了libpng 1.5.10錯誤：提領指向不完全類型

Question

png_read_info (png_ptr, info_ptr); 
{ 
    png_byte color_type = info_ptr->color_type; 
    png_byte bit_depth = info_ptr->bit_depth; 
    ... 

對於最後兩行，我得到

error: dereferencing pointer to incomplete type

有什麼不對？在libpng 1.4中，這總是好的。

Answer 1

的png_info結構是從1.5.0的png.h刪除，現在你應該使用這個指針與png_get_*和png_set_*功能。

如libpng manual規定：

The png_info structure is designed to provide information about the PNG file. At one time, the fields of png_info were intended to be directly accessible to the user. However, this tended to cause problems with applications using dynamically loaded libraries, and as a result a set of interface functions for png_info (the png_get_*() and png_set_*() functions) was developed, and direct access to the png_info fields was deprecated..

例如：

png_uint_32 height; 
height = png_get_image_height(png_ptr, info_ptr);