C语言文件 任意位置读取若干字符

#include <stdio.h>
#include <stdlib.h>
void readBytes(FILE*fp,int offset,int num,char str2[100]);
int main(int argc, char *argv[]) {
	FILE*fp=fopen("file.txt","w+");
    if(fp==NULL){
    	printf("error\n");
    	exit(0);
    }
    
	char str[100],str2[100];
	int offset,num;
	//input
    printf("please input the sentences you want to write to the file\n");
    gets(str);
    fprintf(fp,"%s",str);
    printf("please enter the offset of the first character to be read\n");
    scanf("%d",&offset);
    printf("please enter the number of characters to be read\n");
    scanf("%d",&num);
    //call the function
    readBytes(fp,offset,num,str2);
    fclose(fp);
    
	system ("pause");
	return 0;
}
void readBytes(FILE*fp,int offset,int num,char str2[100]){
	fseek(fp,offset,SEEK_SET);
	fgets(str2,num+1,fp);
    printf("result: ");
	puts(str2);
}