#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#define LENGTH 81
int main()
{
    char mystr[LENGTH];
    int mychar=0;
    FILE *pfile=NULL;
    char *filename="myfile.txt";
   
    printf("Enter an interesting string of up to %d characters:\n",LENGTH-1);
    if(!fgets(mystr,LENGTH,stdin))
    {
        printf("Input from keyboard failed.\n");
        exit(1);
    }
   
    // Creat a new file we can wrrite
    if(fopen(filename,"w"))
    {
        printf("Error opening %s for writing.Program terminated.\n",filename);
        exit(1);
    }
    setvbuf(pfile,NULL,_IOFBF,BUFSIZ);
    for(int i=strnlen(mystr,LENGTH)-1;i>=0;--i)
    {
        fputc(mystr[i],pfile);
    }
    fclose(pfile);
   
    // Open the file for read
    if(fopen(filename,"r"))
    {
        printf("Error opening %s for reading .Program terminated.",filename);
        exit(1);
    }
    setvbuf(pfile,NULL,_IOFBF,BUFSIZ);
   
    // Read a charcaters from the file and display it
    printf("the data read from the file is；\n");
    while((mychar=fgetc(pfile))!=EOF)
    {
        putchar(mychar);
    }
    fclose(pfile);
    pfile=NULL;
    remove(filename);
    return 0;
   
}

楼主，你可以 在if(fopen(filename,"r"))中的filename,后面加上文件格式，比如filename.txt
之前我读文件时也碰到过这种问题最后就是通过以在，文件名，后加上格式解决的