Copying array of characters into a text file

Question

I want to write sentence(s) to a text file using fwrite function. So I have to have these as function arguments:

fwrite( const void *restrict buffer, size_t size, size_t count, FILE *restrict stream )

buffer - pointer to the first object in the array to be written
size - size of each object
count - the number of the objects to be written
stream - pointer to the output stream

As the How to dynamically allocate memory space for a string and get that string from user? said, it is a bad practice to waste memory. I read the answer and got an idea to write a code in my way.

My idea is :

to make an array of character and write to its elements
make that array bigger and bigger using malloc and realloc
Writing continues until it reach the EOF

Unfortunately I encounter a problem. Whenever I build and execute the code it gives me this error:

has stopped working

Heres my code:

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

int main(void)
{
    size_t index=0,size=1;
    char ch;
    FILE *fptr;
    fptr=fopen("E:\\Textsample.txt","w");

    /////////////////Checking///////////////
    if(fptr==NULL)
        {
            free(fptr);
            puts("Error occured!\n");
            exit(EXIT_FAILURE);
        }
    /////////////////Checking///////////////

    char *sentence =(char*) malloc(sizeof(char)) ;
    puts("Plaese write :\n");

    while((ch=getchar()) != EOF)
        {
            sentence[index]=ch;
            index++;
            size++;
            realloc(sentence,size);

            /////////////////Checking///////////////
            if(sentence==NULL)
                {
                    printf("Error Occured!\n");
                    free(sentence);
                    exit(EXIT_FAILURE);
                }
            /////////////////Checking///////////////

        }

    //Copying sentnce(s) to the text file.
    fwrite(sentence,sizeof sentence[0],index,fptr);
    free(sentence);
    free(fptr);
    return 0;
}

Nitpick: You don't need to free a NULL ptr in the Checking code.. — Henno Brandsma, Aug 19 '17 at 08:25
"has stopped working"? What? Maybe run in a debugger and see which line it crashes on. — John Zwinck, Aug 19 '17 at 08:25
@Ceeker, you check it is `NULL` so you are sure it is. No resources need to be freed then. — Henno Brandsma, Aug 19 '17 at 08:31
Also, at the end do `close(ptr)` as it is a `FILE *`. No free needed, it's handled by `fclose` — Henno Brandsma, Aug 19 '17 at 08:36
@HennoBrandsma- Pressing 'ctrl+z' but it still gives me that error. — Pouyan, Aug 19 '17 at 08:38
`getchar()` returns an `int` not a `char`, which is important to successfully detect `EOF`. — alk, Aug 19 '17 at 08:49
You might like to spend a few minutes reading this: https://ericlippert.com/2014/03/05/how-to-debug-small-programs/ — alk, Aug 19 '17 at 08:50
@alk- I had recalled that. But I can't cast it in while section. — Pouyan, Aug 19 '17 at 08:51
Crl+Z puts the program in the background. To be sure of right sizes do `sentence[index] =(ch & 0xff);` — Henno Brandsma, Aug 19 '17 at 08:52
No need to cast. Just do `int ch;` instead of `char ch`, or `int result; while ((result = getchar()) != EOF) { ch = result; ...` — alk, Aug 19 '17 at 08:57
`sentence[0]` is a `char`, so this `sizeof sentence[0]` evaluates to the size of a `char`, which is `1`, always. — alk, Aug 19 '17 at 09:02
You might want to handle EOF separately too, before you get a newline.. — Henno Brandsma, Aug 19 '17 at 09:21

Henno Brandsma · Accepted Answer · 2017-08-19T09:10:45.700

You need to write sentence = realloc(sentence, size), I think.

Also , it's more efficient to double the alloc-ed size, every time we need to. Then we can read as many again as we have read already, so we need fewer reallocations. So then size doubles (it is the size of the allocated buffer, while index keeps track of read characters.

So

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

int main(void)
{
    size_t index=0,size=1;
    int ch;
    FILE *fptr;
    fptr=fopen("./bla.txt","w");

    /////////////////Checking///////////////
    if(fptr==NULL)
    {
            fprintf(stderr, "Error occured!\n");
            exit(EXIT_FAILURE);
    }
    /////////////////Checking///////////////

    char *sentence = malloc(size) ;
    puts("Please write, (close with return) :\n");

    while((ch=getchar()) != '\n')
    {
            sentence[index]=(ch & 0xff);
            index++;;
            if (index >= size){
                    sentence = realloc(sentence,2*size);

                    /////////////////Checking///////////////
                    if(sentence==NULL)
                    {
                    fclose(fptr);
                    fprintf(stderr, "Error Occured!\n");
                    free(sentence);
                    exit(EXIT_FAILURE);
                    }
                    /////////////////Checking///////////////
                    size *= 2; /* update size */
            }
    }

    //Copying sentence(s) to the text file.
    fwrite(sentence,1,index,fptr);
    free(sentence);
    fclose(fptr);
    return 0;

}

which works fine on my system.

I did what you've mentioned but it still doesn't work. – Pouyan Aug 19 '17 at 08:43 — Pouyan, Aug 19 '17 at 08:43

score 0 · Answer 2 · answered Aug 19 '17 at 09:18

You only want to write stdin to a file you already have open. So why not cut out the malloc middle man?

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

int main(void)
{
    int ch;
    FILE *fptr;
    fptr=fopen("./bla.txt","w");

    /////////////////Checking///////////////
    if(fptr==NULL)
    {
            fprintf(stderr, "Error occured!\n");
            exit(EXIT_FAILURE);
    }
    /////////////////Checking///////////////

    puts("Please write, (close with return) :\n");

    while((ch=getchar()) != '\n')
    {       
            fputc(ch, fptr);
    }
    //if you want the newline too 
    fputc('\n', fptr);
    fclose(fptr);
    return 0;

}

Copying array of characters into a text file

2 Answers2