Apple's Extensions To C中：

The power of a closure is its ability to reference local variables and act upon them while executing in a different activation frame. C API often handles this by allowing a context pointer to be passed in and passed along to a corresponding function and having the custom function treat the opaque context value as a custom structure with appropriate parameterization (see qsort_r). This is cumbersome.

和Blocks Programming Topics中：

In many cases, you don’t need to declare block variables; instead you simply write a block literal inline where it’s required as an argument. The following example uses the qsort_b function. qsort_b is similar to the standard qsort_r function, but takes a block as its final argument.

都有對qsort_b和qsort_r的舉例說明，這篇文章是介紹這兩個函數(shù)的使用方法：

輸出char數(shù)組：

- (void)_print_char_array:(char **)charArray len:(int)len
{
    for (int i = 0; i < len; i++) {
        char *tmp = charArray[i];
        NSLog(@"%s", tmp);
    }
}

qsort_b的使用

- (void)test_qsort_b
{
    char *myCharacters[3] = { "TomJohn", "George", "Charles Condomine" };
    NSLog(@"qsort_b before");
    [self _print_char_array:myCharacters len:3];
    qsort_b(myCharacters, 3, sizeof(char *), ^(const void *l, const void *r) {
        char *left = *(char **)l;
        char *right = *(char **)r;
        return strncmp(left, right, 1);
    });
    NSLog(@"qsort_b after");
    [self _print_char_array:myCharacters len:3];
}

qsort_b最后一個參數(shù)就是Objective-C的block

qsort_r的使用

typedef struct RXBlockQSortRParam {
    int a;
    int b;
}RXBlockQSortRParam;

static int _s_compar (void *tmp, const void *l, const void *r) {
    RXBlockQSortRParam *param = (RXBlockQSortRParam *)tmp;
    param->a = 10;
    param->b = 100;
    char *left = *(char **)l;
    char *right = *(char **)r;
    return strncmp(left, right, 1);
}
@implementation RXBlockQSortBRObject
- (void)test_qsort_r
{
    char *myCharacters[3] = { "TomJohn", "George", "Charles Condomine" };
    RXBlockQSortRParam param = {0, 0};
    [self _print_char_array:myCharacters len:3];
    NSLog(@"qsort_r before a:%zd, b:%zd", param.a, param.b);
    
    qsort_r(myCharacters, 3, sizeof(char *), &param, _s_compar);
    
    [self _print_char_array:myCharacters len:3];
    NSLog(@"qsort_r after a:%zd, b:%zd", param.a, param.b);
}
@end
// output
qsort_r before a:0, b:0
qsort_r after a:10, b:100

qsort_b的第四個參數(shù)是額外參數(shù)，是一個void *的指針，在iOS的Framework中有很多這種方式進行參數(shù)傳遞的。