arrays - 初始化从指针目标类型中丢弃 ‘const’ 限定符

Question

我有一个包含字符名称的数组结构。我想使用 qsort 按字母顺序对它们进行排序，但是我一直收到一条错误消息，说“初始化丢弃了指针目标类型中的‘const’限定符”。我相信我的 cmpmi() 函数和 qsort 参数是正确的。任何帮助是极大的赞赏!
我的错误是:

gcc -std=gnu11 -Werror -Wall -o main main.c -lm -g
main.c: In function ‘compmi’:
main.c:18:25: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
  const student_t **p1 = a;
                         ^
main.c:19:25: error: initialization discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
  const student_t **p2 = b;
                         ^
cc1: all warnings being treated as errors
makefile:2: recipe for target 'main' failed
make: *** [main] Error 1

这是我的 qsort 功能:

int compmi(const void *a, const void *b)
{
    const student_t **p1 = a;
    const student_t **p2 = b;
    return strcmp((*p1)->name, (*p2)->name);
}

代码:

int main(int argc, char **argv) {
    unsigned numrecords;
    int i = 0;
    int length_multiplier = 1;
    char *lettergrade;
    //char *input = NULL;
    //char *pItem;
    
    student_t **students = NULL;
    // OPENS THE FILE IN BINARY
    FILE *input_file;
    input_file = fopen("input.bin", "rb");
    // READS THE NUMBER OF RECORDS
    fread(&numrecords, sizeof(u_int32_t), 1, input_file);
    
    // LOOPING THROUGH EACH ENTRY
    for(i = 0; i <= numrecords; i++)
    {
        // ALLOCATES MEMORY
        students = realloc(students, sizeof(student_t *) * length_multiplier);
        students[i] = malloc(sizeof(student_t));
        
        students[i]->name = malloc(sizeof(student_t)* 20);
        fread(students[i]->name, sizeof(student_t), 20, input_file);//READ NAME
        fread(&students[i]->gpa, sizeof(student_t), 1, input_file); // READ GPA
        fread(&students[i]->age, sizeof(u_int32_t), 1, input_file);// READ AGE
        
        length_multiplier++;
        
    }
    //SORTING WITH QSORT
    qsort(*students, numrecords, sizeof(student_t *), compmi);

    // PRINTING OUTPUT
    for(i = 0; i < length_multiplier - 2 ; i++)
    {
        printf("%i of %d:\n", i + 1, numrecords);
        printf("Name: %s\n", students[i]->name);
        //printf("UPDATED GPA USE THIS: %.1f\n", students[i]->gpa);
        printf("GPA:  %.1f \n", students[i]->gpa);
        printf("Age:  %i\n", students[i]->age);
        printf("\n");
    }
    
    // FREEING MEMORY
    for(i = 0; i < length_multiplier; i++)
    {
        free(students[i]);
        
    }
    free(students);
    fclose(input_file);
    
    return 0;
}

Answer 1

变量 a指向 const限定类型，但 p1不指向 const类型(但它指向的是什么)。您需要添加 const *之间s。

int compmi(const void *a, const void *b)
{
    const student_t * const *p1 = a;
    const student_t * const *p2 = b;
    return strcmp((*p1)->name, (*p2)->name);
}