c - 读取c中的输入后如何清除缓冲区？

Question

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

void generarDimension(int, void **, int);
void reallocarDimensiones(int, void **, int);
void preguntarValor(int *);
char *generarCadena(char *);

typedef struct listaAlu{//this is a list of students
    char cedula[7];//this means id
    char *nombreApellido;//this means name and lastname
    short curso;//this means the year of the student in college/school
    unsigned short anho;//and this means year-E.g: 2017
};

int main(void){
    struct listaAlu *alu = NULL;
    int salir=1;
    char c;
    unsigned int cont = 0;
    unsigned int i;
    size_t size = 0;
    struct listaAlu *temp = NULL;
    while(salir!=0){//Condition until user press exit option
        printf("Agregar Alumno?Ingrese 1 para Agregar, y 0 para salir\n");//This means "Add student?Press 1 to add, and 0 to exit
        preguntarValor(&salir);//this verifies user input
        if(salir==1){//if user adds then it asks info of the student
            cont++;
            if ( NULL == ( temp = realloc ( alu, size + 2))) {//realloc for each character input and '\0'
                fprintf ( stderr, "realloc problem\n");
            }//this is for increasing the number of students
            alu = temp;
            printf("Ingrese Cedula:\n");//This means "Enter id"
            scanf("%s", &(alu+(size))->cedula);
            printf("Ingrese Nombre y Apellido:\n");//This means "Enter name and lastname"
            (alu+(size))->nombreApellido = generarCadena((alu+(size))->nombreApellido);
            printf("Ingrese Curso:\n");//This means enter college/school year
            scanf("%hd", &(alu+(size))->curso);
            printf("Ingrese Anho:\n");//and this means "Enter year"
            scanf("%hd", &(alu+(size))->anho);
            size++;
        }
    }
    for(i=0;i<cont;i++){
        printf("\tAlumno %d\n", i+1);
        printf("Nombre:\t%s\n", (alu+i)->nombreApellido);
        printf("Cedula:\t%s\n", (alu+i)->cedula);
        printf("Curso:\t%d\n", (alu+i)->curso);
        printf("Anho:\t%d\n", (alu+i)->anho);
    }//This prints Everything the user entered
    free(alu);
    return 0;
}//Sorry for writing my program entirely on the main >.<

void generarDimension(int bloques, void **ptr, int tamanho){
    void **ptrAux;
    *ptrAux = malloc(bloques * tamanho);
    if(*ptrAux==NULL){
        printf("No se pudo almacenar memoria\n");
    }else{
            *ptr = *ptrAux;
            free(ptrAux);
        }
}//this I didn't use, but it is for generating an array through a pointer
void reallocarDimensiones(int bloques, void **ptr, int tamanho){
    void **ptrAux;
    *ptrAux = realloc(*ptr,bloques * tamanho);
    if(*ptrAux==NULL){
        printf("No se pudo almacenar memoria\n");
    }else{
            *ptr = *ptrAux;
            free(ptrAux);
        }
}//This I didn't either, but it is for reallocating any given pointer

void preguntarValor(int *n){
    long entero, lector;
    *n=-2;
    do{
        printf("\nSi ingresa otro numero o caracter, vuelva a ingresar opcion\n");
        while(!scanf("%d", &entero)){
            while((lector=getchar()) !='\n' && lector !=EOF );
        }
        *n=entero;
    }while(*n>1 || *n<0);//You can change this parameters according to the numbers you want
}//This subrutine scans only integers between 1 and 0

char *generarCadena(char *scaneado){
    char *temp = NULL;
    int in = 0;
    size_t size = 0;

    while ( '\n' != ( in = getchar ( ))) {//loop until newline
        if ( NULL == ( temp = realloc ( scaneado, size + 2))) {//realloc for each character input and '\0'
            fprintf ( stderr, "realloc problem\n");
            return scaneado;
        }
        scaneado = temp;
        scaneado[size] = in;//set input
        scaneado[size + 1] = '\0';//terminate
        size++;
    }
    return scaneado;
}//This subrutine is for geting an input of chars of any given length

这是一个存储大学/学校学生列表的程序，我想知道如何改进此代码，但是在我要求用户输入字符串的部分中，在第二个部分中我无法输入任何内容。如果我尝试输入超过 2 名学生，我的代码不会打印我输入的学生的所有信息

Answer 1

struct listaAlu *temp = NULL;
temp = realloc ( alu, size + 2);//<- wrong size

realloc(alu, size + 2) 将在第一次运行中仅分配 2 个字节。但你需要的远不止这些。所需的总大小是数组中元素的数量乘以结构的大小。即 sizeof(struct listaAlu) ，大约 15 个字节。

realloc( alu, sizeof(struct listaAlu) * (size + 1) );

您不需要将大小增加 2。

确保检查正确位置的输入。当 scanf 失败时清除输入。

要获取名字和姓氏，您可以使用 fgets 和 strdup

generarDimensiones 和 reallocarDimensiones 存在重大问题。您可能想将它们排除在外。

struct listaAlu{
    char cedula[7];//this means id
    char *nombreApellido;//this means name and lastname
    short curso;//this means the year of the student in college/school
    unsigned short anho;//and this means year-E.g: 2017
};

void clear()
{
    int c;
    while((c = getchar()) != '\n' && c != EOF);
}

int main(void)
{
    struct listaAlu *alu = NULL;
    size_t size = 0;
    while(1)
    {
        alu = realloc( alu, sizeof(struct listaAlu) * (size + 1) );
        if (!alu)
        {
            fprintf(stderr, "realloc problem\n");
            break;
        }

        char buf[256] = { 0 };

        printf("Enter id:\n");
        fgets(buf, sizeof(buf), stdin);
        buf[strcspn(buf, "\r\n")] = 0;
        buf[sizeof(alu[size].cedula) - 1] = 0;
        strcpy(alu[size].cedula, buf);

        printf("Enter name and lastname:\n");
        fgets(buf, sizeof(buf), stdin);
        buf[strcspn(buf, "\r\n")] = 0;
        alu[size].nombreApellido = _strdup(buf);

        for(;;)
        {
            printf("Enter college/school year:\n");
            if (scanf("%hd", &alu[size].curso) == 1)
                break;

            printf("input error\n");
            clear();
        }

        for(;;)
        {
            printf("Enter year:\n");
            if (scanf("%hd", &alu[size].anho) == 1)
                break;

            printf("input error\n");
            clear();
        }

        size++;

        printf("Press 1 to continue\n");
        char input;
        scanf(" %c", &input);
        if(input != '1')
            break;
        clear();
    }

    for(size_t i = 0; i < size; i++)
    {
        printf("Alumno %d\n", i);
        printf("Nombre:\t%s\n", alu[i].nombreApellido);
        printf("Cedula:\t%s\n", alu[i].cedula);
        printf("Curso:\t%hd\n", alu[i].curso);
        printf("Anho:\t%hd\n", alu[i].anho);
    }

    for(size_t i = 0; i < size; i++)
        free(alu[i].nombreApellido);
    free(alu);
    return 0;
}

<小时/> 编辑

realloc 不需要包装在不同的函数中。但出于有趣的目的，您可以执行以下操作:

void* my_realloc1(void *ptr, int size)
{
    ptr = realloc(ptr, size);
    if (!ptr) printf("error\n");  
    return ptr;
}

//usage:
alu = my_realloc1(alu, sizeof(struct listaAlu) * (size + 1));
if (!alu) printf("error\n");  

my_realloc1 在这里可以工作，但是没有任何收获。这只是额外的代码行。直接使用realloc并进行必要的错误处理会更简单、更清晰。

另一种方法是传递引用:

void my_realloc2(void **ptr, int size)
{
    *ptr = realloc(*ptr, size);
}

//usage: (*** Note the address `&` operator ***)
my_realloc2(&alu, sizeof(struct listaAlu) * (size + 1));
if (!alu) printf("error\n");

这同样可以工作，但它只是添加了更多代码。

函数generCadena看起来没问题。您可以简化如下:

char *my_getline()
{
    char *buf = NULL;
    int in = 0;
    size_t size = 0;
    while('\n' != (in = getchar()))
    {
        size++;
        buf = realloc(buf, size);
        buf[size - 1] = (char)in;
    }

    size++;
    buf = realloc(buf, size);
    buf[size - 1] = 0;

    return buf;
}

//usage:
printf("Enter name and lastname:\n");
alu[size].nombreApellido = my_getline();

并使用scanf读取6个字符(因为cedula的大小为7)

printf("Enter 6 characters:\n");
scanf("%6s", alu[size].cedula);
alu[size].cedula[6] = '\0';
clear();

为了安全起见，请确保输入以 null 终止，并清除缓冲区。

或者您可以使用我在示例中使用的fgets。这将读取最多 256 个字符:

char buf[256];
fgets(buf, sizeof(buf), stdin);

fgets 有时更容易，因为您不需要清除缓冲区。但是 buf 包含行结束符 \n，在这种情况下您必须删除 \n。