How to use the fopen function in the C language?

The fopen function is used to open a file and returns a pointer to that file.

Function prototype:
FILE* fopen(const char *filename, const char *mode);

Description of parameters:

1. File name: the name of the file that you want to open, which can be either an absolute path or a relative path.
2. mode: The mode for opening a file can be one of the following:

   “r”: Opens the file in read-only mode.
   “w”: Opens the file in write mode, clears the file content if the file already exists, and creates a new file if it doesn’t exist.
   “a”: Opens the file in append mode, appends content at the end of the file if it already exists, and creates a new file if it doesn’t exist.
   “rb”: Opens the file in binary read-only mode.
   “wb”: Opens the file in binary write mode.
   “ab”: Opens the file in binary append mode.
   For more modes, please refer to the C language documentation.

Return value:

1. If the file is successfully opened, a pointer to the FILE structure is returned, which can be used for subsequent file operations.
2. If the file fails to open, return NULL.

For example, the following code demonstrates how to use the fopen function to open a file and read its contents:

#include <stdio.h>

int main() {
    FILE *fp;
    char ch;

    // 打开文件
    fp = fopen("test.txt", "r");
    if (fp == NULL) {
        printf("文件打开失败\n");
        return 1;
    }

    // 读取文件内容
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);
    }

    // 关闭文件
    fclose(fp);

    return 0;
}