fseek函数c语言

In this article, we’ll take a look at using the fseek() function in C/C++.

在本文中，我们将研究在C / C ++中使用fseek（）函数。

fseek() is a very useful function to traverse through a file. We can ‘seek’ to different locations, by moving the file pointer.

fseek（）是遍历文件的非常有用的函数。 通过移动文件指针，我们可以“搜索”到不同的位置。

This enables us to control where we can read and write to and from files.

这使我们能够控制在哪里可以读写文件。

Let’s take a look at using this functions, using some illustrative examples!

让我们使用一些说明性示例来看看如何使用此功能！

C / C ++中fseek（）的基本语法 (Basic Syntax of fseek() in C/C++)

The fseek() function will move the file pointer to a file, based on the option that we give it.

fseek()函数将根据我们提供的选项将文件指针移至文件。

This function is present in the <stdio.h> header file.

<stdio.h>头文件中提供了此功能。

The prototype of the function is as follows:

该函数的原型如下：

#include <stdio.h>int fseek(FILE* fp, int offset, int position);

Usually, if we are moving the pointer, we need to specify the starting position (offset) from which it will move!

通常，如果要移动指针，则需要指定指针从其开始移动的起始位置（ offset ）！

There are three options for choosing position, from where you can use offset to shift the pointer.

有三种选择position选项，您可以在其中使用offset来移动指针。

Here, position can take the following macro values:

在这里， position可以采用以下宏值：

• SEEK_SET -> We place the initial position at the start of the file, and shift from there.SEEK_SET- >我们将初始位置放在文件的开头，然后从那里开始移动。
• SEEK_CUR -> The initial position is taken at the current position of the existing file pointer.SEEK_CUR- >初始位置在现有文件指针的当前位置。
• SEEK_END -> We place the initial position at the end of the file. If you shift the pointer from this position, you will reach EOF.SEEK_END- >我们将初始位置放在文件末尾。 如果将指针从该位置EOF ，将到达EOF 。

If the function executes successfully, it will return 0. Otherwise, it will return a non-zero value.

如果函数成功执行，它将返回0。否则，它将返回非零值。

NOTE: In case of SEEK_END, the offset position is measured backwards, so we’ll be moving from the end of the file!

注意 ：对于SEEK_END ， offset位置是向后测量的，因此我们将从文件末尾开始！

For example, if you try to seek to a position which doesn’t exist, it will fail!

例如，如果您尝试寻找一个不存在的职位，它将失败！

Now that we’ve covered the basic syntax, let’s look at some examples now, using fseek().

既然我们已经介绍了基本语法，现在让我们使用fseek()看一些示例。

For the entire demonstration, we’ll work with the file sample.txt with the following content:

对于整个演示，我们将使用具有以下内容的sample.txt文件：

Hello from JournalDev
This is a sample file
This is the last line.

在C / C ++中使用fseek（）–一些示例 (Using fseek() in C / C++ – Some Examples)

In our first example, we’ll use fseek(), along with fread(), to read from an existing file.

在第一个示例中，我们将使用fseek()和fread()来读取现有文件。

We’ll move the pointer to the start of the file, and place the offset at a dsitance of 5 positions. offset = 5

我们将指针移到文件的开头，并将偏移量放置在5个位置的距离上。 offset = 5

#include <stdio.h>int main() {// Open the fileFILE* fp = fopen("sample.txt", "r");// Move the pointer to the start of the file// And set offset as 5fseek(fp, 5, SEEK_SET);char buffer[512];// Read from the file using fread()fread(buffer, sizeof(buffer), sizeof(char), fp);printf("File contains: %s\n", buffer);// Close the filefclose(fp);return 0;
}

Output

输出量

File contains:  from JournalDev
This is a sample file
This is the last line.

As you can see, it only starts reading from position 5, after the first 5 characters. So we do not see Hello

如您所见，它仅从头5个字符之后的位置5开始读取。 所以我们看不到Hello

Now, we’ll move the pointer to the end, using SEEK_END. We’ll append to the same file, by using fwrite() at the end!

现在，我们将使用SEEK_END将指针移到末尾。 最后，使用fwrite()将附加到同一文件！

#include <stdio.h>int main() {// Open the file for writingFILE* fp = fopen("sample.txt", "a");// Move the pointer to the end of the filefseek(fp, 0, SEEK_END);char text[] = "This is some appended text";// Write to the file using fwrite()fwrite(text, sizeof(buffer), sizeof(char), fp);printf("Appended:%s to the file!\n", text);// Close the filefclose(fp);return 0;
}

Output

输出量

Hello from JournalDev
This is a sample file
This is the last line.
This is some appended text

Indeed, we were able to append the text successfully to the file!

确实，我们能够将文本成功添加到文件中！

结论 (Conclusion)

We learned about using the fseek() function in C / C++, which is quite useful if you want to shift the file pointer.

我们学习了在C / C ++中使用fseek（）函数的知识，如果您要移动文件指针，这将非常有用。

参考资料 (References)

• Linux manual page on the fseek() function in CC语言中fseek（）函数上的Linux手册页

翻译自: https://www.journaldev.com/40749/fseek-function-c-plus-plus

fseek函数c语言