Why I get "implicit declaration of function 'PerformXMLXPathQuery'" warning?

Why I get "implicit declaration of function 'PerformXMLXPathQuery'" warning?

NSString *respons开发者_JAVA技巧e = [request responseString];  
NSString* xPathQuery = [NSString stringWithFormat:@"//ExtendedData"];  
NSArray* myArray = PerformXMLXPathQuery([response dataUsingEncoding:NSUTF8StringEncoding ],xPathQuery);

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The warning indicates that compiler cannot find a prototype declared for the PerformXMLXPathQuery function. Check if you have imported the header file where your function was declared.