Release memory problem

Hi! I had problem in the next code. In it, I want to carry out a tokenizer for getting some parameters values written in a text file.

Code:

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/* Se lee el valor del parámetro Longitud del símbolo Apex */

      p_cadena=p_cadena+2;
      char *param_cpy=(char*)calloc(strlen(&(cadena[p_cadena])),sizeof(char));
      strcpy(param_cpy,&(cadena[p_cadena]));
      char *lg=strtok(param_cpy,",");
      p_cadena=p_cadena+strlen(lg);
      *result=sscanf(lg,"%f",l);      free(param_cpy);

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In the character chain 'param_cpy' I allocate memory in order to copy a fragment of the text chain being descomposed. After i get the token, a want to release this memory. Howewer, when i try doing it, the program crash in execution time. The instruction causing the problem is marked in red in the code above. I don´t understand the reason which this problem occurs for. Someone could help me?

Thank you.

Fernando Gutiérrez

As this is C++ I will not say "You shouldn't cast malloc/calloc"...

Code:

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calloc(strlen(&(cadena[p_cadena])),sizeof(char));

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Leading question: How long is this section, and how long a string does it cope with?

Is there something in a string, that makes it take up a tiny bit more space than you'd immediately think?

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Mats

Your calloc doesn't count the \0 at the end of the string, which will be copied (and which will trash memory when the strcpy happens).

Say
char *param_cpy=calloc(strlen(&(cadena[p_cadena]))+1,sizeof(char));

Also, using calloc instead of malloc has no advantage in this instance, since you're going to overwrite all the data with the following strcpy().

Try allocating 1 or 2 extra bytes. Maybe strcpy() is filling param_cpy and not leaving a NULL at the end, or maybe there's no NULL at the end of cadena[p_cadena] and you're overflowing the buffer?
Use strncpy() to prevent buffer overflow.

> or maybe there's no NULL at the end of cadena[p_cadena] and you're overflowing the buffer?
Then the strlen() itself would have failed.

[QUOTE=Salem;678980]Your calloc doesn't count the \0 at the end of the string, which will be copied (and which will trash memory when the strcpy happens).

Say
char *param_cpy=calloc(strlen(&(cadena[p_cadena]))+1,sizeof(char));
[QUOTE]

I've to review my code, but I think that you are right and that is the reason for that my program crashes. Thank you

Quote:

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Originally Posted by Salem

Also, using calloc instead of malloc has no advantage in this instance, since you're going to overwrite all the data with the following strcpy().

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My question is if using malloc instead of malloc have some advantage.

calloc does more work than malloc (it is after all just malloc + memset), so unless you need that feature, it isn't worth doing IMO.

Also, calloc doesn't necessarily result in valid data, especially where floats and pointers are concerned.
http://c-faq.com/malloc/calloc.html

And once you use malloc instead of calloc, why not use new[] instead?

> calloc does more work than malloc (it is after all just malloc + memset), so unless you need that feature, it isn't
> worth doing IMO.

Not to mention that it confuses people reading your code trying to figure out where you use the initialization, only to finally realize you don't.