ld problem in sqrt function

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

int read_text(char *text, int text_max_size){
    int i;
    char x;
    int length=0;
        
        scanf("%c", &x);
        while(x != '"'){
            scanf("%c", &x);
        }
        for(i=0; i<text_max_size; i++){
            scanf("%c", &x);
            printf("x: %c\n", x);
            if( x == '"'){
                break;
            }
            text[i] = x;
        }
return i;    
}

void print_text(char *text, int n){
    int i;
        printf("Text: ");
        for(i=0; i<n; i++){
            printf("%c", text[i]);
        }
        printf("\n");
}

/*
void store_text(char *text, int text_size, char **c_array, int n){
    int i;
}
*/
int main(void){
    int i, k, n;
    float root;
    char *plaintext, **ch, *ciphertext, *newplaintext;
        printf("Please type k (length): ");
        scanf("%d", &k);
        
        plaintext = (char *)malloc(k*sizeof(char));
        if(plaintext == NULL){
            printf("Error in allocating plaintext space.\n");
            exit(1);
        }
        printf("Type the text: ");
        k = read_text(plaintext, k);
        print_text(plaintext, k);
        
        if(k == 0){
            printf("There is no character...");
        }
        root = sqrt(k);
        
        if((root*2) == k){
            n = root;
        }else{
            n = ((int)root + 1);
        }
        printf("n: %d\n", n);
        
        ch = (char **)malloc(n*sizeof(char *));
        for(i=0; i<n; i++){
            ch[i] = (char *)malloc(n*sizeof(char));
        }
        
        if(ch == NULL){
            printf("Error in allocating ch space.\n");
            exit(1);
        }
        
        ciphertext = (char *)malloc((n^2)*sizeof(char));
        if(ciphertext == NULL){
            printf("Error in allocating ciphertext space.\n");
            exit(1);
        }
        
        newplaintext = (char *)malloc((n^2)*sizeof(char));
        if(newplaintext == NULL){
            printf("Error in allocating newplaintext space.\n");
            exit(1);
        }
        
        printf("Encoding in progres.....\n");
        //store_text(plaintext, k, ch, n);

return 0;
}

and the output is something like this:

/tmp/ccWhTnJp.o: In function `main':
ex1.c:(.text+0x1b2): undefined reference to `sqrt'
collect2: ld returned 1 exit status

I know that the wrong part is when calling the sqrt function...but i do not understand why math.h library does not support it...can anyone help? thanks guys...

[BillyTKid]

Add "-lm" in your commandline.

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thank you....