c - 修改c中文件的现有内容

Question

int main()
{
    FILE *ft;
    char ch;
    ft=fopen("abc.txt","r+");
    if(ft==NULL)
    {
        printf("can not open target file\n");
        exit(1);
    }
    while(1)
    {
        ch=fgetc(ft);
        if(ch==EOF)
        {
            printf("done");
            break;
        }
        if(ch=='i')
        {
            fputc('a',ft);
        }
    }
    fclose(ft);
    return 0;
}

正如人们所看到的，我想编辑 abc.txt，将其中的 i 替换为 a。< br/>该程序工作正常，但当我从外部打开 abc.txt 时，它似乎未经编辑。
有什么可能的原因吗？

为什么在这种情况下，i 之后的字符没有被 a 替换，正如答案所示？

Answer 1

分析

存在多个问题:

1. fgetc()返回 int ，不是char ;它必须返回每个有效的 char值加上一个单独的值 EOF。正如所写，您无法可靠地检测 EOF。如果char是无符号类型，你永远找不到EOF；如果char是有符号类型，您可能会将某些有效字符(通常是 ÿ、y 元音变音、U+00FF、带分音符的拉丁小写字母 Y)误识别为 EOF。

2. 如果在以更新模式打开的文件上切换输入和输出，则必须在读取和写入之间使用文件定位操作( fseek() 、 rewind() ，名义上 fsetpos() )；并且必须使用定位操作或 fflush()介于写作和阅读之间。

3. 关闭您打开的内容是个好主意(现已在代码中修复)。

4. 如果您的写入有效，您将覆盖 i 之后的字符。与 a .

综合

这些变化导致:

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

int main(void)
{
    FILE *ft;
    char const *name = "abc.txt";
    int ch;
    ft = fopen(name, "r+");
    if (ft == NULL)
    {
        fprintf(stderr, "cannot open target file %s\n", name);
        exit(1);
    }
    while ((ch = fgetc(ft)) != EOF)
    {
        if (ch == 'i')
        {
            fseek(ft, -1, SEEK_CUR);
            fputc('a',ft);
            fseek(ft, 0, SEEK_CUR);
        }
    }
    fclose(ft);
    return 0;
}

还有更多错误检查的空间。

注释

输入后输出需要查找

fseek(ft, 0, SEEK_CUR); C 标准要求声明。

ISO/IEC 9899:2011 §7.21.5.3 The fopen function

¶7 When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of- file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.

(强调已添加。)

fgetc()返回 int

引自 ISO/IEC 9899:2011(当前的 C 标准)。

§7.21 Input/output <stdio.h>

§7.21.1 Introduction

EOF which expands to an integer constant expression, with type int and a negative value, that is returned by several functions to indicate end-of-file, that is, no more input from a stream;

§7.21.7.1 The fgetc function

int fgetc(FILE *stream);

¶2 If the end-of-file indicator for the input stream pointed to by stream is not set and a next character is present, the fgetc function obtains that character as an unsigned char converted to an int and advances the associated file position indicator for the stream (if defined).

Returns

¶3 If the end-of-file indicator for the stream is set, or if the stream is at end-of-file, the end-of-file indicator for the stream is set and the fgetc function returns EOF. Otherwise, the fgetc function returns the next character from the input stream pointed to by stream. If a read error occurs, the error indicator for the stream is set and the fgetc function returns EOF.²⁸⁹⁾

²⁸⁹⁾ An end-of-file and a read error can be distinguished by use of the feof and ferror functions.

所以，EOF是一个负整数(通常是-1，但标准没有要求)。 fgetc()函数返回 EOF 或字符值 unsigned char (范围为 0..UCHAR_MAX，通常为 0..255)。

§6.2.5 Types

¶3 An object declared as type char is large enough to store any member of the basic execution character set. If a member of the basic execution character set is stored in a char object, its value is guaranteed to be nonnegative. If any other character is stored in a char object, the resulting value is implementation-defined but shall be within the range of values that can be represented in that type.

¶5 An object declared as type signed char occupies the same amount of storage as a ‘‘plain’’ char object.

§6 For each of the signed integer types, there is a corresponding (but different) unsigned integer type (designated with the keyword unsigned) that uses the same amount of storage (including sign information) and has the same alignment requirements.

§15 The three types char, signed char, and unsigned char are collectively called the character types. The implementation shall define char to have the same range, representation, and behavior as either signed char or unsigned char.⁴⁵⁾

⁴⁵⁾ CHAR_MIN, defined in <limits.h>, will have one of the values 0 or SCHAR_MIN, and this can be used to distinguish the two options. Irrespective of the choice made, char is a separate type from the other two and is not compatible with either.

这证明了我的断言:简单的 char可以是有符号或无符号类型。

现在考虑:

char c = fgetc(fp);
if (c == EOF)
   …

假设fgetc()返回 EOF，并且简单 char是无符号(8 位)类型，EOF 为 -1 。该赋值将值 0xFF 放入 c ，这是一个正整数。进行比较时，c晋升为int (因此为值 255)，并且 255 不是负数，因此比较失败。

相反，假设简单的 char是有符号(8 位)类型，字符集为 ISO 8859-15。如果fgetc()返回 ÿ，分配的值将是位模式 0b11111111，与 -1 相同，因此在比较中，c将转换为-1和比较c == EOF即使读取了有效字符，也会返回 true。

您可以调整细节，但基本论点在 sizeof(char) < sizeof(int) 期间仍然有效。 。有些 DSP 芯片不适用这一点；你必须重新考虑规则。即便如此，基本点依然存在； fgetc()返回 int ，不是char .

如果您的数据确实是 ASCII(7 位数据)，那么所有字符都在 0..127 范围内，并且您不会遇到 ÿ 的误解问题。但是，如果您的char类型是无符号的，你仍然有“无法检测EOF”的问题，所以你的程序会运行很长时间。如果你需要考虑可移植性，你就会考虑到这一点。这些是作为 C 程序员需要处理的专业级问题。您可以相对轻松地找到在您的系统上运行的程序来获取数据，而无需考虑所有这些细微差别。但你的程序无法在其他人的系统上运行。