How to dynamically allocate memory space for a string and get that string from user

Question

I want to read input from user using C program  I don t want to use array like   char names 50     because if the user gives string of length 10  then the remaining spaces are wasted   If I use character pointer like   char  names    then I need to allocate memory for that in such a way of   names    char   malloc 20   sizeof char      In this case also  there is a possibility of memory wastage   So  what I need is to dynamically allocate memory for a string which is of exactly same as the length of the string   Lets assume   If the user input is  stackoverflow   then the memory allocated should be of 14  i e  Length of the string   13 and 1 additional space for   0     How could I achieve this

User · Answer

This is a function snippet I wrote to scan the user input for a string and then store that string on an array of the same size as the user input. Note that I initialize j to the value of 2 to be able to store the '\0' character.

char* dynamicstring() {
    char *str = NULL;
    int i = 0, j = 2, c;
    str = (char*)malloc(sizeof(char));
    //error checking
    if (str == NULL) {
        printf("Error allocating memory\n");
        exit(EXIT_FAILURE);
    }

    while((c = getc(stdin)) && c != '\n')
    {
        str[i] = c;
        str = realloc(str,j*sizeof(char));
        //error checking
        if (str == NULL) {
            printf("Error allocating memory\n");
            free(str);
            exit(EXIT_FAILURE);
        }

        i++;
        j++;
    }
    str[i] = '\0';
    return str;
}

In main(), you can declare another char* variable to store the return value of dynamicstring() and then free that char* variable when you're done using it.

User · Answer

char  load string       char  string    char   malloc sizeof char     string     0    int key  int sizer   2   char sup 2       0     while   key   getc stdin        n         string   realloc string sizer   sizeof char        sup 0     char  key      strcat string sup       sizer      return string      int main       char  str  str   load string     return 0

User · Answer

Below is the code for creating dynamic string     void main       char  str  c    int i   0  j   1     str    char  malloc sizeof char       printf  Enter String          while  c      n            read the input from keyboard standard input     c   getc stdin           re-allocate  resize  memory for character read to be stored     str    char  realloc str  j   sizeof char            store read character by making pointer point to c     str i    c       i        j           str i      0      at the end append null character to mark end of string    printf   nThe entered string is    s   str      free str      important step the pointer declared must be made free

User · Answer

First  define a new function to read the input  according to the structure of your input  and store the string  which means the memory in stack used  Set the length of string to be enough for your input    Second  use strlen to measure the exact used length of string stored before  and malloc to allocate memory in heap  whose length is defined by strlen  The code is shown below   int strLength   strlen strInStack   if  strLength    0        printf    strInStack   is empty  n      else       char  strInHeap    char   malloc  strLength 1    sizeof char        strcpy strInHeap  strInStack     return strInHeap    Finally  copy the value of strInStack to strInHeap using strcpy  and return the pointer to strInHeap  The strInStack will be freed automatically because it only exits in this sub-function

User · Answer

If you ought to spare memory  read char by char and realloc each time  Performance will die  but you ll spare this 10 bytes    Another good tradeoff is to read in a function  using a local variable  then copying  So the big buffer will be function scoped

User · Answer

You can also use a regular expression  for instance the following piece of code   char  names scanf   m   n     amp names    will get the whole line from stdin  allocating dynamically the amount of space that it takes  After that  of course  you have to free names

User · Answer

Here s a snippet which I wrote which performs the same functionality   This code is similar to the one written by Kunal Wadhwa   char  dynamicCharString     char  str  c  int i  str    char  malloc 1 sizeof char     while c   getc stdin  c    n         str i    c      i        realloc str i sizeof char      str i      0   return str

User · Answer

You could have an array that starts out with 10 elements  Read input character by character  If it goes over  realloc another 5 more  Not the best  but then you can free the other space later

User · Answer

Read one character at a time  using getc stdin   and grow the string  realloc  as you go   Here s a function I wrote some time ago  Note it s intended only for text input   char  getln         char  line   NULL   tmp   NULL      size t size   0  index   0      int ch   EOF       while  ch            ch   getc stdin               Check if we need to stop             if  ch    EOF    ch      n               ch   0              Check if we need to expand             if  size  lt   index                size    CHUNK              tmp   realloc line  size               if   tmp                    free line                   line   NULL                  break                            line   tmp                        Actually store the thing             line index      ch             return line

User · Answer

realloc is a pretty expensive action     here s my way of receiving a string  the realloc ratio is not 1 1      char  getAString               define two indexes  one for logical size  other for physical     int logSize   0  phySize   1        char  res  c       res    char   malloc sizeof char           get a char from user  first time outside the loop     c   getchar           define the condition to stop receiving data     while c      n                 if logSize    phySize                        phySize    2              res    char   realloc res  sizeof char    phySize                     res logSize      c          c   getchar                here we diminish string to actual logical size  plus one for  0     res    char   realloc res  sizeof char       logSize   1        res logSize      0       return res

[c] How to dynamically allocate memory space for a string and get that string from user?

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