How to check if a string starts with another string in C?

There’s no standard function for this, but you can define

bool prefix(const char *pre, const char *str)
{
    return strncmp(pre, str, strlen(pre)) == 0;
}

We don’t have to worry about str being shorter than pre because according to the C standard (7.21.4.4/2):

The strncmp function compares not more than n characters (characters that follow a null character are not compared) from the array pointed to by s1 to the array pointed to by s2.”