Trying to read an unknown string length from a file using fgetc()

Question

So yeah, saw many similar questions to this one, but thought to try solving it my way. Getting huge amount of text blocks after running it (it compiles fine).

Im trying to get an unknown size of string from a file. Thought about allocating pts at size of 2 (1 char and null terminator) and then use malloc to increase the size of the char array for every char that exceeds the size of the array.

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

int main()
{
    char *pts = NULL;
    int temp = 0;

    pts = malloc(2 * sizeof(char));
    FILE *fp = fopen("txtfile", "r");
    while (fgetc(fp) != EOF) {
        if (strlen(pts) == temp) {
            pts = realloc(pts, sizeof(char));
        }
        pts[temp] = fgetc(fp);
        temp++;
    }

    printf("the full string is a s follows : %s\n", pts);
    free(pts);
    fclose(fp);

    return 0;
}

Answer 1

You probably want something like this:

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

#define CHUNK_SIZE 1000               // initial buffer size

int main()
{
  int ch;                             // you need int, not char for EOF
  int size = CHUNK_SIZE;

  char *pts = malloc(CHUNK_SIZE);
  FILE* fp = fopen("txtfile", "r");

  int i = 0;
  while ((ch = fgetc(fp)) != EOF)     // read one char until EOF 
  {
    pts[i++] = ch;                    // add char into buffer

    if (i == size + CHUNK_SIZE)       // if buffer full ...
    {
      size += CHUNK_SIZE;             // increase buffer size
      pts = realloc(pts, size);       // reallocate new size
    }
  }

  pts[i] = 0;                        // add NUL terminator

  printf("the full string is a s follows : %s\n", pts);
  free(pts);
  fclose(fp);

  return 0;
}

Disclaimers:

1. this is untested code, it may not work, but it shows the idea
2. there is absolutely no error checking for brevity, you should add this.
3. there is room for other improvements, it can probably be done even more elegantly

Answer 2

Leaving aside for now the question of if you should do this at all:

You're pretty close on this solution but there are a few mistakes

while (fgetc(fp) != EOF) {

This line is going to read one char from the file and then discard it after comparing it against EOF. You'll need to save that byte to add to your buffer. A type of syntax like while ((tmp=fgetc(fp)) != EOF) should work.

pts = realloc(pts, sizeof(char));

Check the documentation for realloc, you'll need to pass in the new size in the second parameter.

pts = malloc(2 * sizeof(char));

You'll need to zero this memory after acquiring it. You probably also want to zero any memory given to you by realloc, or you may lose the null off the end of your string and strlen will be incorrect.

---

But as I alluded to earlier, using realloc in a loop like this when you've got a fair idea of the size of the buffer already is generally going to be non-idiomatic C design. Get the size of the file ahead of time and allocate enough space for all the data in your buffer. You can still realloc if you go over the size of the buffer, but do so using chunks of memory instead of one byte at a time.

Answer 3

Probably the most efficient way is (as mentioned in the comment by Fiddling Bits) is to read the whole file in one go (after first getting the file's size):

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/stat.h>

int main()
{
    size_t nchars = 0; // Declare here and set to zero...
    // ... so we can optionally try using the "stat" function, if the O/S supports it...
    struct stat st;
    if (stat("txtfile", &st) == 0) nchars = st.st_size;

    FILE* fp = fopen("txtfile", "rb"); // Make sure we open in BINARY mode!
    if (nchars == 0) // This code will be used if the "stat" function is unavailable or failed ...
    {
        fseek(fp, 0, SEEK_END); // Go to end of file (NOTE: SEEK_END may not be implemented - but PROBABLY is!)
    //  while (fgetc(fp) != EOF) {} // If your system doesn't implement SEEK_END, you can do this instead:
        nchars = (size_t)(ftell(fp)); // Add one for NUL terminator
    }
    char* pts = calloc(nchars + 1, sizeof(char));

    if (pts != NULL)
    {
        fseek(fp, 0, SEEK_SET); // Return to start of file...
        fread(pts, sizeof(char), nchars, fp); // ... and read one great big chunk!
        printf("the full string is a s follows : %s\n", pts);
        free(pts);
    }
    else
    {
        printf("the file is too big for me to handle (%zu bytes)!", nchars);
    }
    fclose(fp);
    return 0;
}

On the issue of the use of SEEK_END, see this cppreference page, where it states:

• Library implementations are allowed to not meaningfully support SEEK_END (therefore, code using it has no real standard portability).

On whether or not you will be able to use the stat function, see this Wikipedia page. (But it is now available in MSVC on Windows!)