用c语言写一个学生考勤系统，包含以下功能1、 录入学生的缺课记录。 2、 修改某个学生的缺课记录。 3、 查询某个学生的缺课情况。 统计某段时间内，旷课学生姓名及旷课次数，按旷课次数由多到少排序。

好的，这是一个比较复杂的系统，需要用到文件读写功能和字符串处理函数等。以下是一个简单的实现，供参考：

#include <stdio.h>
#include <string.h>

#define MAX_STUDENTS 100
#define MAX_NAME_LEN 20
#define MAX_DATE_LEN 10

typedef struct {
    char name[MAX_NAME_LEN];
    char date[MAX_DATE_LEN];
} AbsenceRecord;

typedef struct {
    char name[MAX_NAME_LEN];
    AbsenceRecord records[30]; // 最多记录30天的缺勤情况
    int num_records;
} Student;

Student students[MAX_STUDENTS];
int num_students = 0;

// 向文件中写入所有学生的缺勤情况
void saveData() {
    FILE *fp = fopen("data.txt", "w");
    if (fp == NULL) {
        printf("Error opening file!\n");
        return;
    }

    for (int i = 0; i < num_students; i++) {
        fprintf(fp, "%s %d\n", students[i].name, students[i].num_records);
        for (int j = 0; j < students[i].num_records; j++) {
            fprintf(fp, "%s %s\n", students[i].records[j].name, students[i].records[j].date);
        }
    }

    fclose(fp);
}

// 从文件中读取所有学生的缺勤情况
void loadData() {
    FILE *fp = fopen("data.txt", "r");
    if (fp == NULL) {
        printf("Error opening file!\n");
        return;
    }

    while (!feof(fp)) {
        char name[MAX_NAME_LEN];
        int num_records;
        fscanf(fp, "%s %d\n", name, &num_records);

        strcpy(students[num_students].name, name);
        students[num_students].num_records = num_records;

        for (int i = 0; i < num_records; i++) {
            char date[MAX_DATE_LEN];
            fscanf(fp, "%s %s\n", students[num_students].records[i].name, date);
            strcpy(students[num_students].records[i].date, date);
        }

        num_students++;
    }

    fclose(fp);
}

// 录入学生的缺课记录
void addRecord() {
    char name[MAX_NAME_LEN];
    printf("Enter student name: ");
    scanf("%s", name);

    int found = 0;
    for (int i = 0; i < num_students; i++) {
        if (strcmp(students[i].name, name) == 0) {
            printf("Enter date (YYYY-MM-DD): ");
            scanf("%s", students[i].records[students[i].num_records].date);
            strcpy(students[i].records[students[i].num_records].name, name);
            students[i].num_records++;
            printf("Record added.\n");
            found = 1;
            break;
        }
    }

    if (!found) {
        strcpy(students[num_students].name, name);
        printf("Enter date (YYYY-MM-DD): ");
        scanf("%s", students[num_students].records[0].date);
        strcpy(students[num_students].records[0].name, name);
        students[num_students].num_records = 1;
        num_students++;
        printf("Record added.\n");
    }

    saveData();
}

// 修改某个学生的缺课记录
void editRecord() {
    char name[MAX_NAME_LEN];
    printf("Enter student name: ");
    scanf("%s", name);

    int found = 0;
    for (int i = 0; i < num_students; i++) {
        if (strcmp(students[i].name, name) == 0) {
            printf("Enter date to edit (YYYY-MM-DD): ");
            char date[MAX_DATE_LEN];
            scanf("%s", date);

            for (int j = 0; j < students[i].num_records; j++) {
                if (strcmp(students[i].records[j].date, date) == 0) {
                    printf("Enter new date (YYYY-MM-DD): ");
                    scanf("%s", students[i].records[j].date);
                    printf("Record updated.\n");
                    found = 1;
                    break;
                }
            }

            if (!found) {
                printf("Record not found.\n");
            }

            break;
        }
    }

    if (!found) {
        printf("Student not found.\n");
    }

    saveData();
}

// 查询某个学生的缺课情况
void queryRecord() {
    char name[MAX_NAME_LEN];
    printf("Enter student name: ");
    scanf("%s", name);

    int found = 0;
    for (int i = 0; i < num_students; i++) {
        if (strcmp(students[i].name, name) == 0) {
            printf("Absence records for %s:\n", name);
            for (int j = 0; j < students[i].num_records; j++) {
                printf("%s\n", students[i].records[j].date);
            }
            found = 1;
            break;
        }
    }

    if (!found) {
        printf("Student not found.\n");
    }
}

// 统计某段时间内旷课学生姓名及旷课次数，按旷课次数由多到少排序
void report() {
    char start_date[MAX_DATE_LEN], end_date[MAX_DATE_LEN];
    printf("Enter start date (YYYY-MM-DD): ");
    scanf("%s", start_date);
    printf("Enter end date (YYYY-MM-DD): ");
    scanf("%s", end_date);

    int num_absences[MAX_STUDENTS] = {0};
    for (int i = 0; i < num_students; i++) {
        for (int j = 0; j < students[i].num_records; j++) {
            if (strcmp(students[i].records[j].date, start_date) >= 0 &&
                strcmp(students[i].records[j].date, end_date) <= 0) {
                num_absences[i]++;
            }
        }
    }

    // 冒泡排序
    for (int i = 0; i < num_students - 1; i++) {
        for (int j = 0; j < num_students - i - 1; j++) {
            if (num_absences[j] < num_absences[j+1]) {
                int temp = num_absences[j];
                num_absences[j] = num_absences[j+1];
                num_absences[j+1] = temp;

                Student temp_student = students[j];
                students[j] = students[j+1];
                students[j+1] = temp_student;
            }
        }
    }

    printf("Absentee report from %s to %s:\n", start_date, end_date);
    for (int i = 0; i < num_students; i++) {
        if (num_absences[i] > 0) {
            printf("%s: %d\n", students[i].name, num_absences[i]);
        }
    }
}

int main() {
    loadData();

    while (1) {
        printf("Enter command (add/edit/query/report/exit): ");
        char command[10];
        scanf("%s", command);

        if (strcmp(command, "add") == 0) {
            addRecord();
        } else if (strcmp(command, "edit") == 0) {
            editRecord();
        } else if (strcmp(command, "query") == 0) {
            queryRecord();
        } else if (strcmp(command, "report") == 0) {
            report();
        } else if (strcmp(command, "exit") == 0) {
            break;
        } else {
            printf("Invalid command.\n");
        }
    }

    return 0;
}

注意，这段代码中的数据持久化方式只是简单地将数据写入文件中，实际应用中可能需要使用数据库等更为高效的方式来存储数据。此外，代码中还有一些不足之处，比如没有对输入进行严格的校验等，需要根据实际需求进行改进。