ping successfully but gethostbyname() returns null - c

Question

I'm programming for a device running Linux OS.
The device runs ping command to youtube.com successfully. However, in source code, if I call gethostbyname("youtube.com"), the function returns NULL.
UPDATE LOG
My code looks like this (this is just a short pseudo code to describe my issue, not full code)
void my_func()
{
struct hostent *hostEntry;
hostEntry = gethostbyname("youtube.com");
if (hostEntry) {
// gethostbyname() resolve host name successfully, no need to care
} else {
printf("Fail to get host, herrno=%d, strErr=%s\n", h_errno, hstrerror(h_errno));
}
}
And the device's output when gethostbyname() returns NULL is
Fail to get host, herrno=1, strErr=Unknown host
This is the device's output when I exit the program and run ping youtube.com
$ ping youtube.com
PING youtube.com (64.233.189.91): 56 data bytes
64 bytes from 64.233.189.91: seq=0 ttl=55 time=57.824 ms
64 bytes from 64.233.189.91: seq=1 ttl=55 time=56.306 ms
64 bytes from 64.233.189.91: seq=2 ttl=55 time=56.790 ms
64 bytes from 64.233.189.91: seq=3 ttl=55 time=56.831 ms
64 bytes from 64.233.189.91: seq=4 ttl=55 time=56.417 ms
What should I check now?

Answer 1

I add res_init() to resolve this issue and fortunately, it works.
#include <resolv.h>
void my_func()
{
struct hostent *hostEntry;
hostEntry = gethostbyname("youtube.com");
if (hostEntry) {
res_init();
hostEntry = gethostbyname("youtube.com");
}
if (hostEntry) {
// gethostbyname() resolve host name successfully, no need to care
} else {
printf("Fail to get host, herrno=%d, strErr=%s\n", h_errno, hstrerror(h_errno));
}
}
This may be the reason why the function helps
The res_init() function reads the configuration files (see resolv.conf(5)) to get the default domain name, search order and name server address(es).