Strlwr function - getting an error in xcode 9.2 - c

Question

I'm trying to convert a string from upper case to lower case to check if it is a palindrome, however I keep getting the error:
"function declaration is not a prototype"
I already added #include <string.h> in the header, but it still doesn't work. How do I get around this issue?
This is the code:
int main (void)
{
char *user_string, *user_string_rev;
/* the malloc function is used to make sure that enough memory is allocated for the string and that it does not overwrite memory boxes of other variables. */
user_string= (char*)malloc(BUFF_SIZE*sizeof(char));
user_string_rev= (char*)malloc(BUFF_SIZE*sizeof(char));
printf("Please enter a string:");
fgets(user_string,BUFF_SIZE, stdin); /* fgets function take the string the user inputs and stores it into user_string. */
user_string_rev=strcpy(user_string_rev, user_string); /*the strcpy takes the string the user inputs and copies it to user_string_rev. */
strlwr(user_string_rev);
palindrome_check(user_string,user_string_rev); /*this is the palindrome function used to check if the two strings are palindromes, it intakes two arguments, the two strings and does not return anything. */
return 0;
}

Answer 1

Replace :
strlwr(user_string_rev);
which is not a standard function with:
int i = 0;
while (user_string_rev[i])
{
if (isalpha(user_string_rev[i]))
user_string_rev[i] |= 32;
++i;
}
Don't forget to add the ctype header at the top of your .c file to use isalpha:
#include <ctype.h>

Answer 2

the following proposed code:
incorporates the comments to the question
cleanly compiles
properly checks for errors
will treat a string that is nothing but a newline as NOT a palindrome
And now the proposed code:
#include <stdio.h> // getline(), printf()
#include <stdlib.h> // free()
#include <ctype.h> // tolower()
#include <string.h> // strlen(), strchr()
// prototypes
void palindrome( char *, size_t length );
int main( void )
{
char *inputStr = NULL;
size_t lengthStr = 0;
printf("Please enter a string:");
if( -1 != getline( &inputStr, &lengthStr, stdin ) )
{
size_t length = strlen( inputStr );
for( size_t i = 0; i < length; i++ )
{
inputStr[i] = (char)tolower( inputStr[i] );
}
char *newline = strchr( inputStr, '\n' );
if( newline )
{
*newline = '\0';
length--;
}
palindrome( inputStr, length );
}
free( inputStr );
return 0;
}
void palindrome( char stringToCheck[], size_t length )
{
size_t index = length - 1; // don't check NUL terminator byte
size_t i;
for( i = 0; i < index; i++ )
{
if( stringToCheck[i] != stringToCheck[ index ] )
{
break;
}
index--;
}
if( i < index )
{
printf( "%s is not a palindrome\n", stringToCheck );
}
else
{
printf( "%s is a palindrome\n", stringToCheck );
}
}