使用man查询C库中的fopen函数使用方法。

在Linux系统中，可以使用`man`命令来查看C库函数的详细帮助文档，例如查阅`fopen`函数的用法。首先打开终端，然后输入`man 3 fopen`（3代表C语言库函数），按回车键。这将打开`man`页面，显示fopen函数的相关信息，包括函数原型、参数说明、返回值以及可能出现的错误码等。

以下是简化的`man`命令输出示例：

NAME
       fopen - open a stream for input or output

SYNOPSIS
       #include <stdio.h>
       FILE *fopen(const char *filename, const char *mode);

DESCRIPTION
       The fopen() function opens a text stream connected to the file named by filename,
       using mode to determine whether the stream is opened for reading (r), writing (w), 
       appending (a) or updating both (r+ or w+). 

       If successful, a pointer to the stream is returned and stored in the FILE pointer variable.
       Otherwise, NULL is returned.

RETURN VALUE
       On success, a pointer to the stream is returned. On error, NULL is returned.

ERRORS
       EBADF, ENOENT - For invalid file descriptors or non-existent files, respectively.

SEE ALSO
       fclose, fread, fwrite, perror, stdio.h