Logging

Syntax#

• NSLog(@“text to log”); // Basic text log
• NSLog(@“data: %f - %.2f”, myFloat, anotherFloat); // Logging text including float numbers.
• NSLog(@“data: %i”, myInteger); // Logging text including integer number.
• NSLog(@“data: %@”, myStringOrObject); // Logging text referencing another String or any NSObject derived object.

Remarks#

For logging various types of objects and data-types refer to: Objective-C, Format Specifiers

Logging

NSLog(@"Log Message!");
NSLog(@"NSString value: %@", stringValue);
NSLog(@"Integer value: %d", intValue);

The first argument of NSLog is an NSString containing the log message format. The rest of the parameters are used as values to substitute in place of the format specifiers.

The formatting works exactly the same as printf, except for the additional format specifier %@ for an arbitrary Objective-C object. This:

NSLog(@"%@", object);

is equivalent to:

NSLog(@"%s", [object description].UTF8String);

NSLog vs printf

NSLog(@"NSLog message");
printf("printf message\n");

Output:

2016-07-16 08:58:04.681 test[46259:1244773] NSLog message
printf message

NSLog outputs the date, time, process name, process ID, and thread ID in addition to the log message. printf just outputs the message.

NSLog requires an NSString and automatically adds a newline at the end. printf requires a C string and does not automatically add a newline.

NSLog sends output to stderr, printf sends output to stdout.

Some format-specifiers in printf vs NSLog are different. For example when including a nested string, the following differences incur:

NSLog(@"My string: %@", (NSString *)myString);
printf("My string: %s", [(NSString *)myString UTF8String]);

NSLog Output Format

NSLog(@"NSLog message");

The message that gets printed by calling NSLog has the following format when viewed in Console.app:

Logging Variable Values

You shouldn’t call NSLog without a literal format string like this:

NSLog(variable);    // Dangerous code!

If the variable is not an NSString, the program will crash, because NSLog expects an NSString.

If the variable is an NSString, it will work unless your string contains a %. NSLog will parse the % sequence as a format specifier and then read a garbage value off the stack, causing a crash or even executing arbitrary code.

Instead, always make the first argument a format specifier, like this:

NSLog(@"%@", anObjectVariable);
NSLog(@"%d", anIntegerVariable);

Empty message is not printed

When NSLog is asked to print empty string, it omits the log completely.

NSString *name = @"";
NSLog(@"%@", name);  // Resolves to @""

The above code will print nothing.

It is a good practice to prefix logs with labels:

NSString *name = @"";
NSLog(@"Name: %@", name);  // Resolves to @"Name: "

The above code will print:

2016-07-21 14:20:28.623 App[87711:6153103] Name: 

Removing Log Statements from Release Builds

Messages printed from NSLog are displayed on Console.app even in the release build of your app, which doesn’t make sense for printouts that are only useful for debugging. To fix this, you can use this macro for debug logging instead of NSLog.

#ifdef DEBUG
#define DLog(...) NSLog(__VA_ARGS__)
#else
#define DLog(...)
#endif

To use:

NSString *value = @"value 1";
DLog(@"value = %@", value);
// little known fact: programmers look for job postings in Console.app
NSLog(@"We're hiring!"); 

In debug builds, DLog will call NSLog. In release builds, DLog will do nothing.

Using __FUNCTION __

NSLog(@"%s %@",__FUNCTION__, @"etc etc");

Inserts the class and method name into the output:

2016-07-22 12:51:30.099 loggingExample[18132:2971471] -[ViewController viewDidLoad] etc etc

NSLog and BOOL type

There is no format specifier to print boolean type using NSLog. One way to print boolean value is to convert it to a string.

BOOL boolValue = YES;
NSLog(@"Bool value %@", boolValue ? @"YES" : @"NO");

Output:

2016-07-30 22:53:18.269 Test[4445:64129] Bool value YES

Another way to print boolean value is to cast it to integer, achieving a binary output (1=yes, 0=no).

BOOL boolValue = YES;
NSLog(@"Bool value %i", boolValue);

Output:

2016-07-30 22:53:18.269 Test[4445:64129] Bool value 1

Logging NSLog meta data

NSLog(@"%s %d %s, yourVariable: %@", __FILE__, __LINE__, __PRETTY_FUNCTION__, yourVariable);

Will log the file, line number and function data along with any variables you want to log. This can make the log lines much longer, particularly with verbose file and method names, however it can help to speed up error diagnostics.

You can also wrap this in a Macro (store this in a Singleton or where you’ll need it most);

#define ALog(fmt, ...) NSLog((@"%s [Line %d] " fmt), __PRETTY_FUNCTION__, __LINE__, ##__VA_ARGS__);

Then when you want to log, simply call

ALog(@"name: %@", firstName);

Which will give you something like;

 -[AppDelegate application:didFinishLaunchingWithOptions:] [Line 27] name: John

Logging by Appending to a File

NSLog is good, but you can also log by appending to a file instead, using code like:

NSFileHandle* fh = [NSFileHandle fileHandleForWritingAtPath:path];
if ( !fh ) {
    [[NSFileManager defaultManager] createFileAtPath:path contents:nil attributes:nil];
    fh = [NSFileHandle fileHandleForWritingAtPath:path];
}
if ( fh ) {
    @try {
        [fh seekToEndOfFile];
        [fh writeData:[self dataUsingEncoding:enc]];
    }
    @catch (...) {
    }
    [fh closeFile];
}