How fread() and fwrite() actually works in C programming ?

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1st can we write structure in text file ?

Yes you can.

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i have seen no good example .

This your design choice. You decide how the structure translate to file and how you get it back to structure.
XML files are used for this all the time. Structures are translates to XML and vice-versa.

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is it practically wrong to write it in text format ?

You design choice. See XML files.

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2nd how these fread() & fwrite works().they operate on a block of data how they get the address of next block .i mean we do have the pointer but file doesnt have any address so how the pointer go to next block ?

Looks like you need to go back to C/C++ basics
The C Programming Language - Wikipedia, the free encyclopedia[^]
https://hassanolity.files.wordpress.com/2013/11/the_c_programming_language_2.pdf[^]
http://www.ime.usp.br/~pf/Kernighan-Ritchie/C-Programming-Ebook.pdf[^]

1st can we write structure in text file ? i have seen no good example .is it practically wrong to write it in text format ?

If, by structure, you mean binary data (eg. not ASCII), no you should not use text mode. Open the file as binary if you want to operate on bytes without the C runtime applying some rules (eg. "cooked" data) that effect carriage return, line feed, and - for some platforms - end of file marker.

2nd how these fread() & fwrite works().they operate on a block of data how they get the address of next block .i mean we do have the pointer but file doesnt have any address so how the pointer go to next block ?

The file system takes care of this. There's an implicit address or file offset where the data is stored. Each call to fwrite causes the offset to move so, by default, you are always appending data to the file. You can also change this offset by using fseek and examine the offset by using ftell. You do not need to worry about how disk sectors, blocks, or clusters are organized.