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Home/ Questions/The munmap_chunk(): invalid pointer error: What should you do?
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Jules Grange
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Jules Grange
Asked: May 12, 20222022-05-12T11:57:25+00:00 2022-05-12T11:57:25+00:00In: c

The munmap_chunk(): invalid pointer error: What should you do?

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As the title says, I am getting the “munmap_chunk(): invalid pointer” error. How can I fix it so the error goes away? Here is my detail:

#include <stdio.h>
 #include <stdlib.h>
 
 char * first()
 {
  char * word = malloc(sizeof(char) * 10);
  word[0] = 'a';
  word[1] = 'b';
  word[2] = '\0';
  return word;
 }
 
 char * second ()
 {
  char * word = malloc(sizeof(char) * 10);
  word = "ab";
  return word;
 }
 
 int main ()
 {
  char * out = first();
  printf("%s", out);
  free(out);
  out = second();
  printf("%s", out);
  free(out);
  return 0;
 }

When I operated it and I received the error text:

Error in `./a.out': munmap_chunk(): invalid pointer: 0x0000000000400714 ***
  ababAborted (core dumped)

I appreciate any help from you.

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    lyytutoria Expert
    2022-06-03T11:17:56+00:00Added an answer on June 3, 2022 at 11:17 am

    The cause: In the function second(), the assignment word = “ab”; assigns a new pointer to word, overwriting the pointer obtained through malloc(). When you call free() on the pointer later on, the program crashes because you pass a pointer to free() that has not been obtained through malloc(). Therefore, assigning string literals does not have the effect of copying their content.

    Solution: Let’s use strcpy()to copy the string literal’s content.

    strcpy(word, "ab");

     

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  2. Jules Bellanger
    2022-05-25T19:49:26+00:00Added an answer on May 25, 2022 at 7:49 pm

    Function char * second

     char * word = malloc(sizeof(char) * 10);
     word = "ab";

    The word = "ab"; second statement changes word to point away to the allocated memory. You are not copying "ab" to an area of heap that malloc has allocated.

    To free, a memory not allocated by malloc and similar functions crashes your program.

    Attempting to free an invalid pointer (a pointer to a memory block that was not allocated by calloc, malloc, or realloc) may affect subsequent allocation requests and cause errors.

    As suggested by others, you should use strcpy.

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