what is this STATIC BYTE be doing here?

[sachinkp21587]

i am working on keil IDE using realview mdk-arm for a project based on LPC1768

i made a header file as below

#ifndef _test_h
#define _test_h

#include "diskio.h"
#include "spi.h"
#include "type.h"
#include "LPC17xx.h"

#define rcvr_spi()		xchg_spi(0xFF)

static
BYTE xchg_spi (BYTE dat);

#endif

and a .c file as follows

#include "test.h"

static volatile
DSTATUS Stat = STA_NOINIT;	/* Disk status */

static volatile
UINT Timer1, Timer2;		/* 1000Hz decrement timer */

static
UINT CardType;

#define xmit_spi(dat) 	xchg_spi(dat)

//#define rcvr_spi_m(p)	SPI2BUF = 0xFF; while (!SPI2STATbits.SPIRBF); *(p) = (BYTE)SPI2BUF;
#define rcvr_spi_m(p)	*(p) = xchg_spi(0XFF);

static
BYTE xchg_spi (BYTE dat)
{
	return (BYTE)SPISend((uint8_t) dat);
}

now i made a main file in which i included this header file.
also by browsing added the c file to projects folder.

now when i make a call to xchg_spi there occurs an error saying

test.h(12): error: #114: function "xchg_spi" was referenced but not defined

the above compiles without any errors when the static is removed.
why so?

 

[alexan_e]

The static keyword limits the visibility of the function to the file were it is declared,
take a look at https://stackoverflow.com/questions/572547/what-does-static-mean-in-a-c-program
https://www.thegeekstuff.com/2012/08/c-static-variables-functions/