Warning: Format string is not a string literal [du

问题:

This question already has an answer here:

• Warning: “format not a string literal and no format arguments” 11 answers

I am getting "Format string is not string literal" warning from following line

NSString *formattedString = [[NSString alloc] initWithFormat:format arguments:valist];

I using this in following function

- (void)logMessage:(NSString *)format
         level:(LoggingLevel)level
withParameters:(va_list)valist {
         if (level >= self.loggingLevel) {
              NSString *formattedString = [[NSString alloc] initWithFormat:format arguments:valist];        
         } 

Any idea how to fix this ? I am using Xcode 4.6.3

回答1:

If you tell the compiler that your method has a format-like argument, using the NS_FORMAT_FUNCTION macro:

- (void)logMessage:(NSString *)format
         level:(LoggingLevel)level
withParameters:(va_list)valist NS_FORMAT_FUNCTION(1,0) {
         if (level >= self.loggingLevel) {
              NSString *formattedString = [[NSString alloc] initWithFormat:format arguments:valist];        
         } 
}

then

• the compiler warning in your method goes away, but
• you get a warning if you call your method with a format string which is not a string literal.

Example:

NSString *abc = @"foo %@ bar";
[self logMessage:abc level:7 withParameters:NULL];

warning: format string is not a string literal [-Wformat-nonliteral]
[self logMessage:abc level:7 withParameters:NULL];
                 ^~~

ADDED: The same applies to the functions mentioned in your comments. They should also be "tagged" with NS_FORMAT_FUNCTION:

+ (void)logVeryFineWithFormat:(NSString *)format, ... NS_FORMAT_FUNCTION(1,2)
{
    va_list ap;
    va_start(ap, format);
    [[self sharedInstance] logMessage:format level:VERY_FINE withParameters:ap];
    va_end(ap);
}

+ (void)say:(NSString *)formatstring, ... NS_FORMAT_FUNCTION(1,2)
{
    va_list arglist;
    va_start(arglist, formatstring);
    // This is not needed: 
    // NSString *litralString = [NSString stringWithFormat:@"%@",formatstring];
    NSString *statement = [[NSString alloc] initWithFormat:formatstring arguments:arglist];
    va_end(arglist);
    [ModalAlert ask:statement withCancel:@"Okay" withButtons:nil];
}

回答2:

Suppress it using:

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wformat-nonliteral"

- (void)logMessage:(NSString *)format
         level:(LoggingLevel)level
withParameters:(va_list)valist {
         if (level >= self.loggingLevel) {
              NSString *formattedString = [[NSString alloc] initWithFormat:format arguments:valist];        
         } 

#pragma clang diagnostic pop

回答3:

The format string should be @"something with some format specifiers to be replaced by the varargs" just like in stringWithFormat: or in NSLog

Remember that @ has a special meaning everywhere in Objective-C. It is a sigil indicating a compiler directive. The compiler will know what to do with the following token ( expanding it or converting contents, sometimes looking for a matching paired directive like @end

回答4:

It may be the right solutio for you just to suppress the warning, as @trojanfoe suggests. It is just that the warning is there for a reason.

In the event that your format string does not match the number and/or type of your arguments, you application may break on runtime. When you provide the format as string literal such as @"My output is: %@" ... then you enable the compiler to check upon types and numer of arguments.

There may be good reasons for using a string varaible as format. However, in that case you are on your own when it comes to avoiding this type of error.

Anyway, I'd suggest re-thinking your algorithm in favor of using string literals.