为了向字符串追加一个换行符，只需使用strcat(line, "\n");
还要注意这些备注：

• 调用strncat(line, temp, strlen(temp))完全等同于strcat(line, temp)。
• 如果文件已经存在，则忘记关闭该文件。
• 应该在main函数之前定义或至少声明set_value函数。

没有理由在你的代码中使用单独的snprintf调用，你可以简单地写：

snprintf(line, sizeof line, "%d, %s, %d, %f\n",
             key, value1, value2, value3);

如果出于某种原因必须一次构造一个字段的输出行，可以通过跟踪输出行中的位置来简化代码，而不是为片段使用单独的数组并使用strcat连接它们：

#include <stdio.h>

int set_value(int key, char *value1, int value2, double value3) {
    char filename[32];
    int status;
    char line[5000];

    // Change directory where we work.
    if (chdir("FilesPractice1") == -1) {
        perror("Error while changing directories.");
        return -1;
    }

    // Convert key to the name of the file.
    snprintf(filename, sizeof filename, "%d.txt", key);

    // Open the file.
    if ((status = open(filename, O_RDONLY)) != -1) {
        // The file already exists, close it and report the error
        close(status);
        // Cannot use `perror` as `errno` was not set
        fprintf(stderr, "Error: key value file %s already exists.\n", filename);
        return -1;
    }

    // Since the file doesn't exist, we proceed to create it.
    if ((status = open(name, O_WRONLY | O_CREAT, 0666)) == -1) {
        perror("Error while creating the tuple.\n");
        return -1;
    }
    // Format the key and values to write them into the file.
#if 0
    // Single call to snprintf
    size_t pos = snprintf(line, sizeof line, "%d, %s, %d, %f\n",
                          key, value1, value2, value3);
#else
    // Separate calls for different parts
    size_t pos = snprintf(line + pos, sizeof line - pos, "%d", key);
    if (pos < sizeof line)
        pos += snprintf(line + pos, sizeof line - pos, ", %s", value1);
    if (pos < sizeof line)
        pos += snprintf(line + pos, sizeof line - pos, ", %d", value2);
    if (pos < sizeof line)
        pos += snprintf(line + pos, sizeof line - pos, ", %f", value3);
    if (pos < sizeof line)
        pos += snprintf(line + pos, sizeof line - pos, "\n");
#endif
    // Check if line is long enough.
    if (pos >= sizeof line) {
        fprintf(stderr, "Buffer size too small: need %zu bytes\n", pos + 1);
    }
    // Write the values into the file.
    ssize_t len = strlen(line);
    ssize_t nwritten = write(status, line, len);
    if (nwritten != len) {
        fprintf(stderr, "Error writing to %s: %zd bytes written, expected %zd.\n",
                filename, nwritten, len);
    }

    // Close the file.
    if (close(status) == -1) {
        perror("Error while closing the file.");
        return -1;
    }

    return 0;
}

int main(void) {
    int res;
    res = set_value(10, "abc", 3, 2.0);
}

“ASCII文本，无行终止符”表示文件末尾没有line ending（CR或LF）。
这不是一个错误，它只是描述了文件的内容。
如果你想像上面说的那样添加行终止符，而不是：

...
n = strlen(line);
line[n + 1] = 0;
...

将这两行更改为下面的LF（Linux约定）行：

strcat(line, "\n");

或者，如果您同时需要CR和LF（Windows约定）：

strcat(line, "\r\n");

编辑：将strncat更改为strcat。