Cannot convert extracted digits to int after extracting from a file

Question

I'm new at C and now I'm trying to extract some digits from a file to a int, but the problem is that although I can extract them from the file and pass them to a char array, I cannot convert them to a int, I don't know why, so here is the code:

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

char *sacardigito(char *vecchar)
{
    char sodigitos[100]={},*ppsodigitos=&sodigitos,*guardarprimeiro=&sodigitos[0];
    do
    {
        if(isdigit(*vecchar)) //if the character inside current address is a digit
        {
            *ppsodigitos=*vecchar; //places the digit inside the char pointer (array sodigitos)
            ppsodigitos++; //increments the pointer
        }
        vecchar++; //increment the pointer address (array vechar)

    }while(*vecchar!='\0');
    printf("\nChar array in function is: %s ",guardarprimeiro); //prints the first position of the array just to make sure only numbers remain
    return guardarprimeiro;
}

int main()
{
    FILE *fp;
    long sonumero;
    int i;
    char vecnumeros[100]={},*retorno;
    fp=fopen("numbers.txt","r"); //open the numbers.txt file
    fgets(vecnumeros,100,(FILE *)fp); //this line passes everything inside to file to a char array
    printf("%s",vecnumeros); //print the char array in order to verify everything is ok
    fclose(fp); //closes the file
    retorno=sacardigito(vecnumeros); //sends to function to retrieve the digits
    printf("\nChar array in main is: %s",retorno); //prints the retuned array
    sscanf(retorno,"%ld",&sonumero); //Convert teh array to digits, I used this function from this user --> http://stackoverflow.com/questions/10204471/convert-char-array-to-a-int-number-in-c
    printf("\nThe numbers in the file are the following: %ld",sonumero); //Now it gives me the error, I don't know why
    return 0;
}

What puzzled me is that the sscanf does not convert the number into a long, but in this other code, it does (it's a function I made just to try to extract only the numbers form a char, and then those numbers from a char into a string):

char *validacao(char *numeros)
{
    char digitos[100]={},*digitospp=&digitos,*inicio=&digitos;
    do
    {
        if(isdigit(*numeros))
        {
            *digitospp=*numeros;
            digitospp++;
        }
        numeros++;

    }while(*numeros!='\0');
    return inicio;


}
int main()
{
    char numeros[100],*retorno;
    long numeroemint;
    setvbuf(stdout, NULL, _IONBF, 0);   //  Necessario  no  eclipse
    printf("Introduza um numero --> ");
    fgets(numeros,20,stdin);
    retorno=validacao(numeros);
    printf("\nO vector de chars e %s",retorno);
    sscanf(retorno,"%ld",&numeroemint); //esta linha transforma o vector de carcacteres numa variável tipo int
    printf("\nO numero %s so com os digitos e --> %ld",numeros,numeroemint);
    return 0;
}

What am I missing?

Kind Regards.

EDIT:

So I changed the code as R Sahu suggested and it works, but I've a question. Another user said something about undefined behavior and automatic variables and pointers... Can someone clarify that? So, isn't a pointer just a memory address, and can't I return a memory address from a function in order for later use?

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

long sacardigito(char *vecchar)
{
    long numeroemint;
    char sodigitos[100]={},*ppsodigitos=&sodigitos[0],*guardarprimeiro=&sodigitos[0];
    do
    {
        if(isdigit(*vecchar)) //if the character inside current address is a digit
        {
            *ppsodigitos=*vecchar; //places the digit inside the char pointer (array sodigitos)
            ppsodigitos++; //increments the pointer
        }
        vecchar++; //increment the pointer address (array vechar)

    }while(*vecchar!='\0');
    printf("\nChar array in function is: %s ",guardarprimeiro); //prints the first position of the array just to make sure only numbers remain
    sscanf(guardarprimeiro, "%ld", &numeroemint);
    printf("\nChar array in function after sscanf is: %ld ",numeroemint);
    return numeroemint;
}

int main()
{
    FILE *fp;
    long retorno;
    char vecnumeros[100]={};
    fp=fopen("numbers.txt","r"); //open the numeros.txt file
    fgets(vecnumeros,100,(FILE *)fp); //this line passes everything inside to file to a char array
    printf("%s",vecnumeros); //print the char array in order to verify everything is ok
    fclose(fp); //closes the file
    retorno=sacardigito(vecnumeros); //sends to function to retrieve the digits
    printf("\nChar array in main is: %ld",retorno); //prints the retuned array
    printf("\nThe numebrs in the file are the following: %ld",retorno); //Now it gives me the error
    return 0;
}

Best Regards.

Answer 1

You have two problems right off the bat. First you're returning a pointer to an array that only exists for the lifetime of the function, causing undefined behavior. Second you're not terminating the string with a null character.

Answer 2

You already know the problem from the comment left by @WhozCraig.

My suggestion to resolve the problem:

Change the return value of sacardigito to long

long sacardigito(char *vecchar)
{
    long numeroemint;
    char sodigitos[100]={},*ppsodigitos=&sodigitos,*guardarprimeiro=&sodigitos[0];
    do
    {
        if(isdigit(*vecchar)) //if the character inside current address is a digit
        {
            *ppsodigitos=*vecchar; //places the digit inside the char pointer (array sodigitos)
            ppsodigitos++; //increments the pointer
        }
        vecchar++; //increment the pointer address (array vechar)

    }while(*vecchar!='\0');
    printf("\nChar array in function is: %s ",guardarprimeiro); //prints the first position of the array just to make sure only numbers remain
    sscanf(guardarprimeiro, "%ld", &numeroemint);
    return numeroemint;
}