IOS NSLog将日志写入文件中

1.首先获取要写入文件的路径
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentDirectory = [paths objectAtIndex:0];
NSString *fileName = [NSString stringWithFormat:@"log.txt"];// 注意不是NSData!
NSString *logFilePath = [documentDirectory stringByAppendingPathComponent:fileName];
// 先删除已经存在的文件
NSFileManager *defaultManager = [NSFileManager defaultManager];
[defaultManager removeItemAtPath:logFilePath error:nil];

// 将log输入到文件

2.重定向输出
freopen([logFilePath cStringUsingEncoding:NSASCIIStringEncoding], "a+", stdout);
freopen([logFilePath cStringUsingEncoding:NSASCIIStringEncoding], "a+", stderr);