Thread: Using free() function

  1. #1
    Registered User
    Join Date
    Jun 2012
    Location
    Here
    Posts
    23

    Using free() function

    Could someone explain what is the most correct usage of free() function in this paricular case:

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    
    
    void reverseStr(const char **str)
        {
        int i, j;
        int word_len = strlen(*str);
    
        char *tmp_word = (char *)malloc(word_len + 1);
    
        if(tmp_word == NULL) return;
    
        for(i=0, j=(word_len - 1); j>=0; i++, j--)
            {
            tmp_word[i] = (*str)[j];
            }
    
        tmp_word[word_len] = '\0';
        *str = tmp_word;
    
        // Putting free(tmp_word) here seems to erase the content of my_str
        // in main() function
    
        return;
        }
    
    
    int main(void)
        {
        const char *my_str = "This is nice sentence.";
    
        printf("%s\n", my_str);
        reverseStr(&my_str);
        printf("%s\n", my_str); // now the string is reversed
    
        return 0;
        }

  2. #2
    Registered User
    Join Date
    Jun 2011
    Posts
    4,513
    Code:
    *str = tmp_word;
    This is not valid syntax for copying a string in 'C'. Try "strcpy()" instead.

  3. #3
    Registered User
    Join Date
    Nov 2010
    Location
    Long Beach, CA
    Posts
    5,909
    You should only free the memory you allocate when you're all done with it. In your case, that would be after the 2nd printf, on line 37. Calling free doesn't erase the contents, but it does free up that memory to be used by something else, which may overwrite the old data, giving the effect of erasing it. Also remember, calling free doesn't change the pointer, so my_str still points to the same bit of memory, but it now contains invalid data, and trying to access it results in undefined behavior.

    Some people suggest that you allocate and free strings in the same context/function, to better keep track of memory usage and reduce the likelihood of memory leaks, but that's not always possible/practical, so diligence on your part is necessary.

  4. #4
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    Location
    New Zealand
    Posts
    6,318
    You should not need to use free in that program because you should not need to use malloc in that program. How about trying to reverse the string in-place?

    You will need to change your main function though because you can't reverse a string literal, you need to declare an array instead, and don't make it const.
    My homepage
    Advice: Take only as directed - If symptoms persist, please see your debugger

    Linus Torvalds: "But it clearly is the only right way. The fact that everybody else does it some other way only means that they are wrong"

  5. #5
    Registered User
    Join Date
    Nov 2011
    Location
    Buea, Cameroon
    Posts
    197
    i think the free function function is probably best for linked lists and in this case i think to free the string would be to make the string NULL..

  6. #6
    Registered User
    Join Date
    May 2012
    Posts
    1,066
    Quote Originally Posted by Nyah Check View Post
    i think the free function function is probably best for linked lists and in this case i think to free the string would be to make the string NULL..
    No, you need free() for every memory block you have allocated in your program. Setting the string to NULL doesn't free the allocated memory block.

    Bye, Andreas

  7. #7
    Ticked and off
    Join Date
    Oct 2011
    Location
    La-la land
    Posts
    1,728
    Usually it is simpler to just return a new dynamically allocated copy:
    Code:
    #include <stdlib.h>
    #include <string.h>
    #include <errno.h>
    #include <stdio.h>
    
    char *reverse(const char *const string)
    {
        char  *rev;
        size_t len, i;
    
        /* We do not accept NULL. */
        if (!string) {
            errno = EINVAL;
            return NULL;
        }
    
        /* Find out the length of the string, */
        len = strlen(string);
    
        /* and allocate memory for a copy. Remember: add '\0' at end! */
        rev = malloc(len + 1);
        if (!rev) {
            errno = ENOMEM;
            return NULL;
        }
    
        /* Copy the string contents in reverse order, */
        for (i = 0; i < len; i++)
            rev[i] = string[len - 1 - i];
    
        /* and add the end-of-string byte. */
        rev[len] = '\0';
    
        /* Return the dynamically allocated string. */
        return rev;
    }
    
    int main(int argc, char *argv[])
    {
        int arg;
        char *reversed;
    
        for (arg = 1; arg < argc; arg++) {
    
            reversed = reverse(argv[arg]);
            if (!reversed) {
                fprintf(stderr, "%s: %s.\n", argv[arg], strerror(errno));
                return 1;
            }
    
            printf("%s reversed is %s.\n", argv[arg], reversed);
    
            free(reversed);
        }
    
        return 0;
    }
    The other approach is to let the user specify a (dynamically allocated) buffer, and the size of that buffer. New input routines in POSIX.2008 (#define _POSIX_C_SOURCE 200809L), getline() and getdelim() use this interface:
    Code:
    #include <stdlib.h>
    #include <string.h>
    #include <errno.h>
    #include <stdio.h>
    
    /* Returns a copy of the string, reversed, in *dataptr.
     * Returns the length of the string, or (ssize_t)-1 with errno set if error.
    */
    ssize_t reverse(char **const dataptr, size_t *const sizeptr, const char *const source)
    {
        char *temp;
        size_t length, i;
    
        /* NULL pointers are not accepted! */
        if (!dataptr || !sizeptr || !source) {
            errno = EINVAL;
            return (ssize_t)-1;
        }
    
        length = strlen(source);
    
        /* Do we need a larger data buffer? */
        if (*sizeptr <= length) {
            /* length + 1, or anything bigger suffices.
             * Experience shows that padding to some next multiple works better.
             * This adjusts *sizeptr to next multiple of 32, adding at least one. */
            *sizeptr = (length | 31) + 1;
    
            temp = realloc(*dataptr, *sizeptr);
            if (!temp) {
                free(*dataptr); /* free(NULL) is perfectly okay. */
                *sizeptr = 0;
                errno = ENOMEM;
                return (ssize_t)-1;
            }
            *dataptr = temp;
        }
    
        /* Copy routine. Note the downwards pointer arithmetic! */
        temp = *dataptr + length - 1;
    
        /* Set the end of string, temp[0] being the last character, */
        temp[1] = '\0';
    
        /* and copy the rest of the string in reverse. */
        for (i = 0; i < length; i++)
            temp[-i] = source[i];
    
        return (ssize_t)length;
    }
    
    int main(int argc, char *argv[])
    {
        char *buffer = NULL; /* You must initialize this to NULL! */
        size_t allocated = 0; /* You must initialized this to zero! */
        ssize_t length;
        int arg;
    
        for (arg = 1; arg < argc; arg++) {
    
            length = reverse(&buffer, &allocated, argv[arg]);
            if (length == (ssize_t)-1) {
                fprintf(stderr, "%s: %s.\n", argv[arg], strerror(errno));
                return 1;
            }
    
            printf("%s (%d bytes): %s (%d bytes)\n", argv[arg], (int)strlen(argv[arg]), buffer, (int)length);
        }
    
        free(buffer);
        buffer = NULL;
        allocated = 0;
    
        return 0;
    }
    If you look carefully, the first implementation is simpler, but uses a lot of allocations. The second implementation is a bit more complex to use, but it dynamically grows the buffer to long enough but does not use extra allocations. If you call the function often, and reuse the same buffer (like in the example main() above), this usually gives a measurable speedup.

  8. #8
    Registered User
    Join Date
    Nov 2011
    Location
    Buea, Cameroon
    Posts
    197
    @Andipersti thanks for the tip...

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. malloc() in function, free() without
    By SashaN in forum C Programming
    Replies: 4
    Last Post: 08-06-2012, 06:29 AM
  2. Create malloc and free function in C?
    By guestcheap in forum C Programming
    Replies: 47
    Last Post: 08-19-2011, 08:17 PM
  3. free function error.
    By brack in forum C Programming
    Replies: 13
    Last Post: 10-28-2010, 01:34 PM
  4. free function
    By rakeshkool27 in forum C Programming
    Replies: 6
    Last Post: 06-06-2010, 03:29 PM
  5. free function
    By vapanchamukhi in forum C Programming
    Replies: 2
    Last Post: 09-18-2008, 02:48 PM

Tags for this Thread