Linux c file operation to a character device file (/dev/uinput) fails - c

Question

I am in the middle of writing a user level C program, that reads a SPI device and translate the results to a keyboard event.
I am now trying to pass emulate key event to /dev/uinput. A weird thing is happening, file operation is failing.
my_kbd.c
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <signal.h>
#include <unistd.h>
#include <errno.h>
#include <linux/input.h>
#include <linux/uinput.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <fcntl.h>
#define PROG_NAME "c2h2_spi_kbd"
int fire_key(__u16);
int setup_uinputfd(const char *);
int close_uinputfd();
void write_uinput(__u16, __u16, __s32);
/*fd for uinput, we do need kernel to support uinput */
static int uinputfd = -1;
int main(int argc, char **argv){
puts("Welcome to use SPI keyboard program v0.1!");
uinputfd = setup_uinputfd(PROG_NAME);
if(uinputfd != -1){
fire_key(KEY_A);
}else{
puts("where is uinput ? do you have permission?\n");
}
close_uinputfd();
exit(0);
}
int fire_key(__u16 key){
write_uinput(EV_KEY, key, 1);
return 0;
}
void write_uinput(__u16 type, __u16 code, __s32 value){
struct input_event ie;
sleep(1);
memset(&ie, 0, sizeof(ie));
ie.type = type;
ie.code = code;
ie.value = value;
if(write(uinputfd, &ie, sizeof(ie)) != sizeof(ie)) puts("ERR1");
memset(&ie, 0, sizeof(ie));
ie.type = EV_SYN;
ie.code = SYN_REPORT;
ie.value = 0;
if(write(uinputfd, &ie, sizeof(ie)) != sizeof(ie)) puts("ERR2");
}
int close_uinputfd(){
close(uinputfd);
return 0;
}
int setup_uinputfd(const char *name){
int fd;
int key;
struct uinput_user_dev dev;
fd = open("/dev/input/uinput", O_RDWR);
if (fd == -1) {
fd = open("/dev/uinput", O_RDWR);
if (fd == -1) {
fd = open("/dev/misc/uinput", O_RDWR);
if (fd == -1) {
fprintf(stderr, "could not open %s\n", "uinput");
perror(NULL);
return -1;
}
}
}
memset(&dev, 0, sizeof(dev));
strncpy(dev.name, name, sizeof(dev.name));
dev.name[sizeof(dev.name) - 1] = 0;
if (write(fd, &dev, sizeof(dev)) != sizeof(dev) ||
ioctl(fd, UI_SET_EVBIT, EV_KEY) != 0
|| ioctl(fd, UI_SET_EVBIT, EV_REP) != 0) {
goto setup_error;
}
for (key = KEY_RESERVED; key <= KEY_UNKNOWN; key++) {
if (ioctl(fd, UI_SET_KEYBIT, key) != 0) {
goto setup_error;
}
}
if (ioctl(fd, UI_DEV_CREATE) != 0) {
goto setup_error;
}
return fd;
setup_error:
fprintf(stderr, "could not setup %s\n", "uinput");
perror(NULL);
close(fd);
return -1;
}
gcc -Wall my_kbd.c && sudo ./a.out
without sleep(1); I never have uinput event happen. with sleep(1);, this works perfect. (emulate keys output to focused program)
I do suspect this could be a cache problem, but fsync(fd); did not help. Please help.