CODEWITHSUNDEEP
cmallocrealloc
cheshire
cheshire

Reputation: 1159

C dynamic length char array

I wanted to write a program in C that will accept a line of any length from stdin and display it or apply any function to that string. For this to work I will need a String (char []) with dynamic length.

This is how I did it:

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

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


    char *line;
    line = malloc(10);
    line[0] = '\0';

    char *str = malloc(10);

    fprintf(stdout, "Please enter your line:\n");

    while(fgets(str, 10, stdin)){

        //check for line break
        if(str[strlen(str)-1] == '\n'){
            str[strlen(str) - 1] = '\0';
            strcat(line, str);
            break;
        }

        strcat(line, str);

        line = realloc(line, strlen(line) + 10);
        str = realloc(str, strlen(str) + 10);
    }

    fprintf(stderr, "you entered %s\n", line);

    //just for testing
    /*
    fprintf(stderr, "\n str= %s \n", str );
    fprintf(stderr, "\n line= %s \n", line);
    */

    free(line);
    free(str);
    exit(EXIT_SUCCESS);
}

However, this looks awful. I need two char arrays. In char *str I will write input from stdin and concatenate it to char *line. str will only hold up to 10Bytes of chars and because of that I need to concatenate everything to line.

Is there a cleaner way to save the output from stdin in this case and apply some function on it? Am I doing this wrong? Can it be done without malloc and realloc?

Upvotes: 0

Views: 1409

Answers (1)

0___________
0___________

Reputation: 67516

This an example. You need to add the malloc & realloc result checking (I did not for the sake of simplicity)

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

#define CHUNK   32 

char *readline(void)
{
    size_t csize = CHUNK;
    size_t cpos = 0;
    char *str = malloc(CHUNK);
    int ch;
    while((ch = fgetc(stdin)) != '\n' && ch != '\r')
    {
        str[cpos++] = ch;
        if(cpos == csize)
        {
            csize += CHUNK;
            str = realloc(str, csize);
        }
    }
    str[cpos] = 0;
    return str;
}

int main()
{
    printf("\n%s\n", readline());

    return 0;
}

working example: https://onlinegdb.com/Sk9r4gOYV

You should also free the allocated memory when not needed anymore.

Upvotes: 1

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