Objective-C结构问题

我有一个学生结构，提供以下代码：

#import <Foundation/Foundation.h>
#import "Grades.m"
#import <stdio.h>

struct Student
{
    NSString *myName;
    struct Grades *myGrades;
};

void setName(struct Student *s, NSString *name);
void ssetGrades(struct Student *s, NSString *gradeList);

void setName(struct Student *s, NSString *name)
{
    s->myName = name;
}

void ssetGrades(struct Student *s, NSString *gradeList)
{
    printf("\n\nWorking\n\n");
    setGrades(s->myGrades, gradeList);
    printf("\n\nWorking");
}

我有一个提供以下内容的成绩结构：

#import <stdio.h>
#import <Foundation/Foundation.h>

#define null NULL

struct Grades
{
    double sgrades[100];
    int length;
};

void setGrades(struct Grades *grades, NSString *gradeList);
void setGrade(int spot, double grade, struct Grades *grades);

void setGrades(struct Grades *grades, NSString *gradeList)
{
    NSString *a = [gradeList substringToIndex:1];
    NSString *b = [gradeList substringWithRange:NSMakeRange(3, 19)];
    int ln = [a integerValue];

    grades->length = ln;

    double grade;
    int x = 1;
    int prev = 1;
    int y;

    int z = 0;

    for(y=0;y<ln;y++)
    {
        while(x < [b length] && [b characterAtIndex:x] != ' ')
        {
            z++;
            x++;
        }

        NSString *sub = [b substringWithRange:NSMakeRange(prev, z)];
        grade = [sub doubleValue];
        printf("%d %d %d %lf\n", y, prev, z, grade);
        prev += z+1;
        x++;
        z=0;
        setGrade(y, grade, grades);
    }
}

void setGrade(int spot, double grade, struct Grades *grades)
{
    grades->sgrades[spot] = grade;
}

最后，我具有以下主要功能：

#import <Foundation/Foundation.h>
#import "Grades.m"
#import "Student.m"
#import <stdio.h>

int main (int argc, const char * argv[])
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

    struct Grades test; 
    setGrades(&test, @"5 - 90 85 95.5 77.5 88");
    toString(&test);
    printf("\nsum = %lf", getSum(&test));   
    printf("\nnum grades = %d", getNumGrades(&test));   
    printf("\nlow grade = %lf", getLowGrade(&test));        
    printf("\nhigh grade = %lf", getHighGrade(&test));


    struct Student stu;
    setName(&stu, @"Billy Bob");
    ssetGrades(&stu, @"5 - 90 85 95.5 77.5 88");

    [pool release];
    return 0;
}

现在，只要我进入ssetGrades (&stu, @"5 - 90 85 95.5 77.5 88")主线，它就会冻结，并说该程序已停止工作。任何猜测为什么，如果是的话，我该如何解决此错误？

注意：这一切都在Windows 7上的Notepad ++中完成