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Array of pointers and pointer for arrays with argv

This is a discussion on Array of pointers and pointer for arrays with argv within the C Programming forums, part of the General Programming Boards category; Good afternoon. I was wondering why this program with a pointer for arrays with char (*argv)[100] doesn't work at all. ...

  1. #1
    Tears of the stars thames's Avatar
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    Array of pointers and pointer for arrays with argv

    Good afternoon. I was wondering why this program with a pointer for arrays with char (*argv)[100] doesn't work at all.

    I understand char (*argv)[100] is declaring a pointer for arrays with n lines and a maximum of 13 chars. And char *argv[] is declaring an array for n char pointers (strings). Therefore, I thought the program below would work as expected, printing thames thames thames when I input:

    Code:
     
      ./strcpy thames thames thames
    My line of thought was: "well, if I input less than 100 characters, everything will work just fine. "

    Code:
     
    #include <stdio.h> 
    #include <stdlib.h> /* for exit and malloc*/
    #include <string.h> 
    
    #ifndef MAXPOINTERS
      #define MAXPOINTERS 20
    #endif 
    
    #ifndef MAXCHARS
      #define MAXCHARS 100
    #endif    
    
    void Strcpy(char*, char*);
    char** Malloc2D(int, int);
    
    void Strcpy(char* s, char* t) 
    { 
       while( (*s++ = *t++) )
       ;
    }
    
    char** Malloc2D(int nPointers, int nChars)
    {
       int i; 
       char** strings;
       
       strings = malloc(nPointers * sizeof(char*));
       
       for(i = 0; i < nPointers; i++) 
       { 
         *(strings + i) = malloc(nChars * sizeof(char));  
       }
       
       return strings;            
    }     
    
    int main(int argc, char (*argv)[100]) 
    { 
       int i, j;
       char** s;
       
       
       if(argc < 2) 
       { 
         printf("Please enter [Executable][string]\n");  
         exit(0);
       }
       
       else if(argc >= MAXPOINTERS) 
       { 
         printf("You can write up to %d words", MAXPOINTERS);
         exit(0);   
       }        
       
       s = Malloc2D(MAXPOINTERS, MAXCHARS);
       
       for(j = 1, i = 0; i < argc - 1; i++, j++) 
       { 
          Strcpy(s[i], argv[j]);  
       }        
       
       printf("\nThe strings which were input are:\n\n");
       for(i = 0; i < argc - 1; i++) 
       { 
          printf("%d.\t%s\n", i + 1, s[i]);   
       }         
       
       return 0;
    }

  2. #2
    SAMARAS std10093's Avatar
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    When i compile it
    Code:
    Macintosh-c8bcc88e5669-9:~ usi$ gcc -Wall px.c -o px
    px.c:38: warning: second argument of ‘main’ should be ‘char **’
    I think here is something you should read.
    Last edited by std10093; 12-02-2012 at 09:10 AM. Reason: damaged url
    Code - functions and small libraries I use


    It’s 2014 and I still use printf() for debugging.


    "Programs must be written for people to read, and only incidentally for machines to execute. " —Harold Abelson

  3. #3
    Tears of the stars thames's Avatar
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    Quote Originally Posted by std10093 View Post
    When i compile it
    Code:
    Macintosh-c8bcc88e5669-9:~ usi$ gcc -Wall px.c -o px
    px.c:38: warning: second argument of ‘main’ should be ‘char **’
    I think [URL="http://www.cprogramming.com/tutorial/c/lesson14.html"]here/URL] is something you should read.
    I knew about this warning, but I'd like to understand why, considering what I said, char (*argv)[50] wouldn't work. will that notation work in another function for what I wanted to print ?

    edit:

    for example, instead of argv I have char (*str)[50] and I want to print thames

    do I have to write char *str[50] to print the output as expected ? why? if I have n letters and I tried to print less than 50 letters?
    Last edited by thames; 12-02-2012 at 09:11 AM.

  4. #4
    Registered User whiteflags's Avatar
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    I think looking up the type would be illuminating.

    char (*str)[50]
    declare str as pointer to array 50 of char

    char *str[50]
    declare str as array 50 of pointer to char

    So there is a difference, and argv is just a list of pointers at least argc long. And argv[argc] is guaranteed to be NULL.
    thames and AndiPersti like this.

  5. #5
    Tears of the stars thames's Avatar
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    I think looking up the type would be illuminating
    I got the idea.

    And argv[argc] is guaranteed to be NULL
    this is interesting, I didn't know that. Thanks.

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