Memory Allocation

Allocating and Deallocating, malloc() and free()

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

int main(){
    int *p = malloc(10*(sizeof(int)));

    for (int i = 0; i< 10; i++){
        p[i] = i*5;
    }

    for (int i = 0; i < 10; i++){
        printf("%d\t", p[i]);
    }
    free(p);
}

Alternative: calloc()

int *p = calloc(10, sizeof(int));
//equivalent to
int *p = malloc(10 * sizeof(int));

Changing Allocated size with realloc()

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

char *readline(FILE *fp){
    int offset = 0;
    int bufsize = 4;

    char *buf;
    int c; //char we read in

    buf = malloc(bufsize);
    if (buf == NULL) { // Error check
        return NULL;
    }


    while (c = fgetc(fp), c != '\n' && c != EOF){
        if (offset == bufsize - 1) {
            char *new_buf = realloc(buf, offset * 2);
            if (new_buf == NULL){
                free(buf);
                return NULL;
            }
            buf = new_buf;
        }
        buf[offset++] = c;
    }

    //We hit new line or End of file

    //If no char read, return null
    if (c == EOF && offset == 0){
        free(buf);
        return NULL;
    }

    if (offset < bufsize - 1){
        char *new_buf = realloc(buf, offset + 1);
        if (new_buf != NULL){
            buf = new_buf;
        }
    }

    //Add the NUL terminator;
    buf[offset] = '\0';

    return buf;
}

int main(){
    FILE *fp = fopen("../hello.txt","r");

    char *line;

    while (line = readline(fp), line != NULL){
        printf("%s\n", line);
        free(line);
    }
    fclose(fp);
}