I'm trying to understand how memory allocation and pointers work, since i find pset5 overwhelming.

I made a simple program that reads characters from an array, and let them be written into both a new text file, and into the terminal.

The programm works, but it is leaking memory. Specifically for each \n encountered in the string, valgrind states that it loses memory in 1 more block. And for each character in the string (of char *c), it states that 1 more byte is leaked.

What am i doing wrong?

Memory leak

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

  int main (void)

     FILE *fp;
     char *c = "One\nTwo\n";

     // Open file for writing (reading and writing works too, we can use 'w+' for that).
     fp = fopen("file.txt", "w");

     // Write data to the file.
     fwrite(c, strlen(c), 1, fp);

     // Seek to the beginning of the file
     fseek(fp, 0, SEEK_SET);

     // close file of the file pointer (the text file).

     // initialize a counter for the amount of characters in the current word that is being read out of the file.
     int char_count = 0;

     // initialize an address for the first character in a string.
     char *buffer_temp_word = NULL;

     // Read and display data, using iterations over each character.
     // Open the file in read mode.
     fp = fopen("file.txt", "r");

     // initiate a for loop.
     // condition 1: getting a character from the fp stream does not equal reaching the end of the file
     // condition 2: the amount of iterations is not above 60 (failsafe against endless loops).
     for (int i = 0; fgetc(fp) != EOF && i <= 60 ; i++)
        //add a counter to the amount of characters currently read.
        // seek the pointer 1 place back (the 'IF' function moves the pointer forward 1 place forward for each character).
        fseek(fp , -1L, SEEK_CUR);
        // get the character value of the current spot that the pointer of the read file points to.
        char x = fgetc(fp);
        buffer_temp_word = realloc(buffer_temp_word, (sizeof(char)) * char_count);

        //the string stores the character on the correct place
        //(the first character starts at memory location 0, hence the amount of characters -1)
        buffer_temp_word[char_count - 1] = x;

        // check for the end of the line (which is the end of the word).
        if(x == '\n')
           //printf("(end of line reached)");
           printf("\nusing memory:");

           // iterate trough characters in the memory using the pointer + while loop, option 2.
           while(*buffer_temp_word != '\n')
              printf("%c", *buffer_temp_word);

           printf("\nword printed succesfully");
           // reset the pointer to the beginning of the buffer_temp_word string (which is an array actually).
           buffer_temp_word = NULL;

           // reset the amount of characters (for the next word that will be read).
           char_count = 0;
        printf("%c", x);

1 Answer 1


You don't free the realloced memory. Your code

           buffer_temp_word = NULL;

means free is called for NULL. You should not change the value of buffer_temp_word there, you could increment a copy instead, or use an index variable.

And 8 bytes is correct, in both cases you lose the memory when you are at a char_count of 4.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .