How to share struct within functions; (warnings: expected 'struct <anonymous> **')

[h4ever]

I don't understand why this happens. I can call the read_png_file and *data is there. But when I call next function so there is no data in there but it is type
(PNGDataPtr *) 0x22ff24 refering to (PNGDataPtr) 0x77c02850 <msvcrt!??9type_info@@QBEHABV0@@Z+5208> Can you explain what happens there and how correct it? I want to pass the struct and share it between functions.

#include <stdio.h>
#include <stdlib.h>
#include <png.h>

typedef struct
{
	FILE *fp;
} PNGData, *PNGDataPtr;


void read_png_file(char* file_name, PNGData * data)
{
}

void write_png_file(char* file_name, PNGDataPtr * data)
{
}

void process_file(PNGDataPtr * data)
{
}

int main(int argc, char **argv)
{
    PNGData data;
    read_png_file(argv[1], &data);
    process_file(&data);
    write_png_file(argv[2], &data);
    return 0;
}

function 'main':|
warning: passing argument 1 of 'process_file' from incompatible pointer type
main.c|19|note: expected 'struct <anonymous> **' but argument is of type 'struct PNGData *'|
warning: passing argument 2 of 'write_png_file' from incompatible pointer type [enabled by default]|
15|note: expected 'struct <anonymous> **' but argument is of type 'struct PNGData *'|


NOTE:
I have removed content of the functions to make it as short as possible. The data variable was filled with values originally but only in the read function.

[naraku9333]

process_file takes a PNGData** but you are passing a PNGData*.