Basic Use of Malloc

[kent lavallie]

Hi. I am an amature c hobbyist and I wish to begin using malloc as I
have been told that running executables from executables is a less
than elegant way to manage memory use. I need basic examples so me and
my compiler can get familliar with malloc. Please give examples that
do not contain nested assignments and the like as I will get lost
quite easily. As an example of my rudimentary style take a look at how
I create an array and assign a pointer to it:

unsigned long arraysize = 50;
char chArray[arraysize];
char *ptrChArray;
ptrChArray = chArray;

I need a few examples of malloc's proper use that are in the same
rudimentary style. Thank you so much for your time. kentinjapan

[Steve Zimmerman]

kent lavallie wrote:

Hi. I am an amature c hobbyist and I wish to begin using malloc as I
have been told that running executables from executables is a less
than elegant way to manage memory use. I need basic examples so me and
my compiler can get familliar with malloc. Please give examples that
do not contain nested assignments and the like as I will get lost
quite easily. As an example of my rudimentary style take a look at how
I create an array and assign a pointer to it:

unsigned long arraysize = 50;
char chArray[arraysize];
char *ptrChArray;
ptrChArray = chArray;

I need a few examples of malloc's proper use that are in the same
rudimentary style. Thank you so much for your time. kentinjapan

Also, you might try

http://www.c-for-dummies.com/lessons/chapter.17

I found the _C for Dummies_ material helpful when I was new to C.

--Steve

[Jeff]

"kent lavallie" <ke**********@y ahoo.ca> wrote in message
news:d6******** *************** ***@posting.goo gle.com...

Hi. I am an amature c hobbyist and I wish to begin using malloc as I
have been told that running executables from executables is a less
than elegant way to manage memory use. I need basic examples so me and
my compiler can get familliar with malloc. Please give examples that
do not contain nested assignments and the like as I will get lost
quite easily. As an example of my rudimentary style take a look at how
I create an array and assign a pointer to it:

unsigned long arraysize = 50;
char chArray[arraysize];
char *ptrChArray;
ptrChArray = chArray;

I need a few examples of malloc's proper use that are in the same
rudimentary style. Thank you so much for your time. kentinjapan

This is an example:

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

int main()
{
char *buffer;
char *string1 = "Hello ";

buffer = malloc(30);
strcpy(buffer, string1);
strcat(buffer, "World.");

puts(buffer);
return 0;
}

--
Jeff

[Nils Petter Vaskinn]

On Tue, 09 Sep 2003 20:56:56 -0700, kent lavallie wrote:

I need a few examples of malloc's proper use that are in the same
rudimentary style. Thank you so much for your time. kentinjapan

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

int main()
{
/* Declare a pointer, TYPE can be anything: int struct double whatever */
TYPE *array;

/* Allocate memory to hold 50 of whatever the type array points to */
array = malloc(50 * sizeof(*array)) ;
if (! array) {
printf("Couldn' t allocate memory\n");
exit(1);
}

do_something(ar ray);

/* We need a bigger array */
TYPE *tmp;
/* Realloc allocates a new block, and copies the contents
(as far as possible */
tmp = realloc(array, 70 * sizeof(*array)) ;
if (! tmp) {
printf("Couldn' t reallocate memory\n");
exit(1);
}

array = tmp;

do_something_el se(array);

/* release the memory when we're done */
free(array);

return 0;
}

hth

--
NPV
"Linux is to Lego as Windows is to Fisher Price." - Doctor J Frink

[Martijn]

Jeff wrote:

buffer = malloc(30);
strcpy(buffer, string1);
strcat(buffer, "World.");

Not a point of critisism to the example, but something to watch out for: be
aware that this allocates space for 30 _bytes_, so don't go and try to store
30 integers in that memory. If that's what you want, try this:

int *buffer;
buffer = malloc(30 * sizeof(int));

Good luck,

--
Martijn
http://www.sereneconcepts.nl

[Irrwahn Grausewitz]

Hi Kent,

good to see you found your way to c.l.c. :)

ke**********@ya hoo.ca (kent lavallie) wrote:

Hi. I am an amature c hobbyist and I wish to begin using malloc as I
have been told that running executables from executables is a less
than elegant way to manage memory use. I need basic examples so me and
my compiler can get familliar with malloc. Please give examples that
do not contain nested assignments and the like as I will get lost
quite easily. As an example of my rudimentary style take a look at how
I create an array and assign a pointer to it:

unsigned long arraysize = 50;
char chArray[arraysize];
char *ptrChArray;
ptrChArray = chArray;

I need a few examples of malloc's proper use that are in the same
rudimentary style. Thank you so much for your time. kentinjapan

Besides the examples I gave to you on c.l.c.moderated and by mail,
here is one more:

/* -------8<----------------- */

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

#define TEXT_SIZE 1000

int main( void )
{
char *text, *copy, *cp;
int i, len, ecount;

/* Allocate some memory, checking for error:
*/
text = malloc( TEXT_SIZE );
if ( text == NULL )
{
printf("Memory allocation for 'text' failed!");
exit( EXIT_FAILURE );
}

/* Read some user input into text:
*/
printf( "\nPlease enter some text: " );
fgets( text, TEXT_SIZE, stdin );

/* Now we step through text, counting 'e's
** as we go, using a pointer-to-char:
*/
ecount = 0;
for ( cp = text; *cp != '\0'; cp++ )
{
if ( *cp == 'e' )
ecount++;
}
printf( "The text you entered contains %d 'e's.\n",
ecount );

/* Now we use array syntax instead, changing
** all 'u's to 'x's as we go:
*/
for ( i = 0; i < TEXT_SIZE; i++ )
{
if ( text[ i ] == 'u' )
text[ i ] = 'x';
}
printf( "'u' changed to 'x': %s", text );

/* Now we allocate just enough space to hold
** the text the user entered (note that we have
** to add 1 to hold the terminating '\0'):
*/
len = strlen( text ) + 1;
copy = malloc( len );
if ( copy == NULL )
{
printf("Memory allocation for 'copy' failed!");
exit( EXIT_FAILURE );
}

/* Copy from 'text' to 'copy':
*/
strcpy( copy, text );
printf( "The copy reads: %s", copy );

return EXIT_SUCCESS;
}

/* -------8<----------------- */

Regards

Irrwahn
--
What does this red button do?

[Joe Wright]

Martijn wrote:

Jeff wrote:

buffer = malloc(30);
strcpy(buffer, string1);
strcat(buffer, "World.");

Not a point of critisism to the example, but something to watch out for: be
aware that this allocates space for 30 _bytes_, so don't go and try to store
30 integers in that memory. If that's what you want, try this:

int *buffer;
buffer = malloc(30 * sizeof(int));

Good luck,

--
Martijn
http://www.sereneconcepts.nl

The clc canon eschews magic numbers and unnecessary type names. Regard..

int *buffer, size = 30;
buffer = malloc( size * sizeof *buffer );
--
Joe Wright mailto:jo****** **@earthlink.ne t
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[Martin Dickopp]

Irrwahn Grausewitz <ir*****@freene t.de> writes:

printf("Memory allocation for 'text' failed!");
exit( EXIT_FAILURE );
It is implementation-defined whether or not the last line written to
a text stream requires a terminating new-line character. For maximal
portability, you might want to write a '\n' as the last character.
printf( "\nPlease enter some text: " );
fgets( text, TEXT_SIZE, stdin );
Add `fflush (stdout);' after the `printf' function call. Otherwise, the
text is not guaranteed to be written before the `fgets' call.
printf("Memory allocation for 'copy' failed!");
exit( EXIT_FAILURE );
Same comment as the first one.
printf( "The copy reads: %s", copy );
return EXIT_SUCCESS;

Same comment as immediately above.

Martin

[Irrwahn Grausewitz]

Martin Dickopp <ex************ *@zero-based.org> wrote:

Irrwahn Grausewitz <ir*****@freene t.de> writes:

printf("Memory allocation for 'text' failed!");
exit( EXIT_FAILURE );
It is implementation-defined whether or not the last line written to
a text stream requires a terminating new-line character. For maximal
portability, you might want to write a '\n' as the last character.

Got me. :)

printf( "\nPlease enter some text: " );
fgets( text, TEXT_SIZE, stdin );
Add `fflush (stdout);' after the `printf' function call. Otherwise, the
text is not guaranteed to be written before the `fgets' call.

Again. Doh!

printf("Memory allocation for 'copy' failed!");
exit( EXIT_FAILURE );
Same comment as the first one.

You're so right... sigh...

printf( "The copy reads: %s", copy );
return EXIT_SUCCESS;

Same comment as immediately above.

Ah, IMHO /this one/ is OK, because I left the '\n' in 'text'
and therefore in 'copy', thus I should be safe in this case,
shouldn't I?

Irrwahn
--
What does this red button do?

[Martin Dickopp]

Irrwahn Grausewitz <ir*****@freene t.de> writes:

Martin Dickopp <ex************ *@zero-based.org> wrote:

Irrwahn Grausewitz <ir*****@freene t.de> writes:

printf( "The copy reads: %s", copy );
return EXIT_SUCCESS;

Same comment as immediately above.

Ah, IMHO /this one/ is OK, because I left the '\n' in 'text' and
therefore in 'copy', thus I should be safe in this case, shouldn't I?

The `fgets' function only stores a newline character if it encounters one
while reading the number of characters it is supposed to read maximally.
If the user enters more characters, or the end-of-file condition or an
error occurs, no newline character is stored.

Which reminds me: Since you don't check the return value of `fgets', it
could also happen that `text' remains unchanged (if end-of-file occurs
before any characters have been read) or becomes indeterminate (if an
error occurs). In both cases, you invoke undefined behavior. :)

Martin