything. I'm pretty sure the problem i have has to do with the pointers because when i execute

Code:

printf("%i",sizeof(*pointer_to_the_struct)

in bwrite function it says 1 and should be 48

Here are the important parts of the code:

Code:

struct superbloque{
    unsigned int firstmbits;
    unsigned int lastmbits
    ...
    unsigned int numbloques;
}

int initSB (unsigned int nbloq){
    struct superbloque SB;
    SB.firstmbits=0;
    SB.lastmbits=TamMB(nbloq)-1;
    ...
    SB.numbloques=nbloq;
    bwrite(0,&SB)
}

int bwrite(unsigned int bloque, void *buf){
    lseek(mifd, bloque*tbloque, SEEK_SET);
    return write(mifd, buf, sizeof(*buf);
}

I know there's a lot of information about this but i've been reading for two days and I can't figure out how to solve the problem.
Thank you very much for your help


Ricardo Galli
Sistemas operativos
Universidad de las Islas Baleares (Palma de Mallorca, Spain)

The size of a void is 1, so only 1 byte is getting written. You need to provide the size of the structure, either:

Code:

int bwrite(unsigned int bloque, void *buf, size_t siz){
    lseek(mifd, bloque*tbloque, SEEK_SET);
    return write(mifd, buf, siz);
}

or

Code:

 int bwrite(unsigned int bloque, struct superbloque  *buf){
     lseek(mifd, bloque*tbloque, SEEK_SET);
     return write(mifd, buf, sizeof(*buf));
 }

if pointer_to_the_struct is a pointer to the structure, then *pointer_to_the_struct is the first byte in the structure. so you a integer of 1
you should try
printf("%i",sizeof(struct xxx))
and this will get you the real size of the structure, and that's what you want