C program malloc with array of strings

Question

At the beginning of a program I need to dynamically allocate memory for an unknown number of strings with unknown number of size to later manipulate with. To get the number of strings from a user I have:

int main(int argc, char *argv[]){

int number = atoi(argv[1]);

So far so good. "number" now holds holds the number that the user inputted on the command line for executing the code. Now here comes the part I don't quite understand. I now need to dynamically store the lengths of the strings as well as the contents of the strings. For example, I want the program to function like this:

Enter the length of string 1: 5
Please enter string 1: hello
Enter the length of string 2: ...

For this I recognize that I will have to create an array of strings. However, I can't quite understand the concept of pointers to pointers and what not. What I would like is perhaps a simplification of how this gets accomplished?

@AsthaSrivastava I would like these strings to later compare lengths, concatenate them, copy them, etc. but I'm familiar with those things. I just don't know how to dynamically allocate them if there are an unknown number with unknown lengths. — ClockwerkSC
– ClockwerkSC, Commented Apr 30, 2018 at 4:46
Enter the length of string 1: 5 then Please enter string 1: hello Be Careful, while you can store 5 characters 'h''e''l''l''o', in an array [5] char, it will NOT be a string of characters. (there is no nul-terminating character at the end -- which would require 6-characters of storage) Any attempt to use the array containing "hello" as a string will result in Undefined Behavior as the string-function madly searches all through your stack looking for a nul-character... — David C. Rankin
– David C. Rankin, Commented Apr 30, 2018 at 6:23
Also, don't use atoi(argv[1]) -- it provides Zero error checking capabilities (what if argv[1] = "duck"?. Instead use strtol which provides solid error handling capabilities. — David C. Rankin
– David C. Rankin, Commented Apr 30, 2018 at 6:25
the first thing the program should be doing is testing argc to assure that the user actually entered a parameter. If no parameter entered, the output a USAGE statement (fprintf( stderr, (USAGE: %s [numberOfStrings]\n", argv[0] );` then call exit( EXIT_FAILURE ) where exit() and EXIT_FAILURE are found in stdlib.h — user3629249
– user3629249, Commented Apr 30, 2018 at 20:25

score 8 · Accepted Answer · 2018-04-30 06:30:39Z

8

You know from the start you will have number strings to store so you will need an array of size number to store a pointer to each string.

You can use malloc to dynamically allocate enough memory for number char pointers:

char** strings = malloc(number * sizeof(char*));

Now you can loop number times and allocate each string dynamically:

for (int i = 0; i < number; i++) {
   // Get length of string
   printf("Enter the length of string %d: ", i);
   int length = 0;
   scanf("%d", &length);

   // Clear stdin for next input
   int c = getchar(); while (c != '\n' && c != EOF) c = getchar();

   // Allocate "length" characters and read in string
   printf("Please enter string %d: ", i);
   strings[i] = malloc(length * sizeof(char));
   fgets(strings[i], length, stdin);
}

edited Apr 30, 2018 at 6:30

answered Apr 30, 2018 at 4:45

user9614249

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9 Comments

ClockwerkSC Over a year ago

I can follow most of this. Just to clarify: a char** would be a pointer to a pointer to char? And as for fgets I am unfamiliar with that function but I can research that. Thank you.

user9614249 Over a year ago

You are right about char**, it is a pointer to a character pointer. In this case, if char* is a string, char** is essentially a dynamically allocated array if strings.

user9614249 Over a year ago

And you can read about fgets here

ClockwerkSC Over a year ago

Haha a pointer to a pointer bends my brain a little bit, but thank you for the answer and for where I can learn about new functions.

David C. Rankin Over a year ago

Unless you have redirected a file holding 10 numbers as input fseek(stdin,0,SEEK_END); will invoke Undefined Behavior as stdin is not seekable unless the input is provided by file-redirection. (e.g. a seekable file) To clear stdin to end of line, just use int c = getchar(); while (c != '\n' && c != EOF) c = getchar();

|

4386427 · Accepted Answer · 2018-04-30 08:15:17Z

1

Since you want to save both the length and the string, I'll suggest that you put them together in a struct. Like

struct string
{
    int length;
    char* str;
};

Now you can dynamically create an array of this struct and dynamically assign memory for the individual strings.

Something like:

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

struct string
{
    int length;
    char* str;
};

int main(void) {
    int i;
    char tmp[128];
    int number = 3;
    struct string* strings = malloc(number * sizeof *strings);

    // read the input
    for (i=0; i<number; ++i)
    {
        printf("length?\n");
        if (fgets(tmp, sizeof tmp, stdin) == NULL)
        {
            printf("error 1");
            exit(1);
        }
        int length;
        if (sscanf(tmp, "%d", &length) != 1)
        {
            printf("error 2");
            exit(1);
        }
        strings[i].length = length;
        strings[i].str = calloc(length + 2, 1);
        printf("string?\n");
        if (fgets(strings[i].str, length + 2, stdin) == NULL)
        {
            printf("error 3");
            exit(1);
        }
        if (strings[i].str[length] != '\n')
        {
            printf("error 4");
            exit(1);
        }
        strings[i].str[length] = '\0';
    }

    // print the strings
    for (i=0; i<number; ++i)
    {
      printf("len=%d str=%s\n", strings[i].length, strings[i].str);
    }

    // Clean up, i.e. free the memory allocated
    for (i=0; i<number; ++i)
    {
        free(strings[i].str);
    }
    free(strings);

    return 0;
}

Note: You should also check that all malloc/calloc are succesful, i.e. doesn't return NULL. For clarity I skipped that.

edited Apr 30, 2018 at 8:15

answered Apr 30, 2018 at 5:53

4386427

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1 Comment

David C. Rankin Over a year ago

By reading the exact number of characters exactly with fgets(strings[i].str, length + 1, stdin) fgets will use the +1 as storage for the nul-character and you will leave each '\n' (generated by the user pressing [Enter]) unread in stdin, this must be removed before your next call to fgets to avoid reading the unread '\n' as your next input.

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