How to use fseek function in C language

The fseek function is used to set the file pointer position in the file stream. Its syntax is fseek(FILE *stream, long int offset, int whence). Depending on the whence parameter, offset is relative to the beginning of the file (SEEK_SET), the current position (SEEK_CUR), or the end of the file (SEEK_END). If the operation is successful, 0 is returned; otherwise, -1 is returned and the errno variable is set to indicate the error.

Usage of fseek function in C language

The fseek function is used to set file reading and writing in the file stream The position of the pointer. The syntax is:

<code class="c">int fseek(FILE *stream, long int offset, int whence);</code>

Among them:

• stream: Pointer to the file stream to be operated on.
• offset: The offset relative to the file starting from the position specified by whence.

• whence: Specify the position relative to offset, there are the following options:

  • SEEK_SET: Start from the beginning of the file.
  • SEEK_CUR: Start from the current file position.
  • SEEK_END: Start from the end of the file.

Usage:

1. Locate to the beginning of the file:

   <code class="c">fseek(stream, 0, SEEK_SET);</code>

2. Locate to a specific location in the file:

   <code class="c">fseek(stream, 100, SEEK_SET); // 定位到文件中的第 101 个字节</code>

3. Move forward from the current position:

   <code class="c">fseek(stream, 50, SEEK_CUR); // 从当前位置向前移动 50 个字节</code>

4. Move backward from the end of the file:

   <code class="c">fseek(stream, -10, SEEK_END); // 从文件末尾向后移动 10 个字节</code>

Return value:

If the operation is successful, the fseek function returns 0. If the operation fails, -1 is returned and the errno variable is set to indicate the error.

Example:

The following example demonstrates how to navigate to a specific location in a file and read data:

<code class="c">#include <stdio.h>

int main() {
  FILE *fp;
  char buffer[100];

  // 打开文件
  fp = fopen("test.txt", "r");

  // 定位到文件中的第 101 个字节
  fseek(fp, 100, SEEK_SET);

  // 从该位置读取数据
  fread(buffer, 1, 50, fp);

  // 关闭文件
  fclose(fp);

  return 0;
}</code>

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