How to use isalnum function correctly in C?

I tried to check if the string character is alnum and then if the string only contains alnum characters to print the string. When I run the program, nothing happens. I have another program with which I read from the input the text I want and I send it with a FIFO. If I do not include the "check" function in the program, it will work, after which it will not.

void    put_alphanum(char *str)
{
while (*str)
{
    if (*str >= '0' && *str <= '9')
        write(1, str, 1);
    else if (*str >= 'A' && *str <= 'Z')
        write(1, str, 1);
    else if (*str >= 'a' && *str <= 'z')
        write(1, str, 1);
    str++;
}
write(1,"\n",1);
}

      

        
        
        
      

    

This is the function I used to print the string. If I am not compatible with the check function, the program works.

int check(char *str)
{

while(*str)
{
if(isalnum(*str)==0)
return 0;

str++;
}

return 1;

}

      

        
        
        
      

    

This is the function I am using to check if a string contains only alnum characters. But if I don't include this feature in my program, the program works. I think this is the problem here. And the main function ()

int     main()
{
int fd;
int len;
char buf[BUFF_SIZE + 1];

mkfifo(FIFO_LOC, 0666);
fd = open(FIFO_LOC, O_RDONLY);
while(1)
{
    while((len = read(fd, buf, BUFF_SIZE)))
    {
        buf[len] = '\0';
        if(check(buf)==1)
             put_alphanum(buf);

    }
}
return (0);
}