a problem about malloc() Post: 25795

Sponsored Content

Top Forums Programming a problem about malloc() Post 25795 by chenhao_no1 on Monday 5th of August 2002 09:31:56 PM

08-05-2002

Registered User

a problem about malloc()

1 . Thanks everyone who read the post.

2 . the programe is that :
#include <stdio.h>
#include <string.h>

void do_it(char *p)
{
p = (char *) malloc(100);

(void )strcpy(p,"1234");
}

int main(void)
{
char *p;

do_it(p);

(void )printf("p = %s \n",p);

return 0;
}

cc -g a.c -o a.out
./a.out
it will show that "memory fault(coredump)"

3 . Why ? I think that malloc() get memory from heap , and the heap can be seen at everywhere in the programe .In other words , it is "global" .

4 Then I write another programe. it works correctly .

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

char *do_it(void)
{
return ((char *) malloc(100));
}

int main(void)
{
char *p;

p = do_it();

strcpy(p, "asdfa");

(void) printf("p = %s \n",p);
}

5 . Then , the point 4 declare that point 3 is correct , but it is not we want .
6 . Then I think the OS has changed the value p when do_it() function return

chenhao_no1

View Public Profile for chenhao_no1

Find all posts by chenhao_no1

10 More Discussions You Might Find Interesting

1. Programming

malloc

hello sir since by mentioning a integer pointer and storing the integers by incrementing the pointer value then what is the purpose of malloc? u can decalre it as in t *p; several integers can be stored by incrementing the value of p, hence what is the diffrence between this...

2. Programming

malloc function

Hello This is a simple program i carried out in my machine i dont know how it is working #include<alloc.h> #include<stdio.h> mian() { int *p,j; p= (int*)malloc(1); for(j=1;j<=580;j++) { *p=j; ++p; } p=p-580; for(j=1;j<=580;j++) { printf("%d",*p); }

3. Programming

malloc for 1 Mb error.

Hi All! Does some one know I am under UNIX system can not allocate more then 1 Mb memory? It broke program down. Any information would be greatly appreciated. Thanks.

4. Programming

When to use Malloc?

Hi! I hope this is the correct forum to post the question even if I'm a newbie... I am a C-newbie (and really on the edge to be a C-addict ;) ) and have a question. When should I use malloc? To state it differently, when should I NOT use malloc? For instance, if I have an array of...

5. Programming

malloc()

Some one please explain me what is Dynamic memory allocation and the use of malloc() function.How do we allocate memory dynamically and also the other way?

6. Programming

Malloc implementation in C

Hey Guys I am trying to implement the malloc function for my OS class and I am having a little trouble with it. I would be really grateful if I could get some hints on this problem. So I am using a doubly-linked list as my data structure and I have to allocate memory for it (duh...). The...

7. UNIX for Dummies Questions & Answers

Kmalloc and malloc

Do kmalloc and malloc allocate from same heap ?

8. Programming

malloc vs new speed

Which one is faster among malloc and new? My understanding is that since new also has to call constructors after allocating memory it must be slower than malloc. Am I correct?

9. UNIX for Dummies Questions & Answers

Help with malloc()

Good day! I'm a newbie in C. I'm trying to get an unlimited input from the user using malloc then printing the inputs after the user presses enter. My code works, but there's a warning that I don't know how to fix. Please help me. Thank you. Here's my code: #include <stdio.h> #include...

10. Programming

Malloc problem with fread() to read file to structure in C

Hello, I am trying to read a text file into linked list, but always got the first and last records wrong. 1) The problem looks related to the initialization of the node temp with malloc(), but could not figure it out. No error/warning at compiling, though. 2) The output file is empty,...

LEARN ABOUT DEBIAN

malloc_trim

MALLOC_TRIM(3)						     Linux Programmer's Manual						    MALLOC_TRIM(3)

NAME

       malloc_trim - release free memory from the top of the heap

SYNOPSIS

       #include <malloc.h>

       void malloc_trim(size_t pad);

DESCRIPTION

       The malloc_trim() function attempts to release free memory at the top of the heap (by calling sbrk(2) with a suitable argument).

       The  pad  argument  specifies  the amount of free space to leave untrimmed at the top of the heap.  If this argument is 0, only the minimum
       amount of memory is maintained at the top of the heap (i.e., one page or less).	A nonzero argument can be used to maintain  some  trailing
       space at the top of the heap in order to allow future allocations to be made without having to extend the heap with sbrk(2).

RETURN VALUE

       The malloc_trim() function returns 1 if memory was actually released back to the system, or 0 if it was not possible to release any memory.

ERRORS

       No errors are defined.

CONFORMING TO

       This function is a GNU extension.

NOTES

       This  function  is  automatically  called by free(3) in certain circumstances; see the discussion of M_TOP_PAD and M_TRIM_THRESHOLD in mal-
       lopt(3).

       This function cannot release free memory located at places other than the top of the heap.

NOTES

       This function only releases memory in the main arena.

SEE ALSO

       sbrk(2), malloc(3), mallopt(3)

COLOPHON

       This page is part of release 3.44 of the Linux man-pages project.  A description of the project, and information about reporting bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2012-03-29							    MALLOC_TRIM(3)