How to write default structure values multiple times using fwrite()

c++

I want to write a default structure, N times, to a file, using fwrite.

typedef struct foo_s {

 uint32 A;
 uint32 B;
 char desc[100];

}foo_t;

void init_file(FILE *fp, int N)
{
   foo_t foo_struct = {0};
   foo_struct.A = -1;
   foo_struct.B =  1;

   fwrite(&foo_struct, sizeof(foo_struct), N, fp);    }

The above code does not write foo_struct N times to the file stream fp.

Instead it writes N*sizeof(foo_struct) bytes starting from &foo_struct to fp.

Can anyone tell how to achieve the same with a single fwrite?

Best Solution

You can't with a single fwrite(). You'd have to use a loop:

int i;
for (i = 0; i < N; ++i)
    fwrite(&foo_struct, sizeof(foo_struct), 1, fp);

The third parameter of fwrite() is the number of objects to write, not the number of times to write a single object.

Explanation

Option 1 will work only if you use C99 (or newer) and it's the "standard way" to do it. Choose this if possible.
Options 2, 3 and 4 will have in practice the same identical behavior. #2 and #3 don't use #defines though, which in my opinion is better.

If you are undecided, go with #1!

Writing data from a linked list to a binary file using fwrite.

Since you have one item of variable length data (a dynamically allocated string pointer) you have to decide how you want to manage it in a binary file. You can set aside a fixed length of space for the name within the structure in your output. Or you can write a zero-terminated string and manage it that way. I'll choose the second case since it is more in line with the dynamic string.

This is a bit verbose since the nature of the structure hasn't been given in the problem statement. So the following will work with structure elements in arbitrary order.

void WriteBinaryFile(ListDataType *Data, int numEl)
{
    FILE *bin_file;

    char name[50];

    printf("\n\nWhat would you like to call the binary file in which the data will be stored?\n"
    printf("NB remember to add \".txt\" after your chosen name!!!\nENTER NAME NOW>>");
    scanf("%s", name);

    bin_file = fopen(name, "wb+");

    do
    {
        //NOTE: include terminating zero in the string output; assumes single-byte chars
        fwrite(Data->component.name, strlen(Data->component.name)+1, 1, bin_file);
        fwrite(&Data->component.int1, sizeof(Data->component.int1), 1, bin_file);
        fwrite(&Data->component.int2, sizeof(Data->component.int2), 1, bin_file);
        fwrite(&Data->component.double1, sizeof(Data->component.double1), 1, bin_file);
        fwrite(&Data->component.double2, sizeof(Data->component.double2), 1, bin_file);
        fwrite(&Data->component.double3, sizeof(Data->component.double3), 1, bin_file);
        fwrite(&Data->component.double4, sizeof(Data->component.double4), 1, bin_file);
        Data = Data->nextPtr;
    } while(Data != NULL);

    fclose(bin_file);
}

If a program needs to read it back, it would start by reading in a name and read characters until it finds a 0. Then it would read the other elements per their data types. That process would be repeated for each structure written. This isn't necessarily portable, though. You have to be careful about size of int times, etc.

Best Solution

Related Solutions

Using boolean values in C

Explanation

Writing data from a linked list to a binary file using fwrite.

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