Error: Conflicting types for 'first'

Question

I made a function where I pass as argument two integers and two pointers of two struct that I created in order to write the Return in a txt file. When running the program, if I give printf() to the function main gives the desired value, but if fprintf() no longer gives this value. How can I solve?

char* primeiraPessoa(int countV, int countF, Visitante *visitante, Funcionario *funcionario){
int hora=0;
char *id;
id = "N/A";
for(int i=0;i<countV;i++){

    if(i==0){

        sprintf(id, "%d", visitante[i].id);
        hora=visitante[i].horaE;

    }else if(hora>visitante[i].horaE){
        sprintf(id, "%d", visitante[i].id);
        hora=visitante[i].horaE;

    }else if(hora==visitante[i].horaE){
        strcat(id,",");
        char idTemp[5];
        sprintf(idTemp, "%d", visitante[i].id);
        strcat(id,idTemp);
    }
}

for(int i=0;i<countF;i++){

    if(hora>funcionario[i].horaE){

        sprintf(id, "%d", funcionario[i].id);
        hora=visitante[i].horaE;

    }else if(hora==funcionario[i].horaE){

        strcat(id,",");
        char idTemp[5];
        sprintf(idTemp, "%d", funcionario[i].id);
        strcat(id,idTemp);

    }
}

return id;
}

To write to txt file I have the function:

void escreveFicheiro(Visitante *visitante,Funcionario *funcionario,int countV,int countF,int countInvalidas,int countBarradas){
char nome [] = "estatisticas.txt";
FILE * fp = fopen(nome, "w");

if (fp) {
    fprintf(fp,"15 - %s\n",primeiraPessoa(countV,countF,visitante,funcionario));
}
    fclose(fp);

}

Answer 1

That one strcat(id,","); will give you trouble. 4 bytes will be allocated to id for the text "N/A" and the null terminator. By doing this you will burst the memory area of the id and start corrupting your program’s memory.

Moreover, id is allocated to the stack. When returning a pointer from something that is allocated on the stack, the result is that you have a pointer to an unallocated memory area, ready to be arbitrarily overwritten by something else.

What you want seems to be to assemble a string with all visitor and employee ids that have a time greater or equal to that of the previous visitor or employee or N/A if there is no.

whereas int goes from -2147483648 to 2147483647, but you’re not interested in the negative side, so you’ll need 10 characters/bytes in the worst case to save a value int. Adding the comma to separate them and the null terminator from the last is 11 characters/bytes for each. In the worst case, you will put the ids of all employees and visitors.

Having a separate function to make the concatenation ends up being much simpler than doing all this within the function primeiraPessoa.

So you can do this:

void concatenar_id(char *c, int id) {
    char temp[11];
    sprintf(temp, "%d", visitante[i].id);
    if (c[0] == 'N') {
        strcpy(c, temp);
    } else {
        strcat(c, ",");
        strcat(c, temp);
    }
}

char* primeiraPessoa(int countV, int countF, Visitante *visitante, Funcionario *funcionario) {
    int hora = 0;
    char *ids = (char *) malloc(11 * (countV + countF));
    strcpy(ids, "N/A");

    for (int i = 0; i < countV; i++) {
        if (i == 0 || hora >= visitante[i].horaE) {
            concatenar_id(ids, visitante[i].id);
            hora = visitante[i].horaE;
        }
    }

    for (int i = 0; i < countF; i++) {
        if (hora >= funcionario[i].horaE) {
            concatenar_id(ids, funcionario[i].id);
            hora = funcionario[i].horaE;
        }
    }

    return ids;
}

In his job escreveFicheiro, you must release the memory allocated with malloc:

void escreveFicheiro(Visitante *visitante, Funcionario *funcionario, int countV, int countF, int countInvalidas, int countBarradas) {
    char nome[] = "estatisticas.txt";
    FILE * fp = fopen(nome, "w");

    if (fp) {
        char *ids = primeiraPessoa(countV, countF, visitante, funcionario);
        fprintf(fp, "15 - %s\n", ids);
        free(ids);
        fclose(fp);
    }
}

In this last function, you are not using the parameters countInvalidas and countBarradas.