return pointer to next word

[Limpor]

Hi there, I'm very new to C program, I'm about to write a function that read input from standard input, and then returns a pointer to the next word, then i can use it later to print out all the words, but when i run it, i got "Segmentation fault " at the end of output line, this is my code, I've been working on this for a few days, but I still cannot get right, can someone can help me? please!

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
/* assume fp just stdin, and rCase is True if the case of the returned word
 * is to be exactly same as in the input
 **/ 
char* readWord (FILE* fp, int rCase)
{
    char buffer[100];
    char* nextWord = NULL;
    int ch, i = 0;
    int wordLength = 0;
 
    while ((ch = fgetc(fp)) != EOF && !(isalpha(ch)))
        ;
 
    while (isalpha(ch)) {
        if (!retainCase) 
            buffer[wordLength++] = tolower(ch);
        else 
            buffer[wordLength++] = ch;
 
        nextWord = buffer;
        buffer[wordLength] = '\0';
        ch = fgetc(fp);
    }
    return nextWord;
}
 
int main(int argc, char *argv[])
{
    char* new;
    while (argc == 1) {
        new = readWord(stdin,0);
        printf("%s\n", new);
    }
    return (EXIT_SUCCESS);
} 

[JosAH]

Note that your 'buffer' array is a local array variable and its allocated on the stack.
When the function returns the stack frame is lost for the posterity and your
buffer array variable is too. Before you want to return that word you should
malloc() enough bytes, copy the word over to that allocated memory and return
whatever malloc() returned.

The caller of your function is responsible then for free()-ing that memory afterwards
(if needed).

kind regards,

Jos

[AdrianH]

Note that your 'buffer' array is a local array variable and its allocated on the stack.
When the function returns the stack frame is lost for the posterity and your
buffer array variable is too. Before you want to return that word you should
malloc() enough bytes, copy the word over to that allocated memory and return
whatever malloc() returned.

The caller of your function is responsible then for free()-ing that memory afterwards
(if needed).

kind regards,

Jos

Jos, what are you saying? "if needed"? One should always cleanup memory that was allocated. I've heard the arguments that the operating system will clean up all allocated memory upon exit, but that is a bunch of crap. It makes for lazy programmers who then forget to implement best practices when it is required.

I'm disappointed Jos, Tisk tisk. ;)

Other than my tirade to Jos, what was said is correct. Further to what Jos said, if you don't want to use malloc(), you can instead allocate the memory in the caller on the stack like you have done in the readWord(), and pass that array and its size to the readWord(). The return value could be then reserved for returning the function's status.

BTW, if this is an assignment, I assume that you were given permission not to check to see if you have exceeded the char array boundary by making the array fairly large?

Further, I don't see how you could have ran the code as this shouldn't compile. retainCase is not defined.

Just adding my 2 cents. ;)


Adrian

[JosAH]

Jos, what are you saying? "if needed"? One should always cleanup memory that was allocated. I've heard the arguments that the operating system will clean up all allocated memory upon exit, but that is a bunch of crap. It makes for lazy programmers who then forget to implement best practices when it is required.

I'm disappointed Jos, Tisk tisk. ;)

From IBM/VM (CMS and TSO) through Harris/VOS, VAX/VMS, quite a bunch
of different Un*xes (some of them even smelled funny), Linux and even MS
Windows, all of them were just clever enough to reap the memory allocated
to the process after it had finished no matter in what horrible way.

Only CPM and MS/DOS (the unprotected memory thingies) were too stupid.
Of course the same counts for a lot of embedded systems too except for
Symbian, that's an extremely clever little embedded OS and an exception
to the embedded rule.

That's why I added the "if needed" phrase ;-)

kind regards,

Jos

[AdrianH]

From IBM/VM (CMS and TSO) through Harris/VOS, VAX/VMS, quite a bunch
of different Un*xes (some of them even smelled funny), Linux and even MS
Windows, all of them were just clever enough to reap the memory allocated
to the process after it had finished no matter in what horrible way.

Only CPM and MS/DOS (the unprotected memory thingies) were too stupid.
Of course the same counts for a lot of embedded systems too except for
Symbian, that's an extremely clever little embedded OS and an exception
to the embedded rule.

That's why I added the "if needed" phrase ;-)

kind regards,

Jos

Yes, but by saying "if needed", you are cultivating bad habbits in programmers.

Sometimes, a function is reused and then the memory leak becomes a persistant problem while the code is running.

They should always think it is needed as this will reduce errors later on in their coding careers.


Adrian

[JosAH]

Yes, but by saying "if needed", you are cultivating bad habbits in programmers.

Sometimes, a function is reused and then the memory leak becomes a persistant problem while the code is running.

They should always think it is needed as this will reduce errors later on in their coding careers.


Adrian

True; I just put the ball on their side so they should take their responsibilities.
I am in the role of the lazy advisor (*) you know and I don't want to spoonfeed ;-)

kind regards,

Jos

(*) "memory leaks are good; otherwise you get bit rust"

[AdrianH]

True; I just put the ball on their side so they should take their responsibilities.
I am in the role of the lazy advisor (*) you know and I don't want to spoonfeed ;-)

kind regards,

Jos

(*) "memory leaks are good; otherwise you get bit rust"

I think you just want to keep your position safe from being overrun by new coders. :p ;) Job security I think its called. :D

Me, I want to build a tower of Bable, without it falling back to the Earth agian. ;)


Adrian

[JosAH]

I think you just want to keep your position safe from being overrun by new coders. :p ;) Job security I think its called. :D

Me, I want to build a tower of Bable, without it falling back to the Earth agian. ;)


Adrian

Duh, I'm the CEO/CTO/CFO/Coffee Lady/Programmer/Janitor of my own
company so "I am the law!" ;-)

kind regards,

Judg^H^H^Hos

[AdrianH]

Duh, I'm the CEO/CTO/CFO/Coffee Lady/Programmer/Janitor of my own
company so "I am the law!" ;-)

kind regards,

Judg^H^H^Hos

Ahh, so you are related to Bill G. eh? Your attidute is so much clearer now, junk is all you know. :D :p


Adrian

[MMcCarthy]

Back to work children :rolleyes:

Actually I quite enjoyed this one :D If you feel like taking your discussion on Memory Allocation to the Software Development forum please feel free. I'm sure you'll have many participants to the discussion on both sides.

Mary