これ以上何も言うまい。
駄作ではあるがこれでも持って帰って今まで悪い夢でも見ていたんだろうとみんなで思おうじゃないか。
そう、全て悪い夢だったのさ。

#include <stdio.h>
#include <stdlib.h>
#define FILENAME_READ "data.txt"
#define FILENAME_WRITE "data_new.txt"
#define NAMESIZE 40
#define REALLOC_UNIT 10
struct person{int age;float height;char name[NAMESIZE];};
int main(){int i,j,k,n;FILE *fp;struct person *s,tmp;
if((fp=fopen(FILENAME_READ,"r"))==NULL){fprintf(stderr,"Fileopen failed (read:"FILENAME_READ").\n");return 1;}
n=0;s=(struct person*)malloc(sizeof(struct person)*REALLOC_UNIT);
while(fscanf(fp,"%d %f %s",&(s[n].age),&(s[n].height),s[n].name)!=EOF)
{n++;if(n%REALLOC_UNIT==0)s=(struct person*)realloc(s,sizeof(struct person)*(n+REALLOC_UNIT));}fclose(fp);
for(i=0;i<n-1;i++){k=i;for(j=i+1;j<n;j++)if(s[k].age>s[j].age)k=j;if(k!=i){tmp=s[i];s[i]=s[k];s[k]=tmp;}}
for(i=0;i<n;i++)printf("%d %f %s\n",s[i].age,s[i].height,s[i].name);
if((fp=fopen(FILENAME_WRITE,"w"))==NULL){fprintf(stderr,"Fileopen failed (write:"FILENAME_WRITE").\n");return 1;}
for(i=0;i<n;i++)fprintf(fp,"%d %f %s\n",s[i].age,s[i].height,s[i].name);fclose(fp);return 0;}