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

main()
{
    FILE *fp;
    char buff[256] ;
    char read_data[3][64];
    char *p;
    char no[6];
    char name[64];
    char s_size[9];
    char dummy_no[6];
    char dummy_name[64];
    long size;
    int  i;

    fp = fopen("file.txt","r");
    if (fp == NULL) {
        printf("file open error\n") ;
        exit(1) ;
    }

    memset(dummy_no,'\0',sizeof dummy_no) ;
    memset(dummy_name,'\0',sizeof dummy_name) ;

    while(fgets(buff,sizeof buff,fp)) {
        i = 0;
        p=strtok(buff,",");
        strcpy(no,p);
        p=strtok(NULL,",");
        strcpy(name,p);
        p=strtok(NULL,",");
        strcpy(s_size,p);

        if (strcmp(dummy_no,no) == 0) {
            size = size + atol(s_size) ;
        } else {
            if (dummy_no[0] != '\0') {
                printf("No[%s] Name[%s] size[%ld]\n",dummy_no,dummy_name,size);
                size = 0;
            }
            size = atoi(s_size);
            strcpy(dummy_name,name);
            strcpy(dummy_no,no);
        }

        memset(read_data,'\0',sizeof read_data) ;
    }
    printf("No[%s] Name[%s] size[%ld]\n",dummy_no,dummy_name,size);
    fclose(fp);
}

000001,yamada.taro,100
000001,yamada.taro,100
000001,yamada.taro,100
000001,yamada.taro,100
000001,yamada.taro,100
000002,satou.jiro,1000
000003,yamamoto.koutaro,123
000003,yamamoto.koutaro,100
000003,yamamoto.koutaro,10
000004,ito.hanako,250

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

typedef struct data{
    char no[64];
    char name[128];
    int size;
}DATA;

int main(void){

    DATA rec = {{'\0'},{'\0'},0};
    DATA old = {{'\0'},{'\0'},0};
    FILE *fp;
    char buf[256];
    int total = 0;
    
    if((fp = fopen("file","r")) == NULL) return 1;
    
    do{
        old.no[0] = '\0';
        total = 0;
        while(!strcmp(old.no,rec.no) || old.no[0] == '\0'){
            old = rec;
            total += rec.size;
            fgets(buf,256,fp);
            if(feof(fp)) break;
            sscanf(buf,"%[^,],%[^,],%d",rec.no,rec.name,&rec.size);
        }
        printf("%s,%s,%d\n",old.no,old.name,total);
    }while(!feof(fp));
    
    fclose(fp);
    
    return 0;
}

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

int main(void)
{
    char buff[256], name[64], prev_name[64];
    int no, prev_no = -1;
    long size, total_size;

    FILE *fp = fopen("file.txt","r");
    if (fp == NULL) printf("file open error\n"), exit(1);

    while (fgets(buff, sizeof buff, fp)) {
        if (sscanf(buff, "%d,%[^,],%ld", &no, name, &size) != 3) continue;
        if (no != prev_no) {
            if (prev_no > 0)
                printf("No[%06d] Name[%s] size[%ld]\n", prev_no, prev_name, total_size);
            prev_no = no;
            strcpy(prev_name, name);
            total_size = size;
        } else
            total_size += size;
    }
    printf("No[%06d] Name[%s] size[%ld]\n", prev_no, prev_name, total_size);
    fclose(fp);
    return 0;
}