How to clear buffer after reading an input in c? - c

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
This is a program to store a college/school student list, I want to know how to improve this code, however in the part where I ask the user to enter strings, on the second one I can't enter anything. And if I try to enter more than 2 students, my code doesn't print all the info of the students I entered

Answer 1

struct listaAlu *temp = NULL;
temp = realloc ( alu, size + 2);//<- wrong size
realloc(alu, size + 2) will allocate only 2 bytes in the first run. But you need much more than that. The total size needed is number of elements in the array multiply by the size of the structure. That is sizeof(struct listaAlu) which is about 15 bytes.
realloc( alu, sizeof(struct listaAlu) * (size + 1) );
You don't need to increment the size by 2.
Make sure to check the input at the right places. Clear the input when scanf fails.
To get first name and last name, you can use fgets and strdup
There are major problems with generarDimension and reallocarDimensiones. You may want to leave those out.
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;
}
Edit
realloc does not need to be wrapped in a different function. But for fun purposes, you can do the following:
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 will work here, however nothing is gained. This is just extra lines of code. It's easier and more clear to use realloc directly and do the necessary error handling.
Another method is to pass a reference:
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");
Again this will work, but it just adds more code.
The function generarCadena seems okay. You could simplify as follows:
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();
And use scanf for to read 6 characters (because size of cedula is 7)
printf("Enter 6 characters:\n");
scanf("%6s", alu[size].cedula);
alu[size].cedula[6] = '\0';
clear();
To be safe, make sure the input is null terminated, and clear the buffer.
Or you can use fgets as I used in the example. This will read up to 256 characters:
char buf[256];
fgets(buf, sizeof(buf), stdin);
fgets is sometimes easier because you don't need to clear the buffer. But buf includes end of line character \n, you have to remove \n in this case.