quick & simple printing with indentation level (spaces, not tabs)

output.c

#include <stdio.h>
#include <stdarg.h>

void outputf(int level, const char *format, ...) {
	FILE *stream = stdout;
	static const int tab_size = 4;
	const int width = level * tab_size + 1;

	va_list args;
	va_start(args, format);
	if (level > 0) {
		fprintf(stream, "%*s", width, " ");
	}
	vfprintf(stream, format, args);
	va_end(args);
}

void output(int level, const char *str) {
	outputf(level, "%s", str);
}

int main() {
	output(0, "no indentation\n");
	for (int i=0; i < 10; i++) {
		outputf(i, "indented by %d\n", i);
	}
	return 0;
}