stdio is a bare-metal replacement for printf and scanf. It has no dependency on libc and is compiled with -nostdlib. All output and input goes directly through the UART driver.
PRINT
void PRINT(const char *s, ...);
% specifier is encountered, it pulls the next argument from the varargs list and routes it to the appropriate alphanumeric conversion or UART output function.
| Specifier | Type | Conversion |
|---|
%d | int | uart_itoa() → uart_puts() |
%f | float (promoted to double in varargs) | uart_ftoa() → uart_puts() |
%s | char * | uart_puts() directly |
%c | char (promoted to int in varargs) | uart_putc() directly |
Implementation
void PRINT(const char *s, ...){
va_list args;
va_start(args, s);
while (*s) {
if(*s == '%'){
char buffer[16];
s++;
if (*s == 'f') {
double v = va_arg(args, double);
uart_ftoa((float)v, buffer);
uart_puts(buffer);
}
if(*s == 'd'){
int v = va_arg(args, int);
uart_itoa(v, buffer);
uart_puts(buffer);
}
if(*s == 's'){
char *v = va_arg(args, char*);
uart_puts(v);
}
if(*s == 'c'){
char v = (char)va_arg(args, int);
uart_putc(v);
}
s++;
}
else{
uart_putc(*s++);
}
}
va_end(args);
}
Usage in user processes
Process 1 (User/P1/main.c) prints a counter as a decimal integer:
PRINT("----From P1: %d\n", i);
User/P2/main.c) prints a character cycling through the alphabet:
PRINT("----From P2: %c\n", c);
READ
void READ(char *type, void *returnv);
| Specifier | Conversion | Output type |
|---|
"%d" | uart_atoi() | int written to *returnv |
"%f" | uart_atof() | float written to *returnv |
Implementation
void READ(char *type, void *returnv){
char input[16];
uart_gets_input(input, sizeof(input));
if(*type == '%'){
type++;
if(*type == 'f'){
*(float *) returnv = uart_atof(input);
}
if(*type == 'd'){
*(int *) returnv = uart_atoi(input);
}
}
}
Usage
int value;
READ("%d", &value);
float temp;
READ("%f", &temp);
Critical section requirement
PRINT must be called between disable_irq() and enable_irq() in user processes. Without this guard, the timer IRQ can preempt mid-print and a context switch to another process will interleave output on the UART, producing garbled characters.
disable_irq();
PRINT("----From P1: %d\n", i);
enable_irq();
PRINT returns so that the scheduler continues to receive timer interrupts normally.