c - 将时间修改标记写入文件顶部

Question

因此，我尝试将“创建时间”和“上次修改时间”时间戳写入报告文件的顶部。代码看起来很简单，但是却行不通。文件的顶部应如下所示:

File created: *timestamp*
Last updated: *timestamp*

首次创建文件时，两个时间戳应该相同。每次对文件进行连续修改时，都应更新第二个文件。然而，第二个时间戳似乎从未改变。这是代码。请注意，asctime() 返回的字符串有一个内置的换行符，我认为这是违反直觉的。

NewFile = 0;
//open for updating, or create
if(   (fid = fopen(FileName,"r+"))   ==    NULL){
    NewFile = 1;
    if(   (fid = fopen(FileName,"w"))    ==    NULL){
        fprintf(stderr,"%s: Could not access file\n",FileName);
        return 1;
    }
}

// Update time
time(&rawtime);
timestruct=localtime(&rawtime);

if(NewFile){
    fprintf(fid,"File created: %32s",asctime(timestruct)); //write the creation timestamp
}else{
    fgets(linebuffer,128,fid); //skip over the creation timestamp
}
printf("Last Updated: %s\n",asctime(timestruct)); //prints correct time to console
fprintf(fid,"Last Updated: %32s\n",asctime(timestruct)); //doesn't seem to have an effect on the file
//fprintf(fid,"Last Updated: %32s\n",asctime(timestruct));

fflush(fid);
fclose(fid);
return 0;

奇怪的是 printf() 语句将正确的时间打印到控制台。但等效的 fprintf() 语句始终显示原始时间。为了进行实验，我添加了一个额外的 fprintf 语句，该语句与第一个语句相同(在上面的代码中注释掉)。这是为了确保 fprintf() 正在执行任何操作。结果是三个始终反射(reflect)创建时间的时间戳。

我已经工作了很长时间，所以我的大脑已经死了。所以也许我错过了一些明显的东西。

Answer 1

尝试使用“w+”打开，读写之间的切换需要fseek。

#include<stdio.h>
#include<stdlib.h>
#include<time.h>

int main() {
    char linebuffer[256] = {0};
    int NewFile = 0;
    time_t rawtime;
    struct tm *timestruct;
    FILE *fid;
    //open for updating, or create
    if(   (fid = fopen("FileName","r+"))   ==    NULL){
        NewFile = 1;
        if(   (fid = fopen("FileName","w+"))    ==    NULL){// open with w+
            fprintf(stderr,"Could not access FileName\n");
            return 1;
        }
    }

    // Update time
    time(&rawtime);
    timestruct=localtime(&rawtime);

    if(NewFile){
        fprintf(fid,"File created: %32s",asctime(timestruct)); //write the creation timestamp
    }else{
        fgets(linebuffer,128,fid); //skip over the creation timestamp
        fseek ( fid, 0, SEEK_CUR);//switch from read to write
    }
    printf("Last Updated: %s\n",asctime(timestruct));
    fprintf(fid,"Last Updated: %32s\n",asctime(timestruct));// now writing

    fclose(fid);

    return 0;
}