input_to_string.c

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

int main(void)
{
    char *content;
    long filesize;
    FILE *fp = fopen("input.txt", "rb");
    if (fp == NULL)
    {
        return 1;
    }
    else
    {
        fseek(fp, 0, SEEK_END);
        filesize = ftell(fp);
        rewind(fp);
        content = malloc((filesize + 1) * (sizeof(char)));
        fread(content, sizeof(char), filesize, fp);
        fclose(fp);
        content[filesize] = 0;
    }
    printf("%s\n", content);
}