Question

我正在编写一个程序，该程序必须反转文件的一半。

例如：

输入：abcdefgh

输出：abcdhgfe

因此，这里我的程序来自用户输入，并存储在文件中。使用fseek得到输入的长度，并创建一个该长度加1的字符数组。在另一个数组中，我以相反的顺序存储输入的后一半。问题是，程序将一些奇怪的字符写入文件。打开文件后，出现错误提示：There was a problem opening the file “filename”. The file you opened has some invalid characters.。我怎么解决这个问题？

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

int main() {
    int fsize; 
    char fname[15];
    char data[fsize+1];

    printf("Enter the name of file you want to create: ");
    scanf("%s", fname);

    FILE *fp;
    fp = fopen(fname, "w+");

    printf("Enter data for that file: ");
    scanf("%s", data);
    fprintf(fp, "%s", data);

    fseek(fp, 0L, SEEK_END); // get length of file
    fsize = ftell(fp);
    fseek(fp, 0L, SEEK_SET); // go back to the beginning of file

    int j = 0, m = (fsize / 2) + 1;
    char data2[m];

    for (int k = fsize - 1; j < k; j++, k--) {
        data2[j] = data[k];
    }
    for (int i = 0; i < fsize / 2; i++) {
        printf("%c", data2[i]);
    }
    printf("\n");
    fprintf(fp, "%s", data2);
    fclose(fp);
    return 0;
}

Answer 1

第一个错误是char data[fsize+1]行，因为fsize尚未初始化。另外，在编译时还不知道数组的大小，因此您应按照in this post所述使用malloc / calloc。对于char data2[m]行也是如此。

另外，您通常应避免使用scanf，因为输入的文件名长度超过14个字符会导致缓冲区fname溢出并导致错误的行为，请参见this post。

Answer 2

卢卡斯·科斯特勒（Lukas Koestler）在回答中说的话：

在

char data[fsize+1];

fsize 是未知的，因此行为是不确定的，请像对 fname 那样使用固定大小。

始终保护（f）scanf，以限制读取的长度以保留在缓冲区中

而且：

检查打开结果。

避免使用变量数组维，实际上，您不需要 data2 。

坦率地说，我不明白为什么您要在文件中写入以了解输入数据的大小，它是一个字符串，只需使用 strlen

您错过了将数据的前半部分写入文件的操作，并且 data2 不是以空字符结尾的，因此fprintf(fp,"%s",data2);的行为未定义

您不能正确管理只有1个字符的空数据。

您的程序可以是：

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

int main()
{
  char fname[15];
  char data[100];

  printf("Enter the name of file you want to create: ");
  scanf("%14s", fname);

  printf("Enter data for that file: ");
  scanf("%99s", data);

  size_t sz = strlen(data);

  if (sz < 2) {
    puts("data too small");
    return -1;
  }

  FILE *fp = fopen(fname, "w+");

  if (fp == NULL) {
    printf("cannot open %s\n", fname);
    return -1;
  }

  size_t m = sz/2;
  size_t i;

  for (i = 0; i != m; ++i) {
    putchar(data[i]);
    fputc(data[i], fp);
  }

  for (i = sz - 1; i >= m; --i) {
    putchar(data[i]);
    fputc(data[i], fp);
  }
  putchar('\n');
  fputc('\n', fp);

  fclose(fp);
  return 0;
}

编译和执行：

pi@raspberrypi:~ $ gcc -g -pedantic -Wextra f.c
pi@raspberrypi:~ $ ./a.out
Enter the name of file you want to create: aze
Enter data for that file: a
data too small
pi@raspberrypi:~ $ ./a.out
Enter the name of file you want to create: aze
Enter data for that file: az
az
pi@raspberrypi:~ $ cat aze
az
pi@raspberrypi:~ $ ./a.out
Enter the name of file you want to create: aze
Enter data for that file: aze
aez
pi@raspberrypi:~ $ cat aze
aez
pi@raspberrypi:~ $ ./a.out
Enter the name of file you want to create: aze
Enter data for that file: azerty
azeytr
pi@raspberrypi:~ $ cat aze
azeytr
pi@raspberrypi:~ $ ./a.out
Enter the name of file you want to create: aze
Enter data for that file: abcdefgh
abcdhgfe
pi@raspberrypi:~ $ cat aze
abcdhgfe

在 valgrind 下执行，请始终使用它

pi@raspberrypi:~ $ valgrind ./a.out
==5321== Memcheck, a memory error detector
==5321== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==5321== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==5321== Command: ./a.out
==5321== 
Enter the name of file you want to create: aze
Enter data for that file: az
az
==5321== 
==5321== HEAP SUMMARY:
==5321==     in use at exit: 0 bytes in 0 blocks
==5321==   total heap usage: 4 allocs, 4 frees, 6,496 bytes allocated
==5321== 
==5321== All heap blocks were freed -- no leaks are possible
==5321== 
==5321== For counts of detected and suppressed errors, rerun with: -v
==5321== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)
pi@raspberrypi:~ $ valgrind ./a.out
==5322== Memcheck, a memory error detector
==5322== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==5322== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==5322== Command: ./a.out
==5322== 
Enter the name of file you want to create: aze
Enter data for that file: azertyu
azeuytr
==5322== 
==5322== HEAP SUMMARY:
==5322==     in use at exit: 0 bytes in 0 blocks
==5322==   total heap usage: 4 allocs, 4 frees, 6,496 bytes allocated
==5322== 
==5322== All heap blocks were freed -- no leaks are possible
==5322== 
==5322== For counts of detected and suppressed errors, rerun with: -v
==5322== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)
pi@raspberrypi:~ $

从字符数组写入文件后，它说“打开文件存在问题”

2 个答案: