Thread: Write dynamic structure to a text file

Originally Posted by deathseeker25

and the array of structures is declared as global variable (because it has to):

Please, tell me why "it has to" be global. 99% of the time, people coming here for help say they have to use globals, but they're wrong. Globals are almost always the wrong way to solve a problem. Declare them in the right scope/function and pass them around as necessary. Read this: Global Variables Are Bad.

Code:

    int tamanho = sizeof &array / sizeof &array[0];

    array = (ContaFicheiro *) malloc(sizeof(ContaFicheiro)*sizeof(array));

These size calculations are not what you want. sizeof &array is the size of a pointer to pointer to ContaFicheiro. sizeof &array[0] is the sizeof a pointer to ContaFicheiro. You can't get the size of an array unless you are in the same scope in which you declare it. I don't really know what you're trying to accomplish with the tamanho calculation, i.e. what you think those sizeof operations return, but it seems tamanho is the number of things you want in your array. The sizeof(ContaFicheiro) in your malloc call is the size of one element, sizeof(array) is the size of a pointer to a ContaFicheiro. The result of multiplying them is some useless number. Your malloc call should look something more like:

Code:

array = malloc(tamanho * sizeof(*array));

Notice, I use *array in the sizeof. That is saying "size of the thing pointed to by array", or "size of one array element". You multiply that by the number of items you want for your array.

Also, don't cast the return value of malloc. Read this: FAQ > Casting malloc - Cprogramming.com

Code:

            fprintf(fp, "%u\t%s\t%s\t%f\n", array[i].numero, 
                            array[i].nomeUtilizador, 
                            array[i].PIN, 
                            array[i].saldo);

The thing is that it doesn't print any information about the structure. Can you help me with this ?

What data do you expect to be in the structure at this point? You allocated a bunch of memory (the malloc call), but never put any data in it.

Originally Posted by anduril462

Please, tell me why "it has to" be global. 99% of the time, people coming here for help say they have to use globals, but they're wrong. Globals are almost always the wrong way to solve a problem. Declare them in the right scope/function and pass them around as necessary. Read this: Global Variables Are Bad.

It has to be global because it's what's specified in the project sheet. This is a college project and there are some things I just have to follow, even though, after reading the article you suggested, I agree that yeah, global variables are quite bad.

Originally Posted by anduril462

Code:

    int tamanho = sizeof &array / sizeof &array[0];

    array = (ContaFicheiro *) malloc(sizeof(ContaFicheiro)*sizeof(array));

These size calculations are not what you want. sizeof &array is the size of a pointer to pointer to ContaFicheiro. sizeof &array[0] is the sizeof a pointer to ContaFicheiro. You can't get the size of an array unless you are in the same scope in which you declare it. I don't really know what you're trying to accomplish with the tamanho calculation, i.e. what you think those sizeof operations return, but it seems tamanho is the number of things you want in your array.

So, how exactly can I know what's the size of the array *array ? I understand the tamanho calculations are not correct, but how will I know then the size of it so I can use it in the for cycle ?

Originally Posted by anduril462

The sizeof(ContaFicheiro) in your malloc call is the size of one element, sizeof(array) is the size of a pointer to a ContaFicheiro. The result of multiplying them is some useless number. Your malloc call should look something more like:

Code:

array = malloc(tamanho * sizeof(*array));

Notice, I use *array in the sizeof. That is saying "size of the thing pointed to by array", or "size of one array element". You multiply that by the number of items you want for your array.

Also, don't cast the return value of malloc. Read this: FAQ > Casting malloc - Cprogramming.com

What data do you expect to be in the structure at this point? You allocated a bunch of memory (the malloc call), but never put any data in it.

At this point there is data in the structure, data that was read from a file exactly with the same string format to save data.

Originally Posted by deathseeker25

It has to be global because it's what's specified in the project sheet. This is a college project and there are some things I just have to follow, even though, after reading the article you suggested, I agree that yeah, global variables are quite bad.

Well, your professor is a dunderhead. You can even tell him I said so .

So, how exactly can I know what's the size of the array *array ? I understand the tamanho calculations are not correct, but how will I know then the size of it so I can use it in the for cycle ?

How am I supposed to know? It depends on the project specifications. Actually, your next question, and my answer, takes care of this issue.

At this point there is data in the structure, data that was read from a file exactly with the same string format to save data.

At that point, there is on longer data in the structure. You read it in from a file, passed a pointer to that data, then allocated new memory, with no data in it, and overwrote the pointer. You no longer have access to the data you read it, only to the newly allocated memory. If you already have the data in array, you don't need that malloc call at all. It also sounds like, if you already read the data, then you already know how many items in your array, so you could just pass tamanho in as a parameter, no need to calculate it. Or perhaps this is what the ultimaConta variable is for, and you don't even need tamanho.