question about reading MSP430F5418A internal flash

Unless the flash controller is actively writing to flash, you can read all flash content by just reading it like any other memory location. For reading, there is no difference between flash or ram or hardware registers (except that the CPU cannot fetch a program instruction from hardware registers - it will cause a reset due to illegal instruction fetch exception)

So any way that allows you to read from memory address 0x1800 will return the content of the flash at 0x1800.

Nick Carter said:

I know that before writing flash I need to erase it. Owing to I write a long word in the information memory segment D, so I need to erase this segment before using it. Right?

Yes and no. You need to erase the whole segment, if you want to write a '1' bit where there is currently a '0' bit. However, you may erase the segment (so it is all bits 1) and then write on byte to is, then another byte or word. You can even overwrite a byte, so it is e.g. written 0x80 first time, and then 0x00 (no change from any bit from 0 to 1 required).

But there is a limit on how long programming voltage may be applied to a 64 byte block or else bits may turn form 1 to 0 by themselves through leakage currents.
On older MSPs, this limit was so short that a maximum of ~50 individual write operations was possible (whcih did not necessarily have to happen all at the same time) before the whoel segment (in case of main flash with 8 64byte blocks, for info memory with one block) had to be erased.

On newer MSPs this has relaxed a bit. But whiel it is now enough to write the whoel block byt eby byte without exceeding the limit, there's still a tight limit for the number of total writes between two erases.

Nick Carter said:

Or only one segment has been erased? If one segment has been erased, what value should I write to the flash if I wanna erase other segments?

if hte flash controller is in erase mode, any write (the value itself is a don't care) causes the whole segment that belongs to this address to be erased. In case of the info memory, it is just one of the INFO segments (64 or 128 bytes, depending on MSP), in case of main flash it is 512 bytes.
You need to write somewhere into the segment syou want to erase and the whole segment at this address gets set back to 0xff.

Nick Carter said:

For example, if I wanna erase  information memory segment C or B or A, what value should I write to the flash for the dummy write cycle?

gpudFlash_ptrD is a pointer. The value you need to assing to it must be somewhere in the address range of the segment you want to erase. (0x1800 to 0x183f for INFOD, I think).

If you assign a value to *gpudFlash_ptrD (note the '*' before it), it gets written to the address this pointer ponts to. The value itself is unimportant for the erase. The fact that you write trigers the erase (the flash controller picks the address from the write instruction), the value itself is ignored.