Start C function in a specific address by using BL51

I am designing a program that I could update the program by receiving the code through RS232. In order to reach this goal, I planed to write a code iapUpdate and then relocate it to somewhere else. However, when I tried to relocate it by BL51, it just said "Can't find segment iapUpdate".

For those functions in assembly, We used the format as follows to define
the segment that we could locate by BL51:
in assembly code
; segments in this library
?PR?iap_read_manufacturer_id?RX2IAPLIB SEGMENT CODE
?PR?iap_read_device_id?RX2IAPLIB SEGMENT CODE

BL51
?PR?iap_read_manufacturer_id?RX2IAPLIB(0x4000),
?PR?iap_read_device_id?RX2IAPLIB(0x4100),

How could we do the same thing while we have a function called iapUpdate in C part? Thanks.