structs memory allocation

[fervent locust]

Could someone explain to me what is happening here and why ?

[bronze flume]

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Remember to ask specific questions, provide necessary details, and reduce your question to its simplest form. For tips on how to ask a good question run !howto ask.

[fervent locust]

    // Von Mattis
    int extra_bytes = ((width * 3) % 4) ? (4 - ((width * 3) % 4)) : 0;    // variable for extra bytes

    // 3.2 Wichtigster Teil - Memory allocation
    // 3.2.1 Speicher für die Struktur
    tBmp* p_tBmp = (tBmp*)calloc(4, sizeof(tBmp)); 
    if (p_tBmp == NULL) {
        printf("Speicherzuweisung in der Datei %s in Zeile %d ist fehlgeschlagen!", __FILE__, __LINE__);
        exit(EXIT_FAILURE);
    }

    // 3.2.2 Speicher für die Daten der Bitmap
    p_tBmp->data = (char*)calloc(4, sizeof(tFileHeader) + sizeof(tInfoHeader) + (3 * width + extra_bytes) * height * sizeof(BYTE));
    if (p_tBmp->data == NULL) {
        printf("Speicherzuweisung in der Datei %s in Zeile %d ist fehlgeschlagen!", __FILE__, __LINE__);
        exit(EXIT_FAILURE);
    }

    // 3.2.3 Speicher für die Pixel 
    p_tBmp->pixel = (tRgb**)calloc(width, sizeof(tRgb*));
    if (p_tBmp->pixel == NULL) {
        printf("Speicherzuweisung in der Datei %s in Zeile %d ist fehlgeschlagen!", __FILE__, __LINE__);
        exit(EXIT_FAILURE);
    }
    


    if (p_tBmp->pixel != NULL){
        p_tBmp->pFile = (tFileHeader*)p_tBmp->data;
        p_tBmp->pInfo = (tInfoHeader*)(p_tBmp->data + sizeof(tFileHeader));
        p_tBmp->pixel[0] = (tRgb*)(p_tBmp->data + sizeof(tFileHeader) + sizeof(tInfoHeader));
    }
    else{
        printf("Speicherzuweisung in der Datei %s in Zeile %d ist fehlgeschlagen!", __FILE__, __LINE__);
        exit(EXIT_FAILURE);
    } ```

I don´t get this part:

        p_tBmp->pFile = (tFileHeader*)p_tBmp->data;
        p_tBmp->pInfo = (tInfoHeader*)(p_tBmp->data + sizeof(tFileHeader));
        p_tBmp->pixel[0] = (tRgb*)(p_tBmp->data + sizeof(tFileHeader) + sizeof(tInfoHeader));
    }  ```

Without it, p_tBmp would be NULL pointer

[fervent locust]

@native cairn @safe dome could you guys help me out ?

[native cairn]

👋

[native cairn]

!f

[bronze flume]

tBmp* createBmp(int width, int height) {
  // Von Mattis
  int extra_bytes = ((width * 3) % 4) ? (4 - ((width * 3) % 4))
                                      : 0;  // variable for extra bytes

  // 3.2 Wichtigster Teil - Memory allocation
  // 3.2.1 Speicher für die Struktur
  tBmp* p_tBmp = (tBmp*)calloc(4, sizeof(tBmp));
  if (p_tBmp == NULL) {
    printf("Speicherzuweisung in der Datei %s in Zeile %d ist fehlgeschlagen!",
           __FILE__, __LINE__);
    exit(EXIT_FAILURE);
  }

  // 3.2.2 Speicher für die Daten der Bitmap
  p_tBmp->data =
      (char*)calloc(4, sizeof(tFileHeader) + sizeof(tInfoHeader) +
                           (3 * width + extra_bytes) * height * sizeof(BYTE));
  if (p_tBmp->data == NULL) {
    printf("Speicherzuweisung in der Datei %s in Zeile %d ist fehlgeschlagen!",
           __FILE__, __LINE__);
    exit(EXIT_FAILURE);
  }

  // 3.2.3 Speicher für die Pixel
  p_tBmp->pixel = (tRgb**)calloc(width, sizeof(tRgb*));
  if (p_tBmp->pixel == NULL) {
    printf("Speicherzuweisung in der Datei %s in Zeile %d ist fehlgeschlagen!",
           __FILE__, __LINE__);
    exit(EXIT_FAILURE);
  }

  if (p_tBmp->pixel != NULL) {
    p_tBmp->pFile = (tFileHeader*)p_tBmp->data;
    p_tBmp->pInfo = (tInfoHeader*)(p_tBmp->data + sizeof(tFileHeader));
    p_tBmp->pixel[0] =
        (tRgb*)(p_tBmp->data + sizeof(tFileHeader) + sizeof(tInfoHeader));
  } else {
    printf("Speicherzuweisung in der Datei %s in Zeile %d ist fehlgeschlagen!",
           __FILE__, __LINE__);
    exit(EXIT_FAILURE);
  }

MK61

Formatted by Zelis 🔱

[safe dome]

Don't ping people unless they are already part of the thread

[fervent locust]

sorry

didn´t know that..won´t do that again

[native cairn]

What exactly is the question?

[native cairn]

This doesn't change p_tBmp

[fervent locust]

But if I don´t use that, I get an error, which says that I am working with a NULL pointer

[fervent locust]

I wrote this part

[fervent locust]

And this I got from someone else, but I don´t really understand whats that part of code is doing

[native cairn]

It checks if p_tBmp->pixel != NULL and then set some bmp properties / the first pixel

[fervent locust]

But didn´t I already allocate memory for it (above) p_tBmp->pFile = (tFileHeader*)p_tBmp->data; I mean like this part

I already allocated memory for p_tBmp->pFile

[native cairn]

where did you allocate memory for p_tBmp->pFile?

[fervent locust]

😅 Oh okay, you are right

so this sets the pointers on memory for the first pixel

Is something like that always necessary or did you have use something similiar

Because I never saw allocatig memory at the and like this

[native cairn]

There's a million ways to do everything

[fervent locust]

thank you for your time