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/*
* playground_05-25-2011.c
*
*
* Playing around with unions and macros, and how both
* of these things look in the memory
*
*/
#include <stdio.h>
#include <stdint.h>
#define EXP(base, exp, ret) do { \
int i; \
int acc = 1; \
for (i = 0; i < exp; i++) \
acc *= base; \
ret = acc; \
} while(0) \
#define MUL(base, mult, ret) do { \
int i; \
int acc = 0; \
for (i = 0; i < mult; i++) \
acc += base; \
ret = acc; \
} while(0) \
/* trace a variable, requires ANSI compliant compiler */
#define TRACE(var, fmt) \
printf("TRACE: " #var " = " #fmt ", %s:%d\n", var, __FILE__, __LINE__)
#pragma pack(1)
struct UART_tag
{
union
{
struct
{
uint32_t baud_rate; /* bits [0:31] */
uint8_t tx_en : 1;
uint8_t rx_en : 1;
uint8_t parity : 1; /* bit [34], 35th */
uint32_t : 29;
} B;
uint64_t R;
} SET;
};
typedef struct UART_tag UART_t;
void UART_info(UART_t *dev)
{
printf("baud rate: %u\n"
"tx enabled: %s\n"
"rx enabled: %s\n"
"parity enabled: %s\n", dev->SET.B.baud_rate,
dev->SET.B.tx_en ? "yes" : "no",
dev->SET.B.rx_en ? "yes" : "no",
dev->SET.B.parity ? "yes" : "no");
}
void dumpHex(UART_t *p, unsigned int sz)
{
int i;
for (i = 0; i < sz; i++)
{
if (i % 4 == 0)
printf("%s0x%08x: ", i == 0 ? "" : "\n", i);
printf(" 0x%02x", ((unsigned char *) p)[i] );
}
printf("\n\n");
}
int main(int argc, char **argv)
{
/* assigning a struct within the union */
puts("\t assigning a struct within the union");
UART_t myUart;
myUart.SET.R = 0;
myUart.SET.B.baud_rate = 19200;
myUart.SET.B.tx_en = 1;
myUart.SET.B.rx_en = 1;
myUart.SET.B.parity = 0;
UART_info(&myUart);
puts("");
puts("\t this is how UART_t looks in the memory");
dumpHex(&myUart, sizeof(UART_t));
/* disable a flag within the struct */
puts("\t disable a flag within the struct");
myUart.SET.B.tx_en = 0;
UART_info(&myUart);
puts("");
/* modifying the whole structure at once */
puts("\t modifying the whole structure at once");
myUart.SET.R = 0;
UART_info(&myUart);
puts("");
/* enable the parity bit */
myUart.SET.R |= (uint64_t) \
((uint64_t) 1 << (64 - 30)); /* this is inverted compared
* to how it would be done on
* a register */
puts("\t setting the parity bit to ON using logical OR operator");
UART_info(&myUart);
puts("");
printf("size of UART_t is: %lu bytes\n", sizeof(UART_t));
printf("size of UART_tag is: %lu bytes\n", sizeof(myUart.SET.B));
int x;
EXP(2, 20, x);
printf("a megabyte is %d bytes\n", x);
int y;
MUL(x, 4, y);
printf("4 megabytes is %d bytes\n", y);
/* endianess */
int z = 1;
if (*((unsigned char *) &z) == 1)
puts("machine has a little-endian byte ordering");
else
puts("machine has a big-endian byte ordering");
TRACE(y, %d);
return 0;
}
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