c - How to use fscanf() for two strings (of ANY length) and dynamically allocate/de-allocate the memory properly

Question

I need to read an input .txt file and print out two separate strings from each line in the file. I used a while loop and a fscanf function to get each string and ignore blank space between. If the strings in a line of the input file are too long, I get a segmentation fault. However, I am also getting a munmap_chunk(): invalid pointer error when I run my executable.

If I don't allocate memory for string1 and string2, fscanf doesn't work properly. I believe fscanf is changing the pointers to string1 and string2, which is causing the munmap_chunk() error. However, I need to de-allocate the memory I gave string1 and string2 so I don't have memory leaks.

How do I scan this file for strings (of ANY length) and de-allocate the memory properly?

int main(int argc, char *argv[]) 
{
  char *string1;
  char *string2;
  string1 = (char *)malloc(sizeof(string1)); //these strings need memory allocated for the fscanf to function properly
  string2 = (char *)malloc(sizeof(string2)); 
  FILE* file = fopen(argv[1], "r");
  
  while (fscanf(file, "%s %s", string1, string2) != EOF)
    {
      printf("%s %s
", string1, string2);
    }
  fclose(file);

  //Deallocating memory
  free(string1);
  free(string2);
  return 0;
}

question from:https://stackoverflow.com/questions/66055845/how-to-use-fscanf-for-two-strings-of-any-length-and-dynamically-allocate-de

Answer 1

'fscanf' does not change pointers but it can corrupt memory if you do not allocate enough space for your input.

And you are not allocating the memory correctly: string1 and string2 are pointers, so all you are allocating is a size of a pointer (4 or 8 bytes depending on your system).

If you need to read a line from a file and you do not know the maximum length of the line in advance, you can not use fscanf.

You need to allocate a starting buffer, say something like:

string1 = malloc(512 * sizeof(char));

Were 512 is an arbitrary but reasonably large length for a line. You then use fread to read one byte at a time in a loop, and check for end of line (usually ' ').

You must also count how much you read, and if the line is longer than 512 bytes, use realloc to increase the size of your buffer, like so:

if (bytesRead == (string1Size - 1) && curByte != '
') {
    string1Size += 512;
    string1 = realloc(string1, string1Size);
}

Here, bytesRead is an int variable counting how many bytes you successfully read so far, and string1Size is also int variable used to track the size of string1 buffer.