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malloc and array of pointers

This is a discussion on malloc and array of pointers within the C Programming forums, part of the General Programming Boards category; Hi all. I am trying to compile the below program and getting these errors: Code: |In function ‘main’:| 18|error: incompatible ...

  1. #1
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    malloc and array of pointers

    Hi all.

    I am trying to compile the below program and getting these errors:

    Code:
    |In function ‘main’:|
    18|error: incompatible types when assigning to type ‘char *[30]’ from type ‘char *’|
    19|warning: passing argument 1 of ‘strcpy’ from incompatible pointer type [enabled by default]|
    /usr/include/string.h|128|note: expected ‘char * __restrict__’ but argument is of type ‘char **’|
    20|error: lvalue required as increment operand|
    ||=== Build finished: 3 errors, 1 warnings ===|
    Why are the types incompatible with malloc? filename == filename[0]? So what is the problem? My reference books has not provided any solutions. Thanks.

    Code:
    #include <stdio.h>
    #include <dirent.h>
    #include <stdlib.h>
    #include <string.h>
    
    int main(void)
    {
        DIR *dirname;
        struct dirent *direntries;
        char *filename[30];
    
        dirname = opendir("/home/test/Downloads");
    
        if(dirname != NULL)
        {
            while((direntries = readdir(dirname)))
            {
                filename = (char *) malloc(strlen(direntries->d_name));
                strcpy(filename, direntries->d_name);
                filename++;
            }
            (void) closedir(dirname);
        }
        else
        {
            perror("Error opening directory");
        }
    
        return 0;
    }
    Last edited by cfanatic; 09-12-2012 at 11:56 PM.
    IDE: Code::Blocks | Compiler Suite for Windows: TDM-GCC (MingW, gdb)

  2. #2
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    "filename" is an array of strings (char*[30]), you try to assign it a string (char*). You have to either dereference (*filename = (char*)malloc...) it or specify an index (filename[i]=...).

    But what you're doing is wrong, "filename", being an array, is a readonly value, you can't reassign it, that's why you get that error line 20 with filename++, your compiler tells you it's not a lvalue. You'll have to either access that array using indices, or declare an additional pointer (char**) to walk it using an increment operator.
    Indeed, in assembly, an array is just the address of its first element, but as you see here, they are not equivalent.
    Last edited by root4; 09-13-2012 at 12:14 AM.
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  3. #3
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    I had tried *filename = ...., and file[0] = .....

    But that gives me different warnings:

    Code:
    |In function ‘main’:|
    19|warning: passing argument 1 of ‘strcpy’ from incompatible pointer type [enabled by default]|
    /usr/include/string.h|128|note: expected ‘char * __restrict__’ but argument is of type ‘char **’|
    20|error: lvalue required as increment operand|
    ||=== Build finished: 2 errors, 1 warnings ===|
    I tried &filename[0] = ... , and that gives even more errors.
    IDE: Code::Blocks | Compiler Suite for Windows: TDM-GCC (MingW, gdb)

  4. #4
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    You should be putting the returned address into the first element in the array - filename[0].
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  5. #5
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    filename == filename[0]?
    This assumption is wrong.

    filename == &filename[0]
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  6. #6
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    You still have errors because "filename" is an array, not a pointer, I've edited my previous post, please re-read it.
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  7. #7
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    I still can't get it work. I am going to reread the section on pointers again. Thanks for the replies.
    IDE: Code::Blocks | Compiler Suite for Windows: TDM-GCC (MingW, gdb)

  8. #8
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    What have you changed?

  9. #9
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    Code:
    #include <stdio.h>
    #include <dirent.h>
    #include <stdlib.h>
    #include <string.h>
    
    int main(void)
    {
        DIR *dirname;
        struct dirent *direntries;
        char *filename[30];
        int x = 0;
    
        dirname = opendir("/home/phuong/Downloads");
    
        if(dirname != NULL)
        {
            while((direntries = readdir(dirname)))
            {
                filename[x] = (char *) malloc(strlen(direntries->d_name));
                strcpy(filename[x], direntries->d_name);
                x++;
            }
            (void) closedir(dirname);
        }
        else
        {
            perror("Error opening directory");
        }
    
        return 0;
    }
    IDE: Code::Blocks | Compiler Suite for Windows: TDM-GCC (MingW, gdb)

  10. #10
    TEIAM - problem solved
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    Code:
    filename[x] = (char *) malloc(strlen(direntries->d_name)+1);
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  11. #11
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    It's ok, just be sure to reset the array content:
    Code:
    char* filename[30] = {};
    also fix what click_here said above.
    If you run your code and print the content of the array, with e.g.
    Code:
    for(size_t i = 0; i < sizeof(filename)/sizeof(*filename) && filename[i]; ++i) fprintf(stderr, "* %s\n", filename[i]);
    you'll see the list of your files.
    Do you expect something else?
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  12. #12
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    Quote Originally Posted by root4 View Post
    Code:
    for(size_t i = 0; i < sizeof(filename)/sizeof(*filename) && filename[i]; ++i) fprintf(stderr, "* %s\n", filename[i]);
    I just used a boring for loop.
    Code:
    #include <stdio.h>
    #include <dirent.h>
    #include <stdlib.h>
    #include <string.h>
    
    int main(void)
    {
        DIR *dirname;
        struct dirent *direntries;
        char *filename[30];
        int i, x = 0;
    
        dirname = opendir("/home/aspire/Documents");
    
        if(dirname != NULL)
        {
            while((direntries = readdir(dirname)))
            {
                filename[x] = (char *) malloc(strlen(direntries->d_name) + 1);
                strcpy(filename[x], direntries->d_name);
                x++;
            }
            (void) closedir(dirname);
        }
        else
        {
            perror("Error opening directory");
        }
    
        for(i = 0; i < x; i++)
        {
            printf("%s\n", filename[i]);
            free(filename[i]);
        }
    
        return 0;
    }
    IDE: Code::Blocks | Compiler Suite for Windows: TDM-GCC (MingW, gdb)

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