Hello, I’m trying to do a data structures exercise in c, but there’s an error and I can’t find it. Follow the statement and the code

Question

4. Write in C a system for enrollment and visualization of students.

/// To store all students, implement a dynamic vector of pointers for structs, with a size of 100 records

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

#define qtdalunos 1

typedef struct{

char* nome;
char* matricula;
char* nascimento;
char* rg;

}aluno;

int i;

int main(){


aluno* alunos = (aluno*)malloc(sizeof(aluno)*100);

alunos->nome = (char*)malloc(50);
alunos->matricula = (char*)malloc(20);
alunos->nascimento = (char*)malloc(10);
alunos->rg = (char*)malloc(20);

void lerdados(aluno dados[qtdalunos]);
void printdados(aluno dados[qtdalunos]);

aluno classe[qtdalunos];

lerdados(classe);
printdados(classe);

free(alunos->nome);
free(alunos->matricula);
free(alunos->nascimento);
free(alunos->rg);
free(alunos);

alunos->nome = NULL;
alunos->matricula = NULL;
alunos->nascimento = NULL;
alunos->rg = NULL;
alunos = NULL;

getchar();
return 0;
}
void lerdados(aluno dados[qtdalunos]){

printf("\t\t Nome \\ Matricula \\ Data de Nascimento \\ RG ");

for(i=0; i<qtdalunos; i++){
    printf("\nInforme os dados do aluno %d:\n", i+1);
    scanf("%s %s %s %s", &dados->nome, &dados->matricula, &dados->nascimento, &dados->rg);
}

} void printdados(student data[qtdalunos]){

printf("\t\t Nome\\Matricula\\Data de Nascimento\\Nota Final\\Frequencia\n");

for(i=0; i<qtdalunos; i++){
    printf("%s %s %s %s \n", &dados->nome, &dados->matricula, &dados->nascimento, &dados->rg);
}
}

Answer 1

There are several errors in your code and, as you have not specified any in particular, we will step by step by each.

1. Prototype of declared functions within the main function body

You are declaring the prototype of the functions lerdados and printdados within the main function body (main). Although some compilers accept this statement, the most appropriate is to declare the prototypes in one place before the function main and not inside it.

2. Passing parameters to functions

The statement void lerdados(aluno dados[qtdalunos]) says that you will pass the array by value, which will generate problems in future allocations and releases for this vector. The correct one here would be to pass the array by reference and add an extra parameter informing the amount of positions that exist in it. The syntax would look like in the example below:

void lerdados(aluno dados[], int qtde);
void printdados(aluno dados[], int qtde);

3. Memory allocation outside the loop

aluno* alunos = (aluno*)malloc(sizeof(aluno)*100);
alunos->nome = (char*)malloc(50);
alunos->matricula = (char*)malloc(20);
alunos->nascimento = (char*)malloc(10);
alunos->rg = (char*)malloc(20);
lerdados(classe);

You declare an array of 100 positions, initialize the first position and call the function lerdadoswhich will potentially try to read n positions, but passes the vector by value and not by reference. In addition, if you are going to read the data inside the loop, each student should preferably be allocated here as well.

void lerdados(aluno dados[qtdalunos]){
  printf("\t\t Nome \\ Matricula \\ Data de Nascimento \\ RG ");
  for(i=0; i<qtdalunos; i++){
      printf("\nInforme os dados do aluno %d:\n", i+1);
      scanf("%s %s %s %s", &dados->nome, &dados->matricula, &dados->nascimento, &dados->rg);
}

4. Data reading

Here you try to read 4 strings, but the syntax is wrong.

scanf("%s %s %s %s", &dados->nome, &dados->matricula, &dados->nascimento, &dados->rg)

See, the %s need a buffer address to put the text read by scanf. The syntax used (&dados->nome, for example) will always return the address of the first record. You would have to take into account the index of the loop. For example:&dados[i].nome.

5. memory release outside the loop

The same thing as the previous functions, you are only releasing 1 vector item. The program ends leaving the other 99 pointers to students still allocated.

I know it’s a lot of stuff and the C language can be a little confusing at first, but don’t be discouraged. Follow a functional version of the code with all the corrections suggested above. Good studies.

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

#define QT_ALUNOS 2
#define MAX_ALUNOS 100
typedef struct {
    char* nome;
    char* matricula;
    char* nascimento;
    char* rg;
}aluno;

void lerdados(aluno dados[], int qtde);
void printdados(aluno dados[], int qtde);
void liberardados(aluno dados[], int qtde);

int main() {

    setlocale(LC_ALL, "pt-BR");  //para funcionar acentos e o ç

    aluno* classe = (aluno*) malloc(sizeof(aluno) * MAX_ALUNOS);
    lerdados(classe, QT_ALUNOS);
    printdados(classe, QT_ALUNOS);
    liberardados(classe, QT_ALUNOS);

    getchar();
    return 0;
}

void alocar_aluno(aluno *a) {
    a->nome = (char*)malloc(50);
    a->matricula = (char*)malloc(20);
    a->nascimento = (char*)malloc(10);
    a->rg = (char*)malloc(20);
}

void liberar_aluno(aluno* a) {
    free(a->nome);
    free(a->matricula);
    free(a->nascimento);
    free(a->rg);

    a->nome = NULL;
    a->matricula = NULL;
    a->nascimento = NULL;
    a->rg = NULL;
}

void lerdados(aluno dados[], int qtde){
    int i;  
    for (i = 0; i < qtde; i++) {
        aluno *item = &dados[i];
        alocar_aluno( item );
        printf("Informe os dados do aluno %i de %i:\n", i+1, qtde);
        printf("\tNome           : "); scanf("%s", item->nome);
        printf("\tMatrícula      : "); scanf("%s", item->matricula);
        printf("\tDt. Nascimento : "); scanf("%s", item->nascimento);
        printf("\tRG             : "); scanf("%s", item->rg);
        printf("\n");
    }
}

void printdados(aluno dados[], int qtde) {
    int i;
    printf("Listando os %i alunos da classe:\n", qtde);
    for (i = 0; i < qtde; i++) {
        printf("\t%i\tNome : %s, Matríula: %s, Dt. Nascimento: %s, RG: %s\n ", i,
            dados[i].nome,
            dados[i].matricula,
            dados[i].nascimento,
            dados[i].rg);
    }
}

void liberardados(aluno dados[], int qtde) {
    int i;  
    for (i = 0; i < qtde; i++) {
        liberar_aluno(&dados[i]);
    }
}