GIF画像からXMPを削除する

//rmxmp.c

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

#define TYPE_GIF 1

int fileType(char*);
int removeXmp_GIF(char*);

int main(int argc, char *argv[])
{
  if(0 < argc){
    for(int i=1; i < argc; i++){
      switch(fileType(argv[i])){
      case TYPE_GIF:
        removeXmp_GIF(argv[i]);
        break;
      }
    }
  }
  return 0;
}

int fileType(char* fileName){
  FILE* fp = NULL;
  if((fp = fopen(fileName, "r")) == NULL){
    return -1;
  }

  char data[7] = {0};
  fread(data, 1, 6, fp);
  if(strcmp("GIF89a", data) == 0){
    //GIF
    return TYPE_GIF;
  };

  // other files


  fclose(fp);
  return -1;
}

int removeXmp_GIF(char* fileName){
  char xmp[] = {0x0b, 0x58, 0x4d, 0x50, 0x20, 0x44,
                    0x61, 0x74, 0x61, 0x58, 0x4d, 0x50};
  bool exists = false;
  int c;
  long s, e, f;

  FILE* fp = NULL;
  if((fp = fopen(fileName, "r")) == NULL){
    return -1;
  }

  fseek(fp, 0, SEEK_END);
  f = ftell(fp);
  rewind(fp);

  while((c=fgetc(fp)) != EOF){
    if(c == 0x21 && fgetc(fp) == 0xff){
      // application extension
      char data[12] = {0};
      fread(data, 1, 12, fp);
      if(memcmp(data, xmp, 12) == 0){
        s = ftell(fp) - 14;
        exists = true;
        break;
      }
    }
  }
  if(!exists){
    return 1;
  }

  // XMPデータサイズの計測
  while((c=fgetc(fp)) != EOF){
    if(c != 0x00){
      fseek(fp, c, SEEK_CUR);
    }
    else{
      e = ftell(fp);
      break;
    }
  }

  char* remain = (char *)malloc((size_t)(f - (e - s)));

  // XMP以外を読み込み
  fseek(fp, 0, SEEK_SET);
  fread(remain, 1, s, fp);

  fseek(fp, e, SEEK_SET);
  fread(remain+s, 1, f-e, fp);

  fclose(fp);

  // 新規作成
  char tmp[ L_tmpnam ];
  tmpnam(tmp);
  if((fp = fopen(tmp, "w+")) == NULL){
    return -1;
  }
  fwrite(remain, 1, (f - (e - s)), fp);

  free(remain);
  fclose(fp);

  // 置き換え
  remove(fileName);
  rename(tmp, fileName);

  return 0;
}