can not get dynamic string allocation to work

Question

hey guys i have looked around for a solution and tried everything i can think of im new to pointers and dynamic strings and i could really do with some help with problem. im currently learning c and i need to get the user to input a dynamic size for the string length . i need to make it so the users input can not be bigger then 100 . here's where i am at currently . i have the code booting but if i try set the size to let's say 5 i can still input way more chars into the string. cheers really appreciate any help .

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main () {
    int  maxSize = 100; 
    char *name = (char*) malloc (maxSize * sizeof (char));
    int n; 
    char text1[]= ("input string size\n");

    printf ("your string lenght is %d\n", strlen(name));

    //getting size 
    n=intPrintScanner(text1);
    printf ("your size is %d\n",n);

    name = realloc (name,  sizeof (char) * n);

    //printing results 
    printf ("your string lenght is %d\n",strlen (name));
    scanf("%s", name);
    printf("your string is %s",name);

    free(name);

    fflush(stdin);
    printf("press any key to close");
    getchar();
    return (0);
}

Answer 1

Bugs:

• You never assign any data to name so it just contains garbage. You can't use strlen on it before you have stored valid data there either. You can store a string inside name by for example using strcpy.

• When using realloc, there's no guarantee that the old pointer is the same as the returned pointer. Also, you need error handling. Implement it like this instead:

  char* tmp = realloc (name, n);
  if(tmp == NULL) 
  { 
    /* actual error handling here in case realloc fails */ }
  }
  name = tmp; // assign pointer to the new area only if allocation succeeded
  
  

• fflush(stdin); is not well-defined, never use fflush on input streams. Instead you should discard unwanted line feed characters from stdin - which could be as trivial as an extra getchar() call just after reading something. Check out How to read / parse input in C? The FAQ for lots of general good advise regarding how to take input from stdin.

Cosmetic/style:

• No need for parenthesis here: char text1[]= ("input string size\n");. All it achieves it to make the code look strange.

• The correct form of main is int main (void). The int main() is obsolete style.

• There is no need to wrap the expression passed to return in a parenthesis.

• There is never a need to multiply something with sizeof (char), since sizeof (char) is by definition always 1 no matter system.

• There is no need to cast the result of malloc.

Answer 2

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

char* read_until(int fd, char end) {
      int i = 0, size;
      char c = '\0';
      char* string = (char*)malloc(sizeof(char));

     while (1) {
          size = read(fd, &c, sizeof(char));

          if (c != end && size > 0) {
              string = (char*)realloc(string, sizeof(char) * (i + 2));
              string[i++] = c;
          } else {
              break;
          }
      }

                string[i] = '\0';
        return string;
 }

int main()
{
        char *name;
        int correct=0;

        do{
                write(1,"Put a name: ",strlen("Put a name: "));
                name = read_until(STDIN_FILENO,'\n');

                if(strlen(name) > 99){
                        write(1,"Error\n",strlen("Error\n"));

                }else{
                        correct=1;
                }

        }while(correct != 1);

        write(1,name,strlen(name));
       
        free(name);

}

Try using write and read instead of printf and scanf, it is better for allocating dynamic memory, read and try to understand the read_until function, there are better ways to do main.