How to write a string in a stream using physical files?

Question

When inserting in a text file the characters of a string in C, blank spaces are ignored. How should I make long sentences within a string have separate words?

Follows the code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(){
    int op = 10, k;
    char str[100], c, arquivo[20];
    FILE *fluxo;
    do{
        printf("Digite:\n1- Criar arquivo\n2- Inserir no arquivo\n3- Ler arquivo\n4- Fechar fluxo\n5- Remover arquivo");
        scanf("%d",&op);
        switch(op){
            case 1:
                printf("\nDigite o nome do arquivo: \n");
                scanf("%s",arquivo);
                fluxo = fopen(arquivo,"w");
                if(!fluxo)
                    printf("\n***Erro ao abrir/criar arquivo!***\n");
            break;
            case 2:
                printf("\nDigite o texto a ser gravado:\n");    
                scanf("%s",str);
                for (k=0;str[k];k++){
                    c = str[k];
                    putc(c, fluxo);
                }
                    //fprintf(fluxo, "%s",str);             
            break;
            case 3:
                fclose(fluxo);
                fluxo = fopen(arquivo, "r");
                while(!feof(fluxo)){
                    fscanf(fluxo,"%c", &c);
                    printf("%c",c);
                }
            break;
            case 4:
                fclose(fluxo);
            break;
            case 5:
                remove(arquivo);
            break;
        }
    }while(op!=0);
    fclose(fluxo);

}

Answer 1

According to the function documentation scanf:

Whitespace Character: the Function will read and ignore any whitespace characters encountered before the next non-whitespace Character (whitespace characters include Spaces, newline and tab characters -- see isspace).

Freely translating:

"White character: the function will read and ignore any white character found before the next non-white character (white characters include space, new line and tab)."

What happens then is that your reading is being "broken" in the text spaces. Assuming the test "This is a test." the first reading will simply return "This", and the rest will be kept in the stream to return in the next readings "eh", "um" and "test." respectively.

I figured you could include a blank space between each reading, but @C.E.Gesser well recalled that there is no way to know if there are one or more spaces between words. Therefore, I believe that the ideal is to use the fgets indicating the stdin as a source for reading and the maximum size of the data buffer:

fgets(str, 100, stdin);

Answer 2

The problem is that the scanf("%s",str); reads the string and ignores whitespace. A solution to this is " %[^\n]", instead of %s, which will read the entire line written by the user until it finds a character \n, including the spaces.

scanf(" %[^\n]", arquivo);

Or

scanf("%[^\n]%*c", arquivo);

This second adds a %*c to suppress the ENTER that the user will give to confirm the end of the string (reference).

---

In the test I did here with your code, I also changed the scanf("%d",&op); for scanf("%d%*c",&op); for the ENTER after typing the index, "swallow" the chosen options.

---

Note: Consider using fgets instead of scanf, if possible. When using scanf, should be attentive to some details to avoid overflow (as presented here).