XCode

what should i do ?

strrev is not a Standard C function and is not included in the libraries provided with Xcode.
You may need to define it by yourself.

how can i define it ? can you guide me with me that :) @OOPer

how can i define it ?

You can find many articles defining strrev, but it would be better for you to do it by yourself if you are learning C-programming.

thanks for the help but after i defined it , i have another error which is " Incompatible pointer to integer conversion passing 'char [100]' to parameter of type 'char' " . do you have any idea what should i do ?

do you have any idea what should i do ?

You should show your code.

here's my code :)

#include <stdio.h>
#include <string.h>
int main()
{
    char word[100];
    char words[100];
    char strrev(char word);

    printf("Input your word:");
    gets(word);

    strcpy(words,word);

    printf("Reversed:");
    printf("%c",strrev(words));

    if((strcmp(word,words)==0))
printf("It is a palindrome word");
    else
        printf("It is not a palindrome word");

    return 0;

}

here's my code :)

Thanks. It's a little bit far from I imagined...

It should be something like this:

Code Block 
#include <stdio.h>
#include <string.h>
char *strrev(char *word) {
    //... <- You need to implement `strrev` by yourself
}
int main(int argc, const char * argv[]) {
    char word[100];
    char words[100];
    printf("Input your word:");
    gets(word); //<- `gets` is unsafe. Better not use it.
    strcpy(words,word);
    printf("Reversed:");
    printf("%s\n", strrev(words));
    if( strcmp(word,words) == 0 ) {
        printf("It is a palindrome word\n");
    } else {
        printf("It is not a palindrome word\n");
    }
    return 0;,
}

If you want to practice C-programming with Xcode, you should better find a better textbook which is written for Standard C.