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  • 1 Post By std10093

Mallocin' strlen(argv) into a char*

This is a discussion on Mallocin' strlen(argv) into a char* within the C Programming forums, part of the General Programming Boards category; edit: gotcha! Code: s = malloc(strlen(argv[1]) * sizeof(char)); Good afternoon friends. How can I malloc argv in main? Code: #include ...

  1. #1
    Tears of the stars thames's Avatar
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    Question Mallocin' strlen(argv) into a char*

    edit:

    gotcha!

    Code:
     
    s = malloc(strlen(argv[1]) * sizeof(char));
    Good afternoon friends. How can I malloc argv in main?

    Code:
     
    #include <stdio.h> 
    #include <stdlib.h> /* for exit() */
    #include <string.h> 
    
    void Strcpy(char*, char*);
    
    void Strcpy(char* s, char* t) 
    { 
       while ( (*s++ = *t++) )
       ;     
    }
    
    int main(int argc, char** argv) 
    { 
       char* s = malloc(strlen(argv) * sizeof(char));
       
       if(argc < 2) 
       { 
         printf("Please enter [Executable][string]\n");  
         exit(0);
       }
       Strcpy(s, *(argv + 1));
       printf("Clone is %s\n", s);
       
       free(s);
       return 0;
    }
    Last edited by thames; 12-01-2012 at 08:39 AM.

  2. #2
    C++ Witch laserlight's Avatar
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    I expect that you would malloc space to store pointers to char, then malloc space for each string.
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  3. #3
    Registered User camel-man's Avatar
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    Code:
    Strcpy(s, *(argv + 1));
    I think you need to change that to
    Code:
     Strcpy(s, (*argv) + 1);
    Remember to dereference first.

  4. #4
    Tears of the stars thames's Avatar
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    Quote Originally Posted by laserlight View Post
    I expect that you would malloc space to store pointers to char, then malloc space for each string.
    how would I do that ?

    Now I'm getting /strcpy from the output.

    Code:
     
    ./strcpy thames
    Clone is /strcpy
    Code:
     
    #include <stdio.h> 
    #include <stdlib.h> /* for exit() */
    #include <string.h> 
    
    void Strcpy(char*, char*);
    
    void Strcpy(char* s, char* t) 
    { 
       while( (*s++ = *t++) )
       ;
    }
    
    int main(int argc, char** argv) 
    { 
       char* s;
       
       if(argc < 2) 
       { 
         printf("Please enter [Executable][string]\n");  
         exit(0);
       }
       
       s = malloc(strlen(argv[1]) * sizeof(char));
       
       Strcpy(s, (*argv) + 1);
       printf("Clone is %s\n", s);
       
       free(s);
       return 0;
    }
    Last edited by thames; 12-01-2012 at 10:02 AM.

  5. #5
    SAMARAS std10093's Avatar
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    Quote Originally Posted by thames View Post
    how would I do that ?
    Read the comments.

    Code:
    #include <stdio.h>
    #include <string.h> /* strcpy */
    #include <stdlib.h> /* malloc */
    
    char **get(int n, int m) /* Malloc the 2D array (n x m) */
    {
        int i;
        char** table;
    
        /* Malloc n char pointers */
        table=malloc(n*sizeof(char *));
    
        /* For every char pointer, malloc space for the string
        * that they are going to point to    */
        for(i=0 ; i<n ; i++)
             /* *(table+i) is equivalent to table[i] */
            *(table+i)=malloc( m*sizeof(char) );
        return table;
    }
    
    int main(void)
    {
            int numberOfcharPtrs = 5;
    
            int spaceForEveryString = 20;
    
            int i;
    
            char** arrayOfStrings;
    
            arrayOfStrings = get( numberOfcharPtrs, spaceForEveryString);
    
            /* Fill the array with strings */
            for( i = 0 ; i < numberOfcharPtrs ; i++)
                    strcpy( arrayOfStrings[i] , "Example for thames from Brazil");
    
    
            /* Print all the strings */
            for( i = 0 ; i < numberOfcharPtrs ; i++)
                    printf("At index %d we have stored : %s\n",i,arrayOfStrings[i]);
    
            return 0;
    }
    thames likes this.
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    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

  6. #6
    Tears of the stars thames's Avatar
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    I'm getting a segmentation fault when I try to input a name:

    Code:
     
      ./strcpy thames thames
      Segmentation fault

    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) 
    { 
       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 < MAXCHARS; i++, j++) 
       { 
          Strcpy(s[i], argv[j]);  
       }        
       
       printf("The strings which were input are:\n");
       for(i = 0; i < MAXPOINTERS; i++) 
       { 
          printf("%d.\t%s", i, s[i]);   
       }         
       
       return 0;
    }

  7. #7
    SAMARAS std10093's Avatar
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    Code:
    for(j = 1, i = 0; i < MAXCHARS; i++, j++)
    How many strings should you copy inside your array?
    - I should copy inside be array as many strings as the user gives in the cmd

    How many does he input?
    - argc
    No
    - why?
    Because argc contains the name of the executable too, so the number is
    Code:
    argc-1
    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

  8. #8
    Tears of the stars thames's Avatar
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    Quote Originally Posted by std10093 View Post

    How many strings should you copy inside your array?
    - I should copy inside be array as many strings as the user gives in the cmd
    I'm using j for that purpose (to start argv at index 1) . I fixed the for:

    Code:
     
    for(j = 1, i = 0; i < MAXPOINTERS; i++, j++) 
       { 
          Strcpy(s[i], argv[j]);  
       }
    But I'm still getting the same error.

    edit:

    okey-dokey, I fixed this else as you told me:

    Code:
     
    else if(argc >= MAXPOINTERS) 
       { 
         printf("You can write up to %d words", MAXPOINTERS);
         exit(0);   
       }
    The current code:

    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) 
    { 
       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 < MAXPOINTERS; i++, j++) 
       { 
          Strcpy(s[i], argv[j]);  
       }        
       
       printf("The strings which were input are:\n");
       for(i = 0; i < MAXPOINTERS; i++) 
       { 
          printf("%d.\t%s", i, s[i]);   
       }         
       
       return 0;
    }
    Last edited by thames; 12-01-2012 at 01:44 PM.

  9. #9
    SAMARAS std10093's Avatar
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    argv[0] containts the executable name, argv[1] will contain the first string and so on...

    So what I said before in code is this
    Code:
    for(j = 1, i = 0; i < argc-1; i++, j++)
    Of course do not forget to act likewise for the printing part of your code
    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

  10. #10
    Tears of the stars thames's Avatar
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    I think I understood. I can't use MAXPOINTERS because I don't know how many pointers were entered. The remaining will cause a Segmentation Fault. A common problem.

    many thanks.

    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) 
    { 
       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");
       for(i = 0; i < MAXPOINTERS; i++) 
       { 
          printf("\n%d.\t%s", i + 1, s[i]);   
       }         
       
       return 0;
    }
    Now, I think I won't forget the segmentation fault problems are because of out of bounds access.

  11. #11
    SAMARAS std10093's Avatar
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    Isn't it nice when you find the solution by yourself? ( With some ideas from other people, but still it seems that you did understand )

    A last tip : I wouldn't print all the pointers. I would print only how many input I have.
    In code
    Code:
    printf("\nThe strings which were input are:\n");
       for(i = 0; i < MAXPOINTERS; i++) 
       { 
          printf("\n%d.\t%s", i + 1, s[i]);   
       }
    This can achieved by modifying the red part

    You know you are welcome. Very glad that I helped
    thames likes this.
    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

  12. #12
    Tears of the stars thames's Avatar
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    This can achieved by modifying the red part
    heh [trollface]

    You know you are welcome. Very glad that I helped
    sure thing! many thanks.

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