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Daniel Briceño
Asked: 2022-03-24 06:07:26 +0800 CST 2022-03-24 06:07:26 +0800 CST

Problem with freeing dynamic memory

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I have this code:

char* texto=(char*)malloc(11);//Retorno un arrays de 10 char
strcpy(texto,"Hello World");
texto[4]='\0';//Recortamos la cadena.
for (int i=5;i<11;i++)
 free(&texto[i]);//Liberamos el espacio que no nesecitamos.

This stops the executable for me. But if I did this:

texto[7]='\0';
for (int i=8;i<11;i++)
 free(&texto[i]);

The executable ends perfectly. Because? And how do I solve it?
I got this page: Uncle C(GCC) There I could see this error:

double free or corruption (top)
/srv/wrappers/c-gcc: line 5:  3868 Aborted                 (core dumped) ./.bin.tio "$@" < .input.tio

But why if I am freeing memory that I reserve to trim the string?
Any help or advice. The idea is just to trim the string and free up the space I don't need.

c

1 Answers

  1. Best Answer

    Memory management does not work as you think. When you ask for memory:

    char* texto=(char*)malloc(11);
//    ^^^^^ <---- Bloque de 11 char:
/*

              | 0| 1| 2| 3| 4| 5| 6| 7| 8| 9|10|
texto ------> |  |  |  |  |  |  |  |  |  |  |  | 
*/

    You ask for an entire block of memory, this block is pointed to by the return of mallocand refers to that block of 11 characters. All memory requested by mallocmust be freed by freeand what you've asked for with mallocis that block of 11 characters so you can't free part of them in the same way that if you buy a loaf of bread you can't return half of it to have bought half!

    However, you can reallocate the memory, for that the function is used realloc:

    char* texto=(char*)malloc(11);//Retorno un arrays de 10 char
strcpy(texto,"Hello World");
texto[4]='\0';//Recortamos la cadena.
texto = realloc(texto, 5);//descartamos de la sexta posición en adelante.
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