在 C/C++ 中声明和使用 FILE * 指针的正确方法是什么？
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What is the correct way to declare and use a FILE * pointer in C/C++?
提问by paxdiablo
What is the correct way to declare and use a FILE * pointer in C/C++? Should it be declared global or local? Can somebody show a good example?
在 C/C++ 中声明和使用 FILE * 指针的正确方法是什么？它应该被声明为全局还是本地？有人可以举一个很好的例子吗？
回答by paxdiablo
It doesn't matter at all whether it's local or global. The scope of the file pointer has nothing to do with its use.
无论是本地的还是全球的，都没有关系。文件指针的范围与其使用无关。
In general, it's a good idea to avoid global variables as much as possible.
一般来说，尽可能避免使用全局变量是个好主意。
Here's a sample showing how to copy from input.txtto output.txt:
这是一个示例，展示了如何从input.txtto复制output.txt：
#include <stdio.h>
int main(void) {
FILE *fin, *fout; int c;
// Open both files, fail fast if either no good.
if ((fin = fopen("input.txt", "r")) == NULL) {
fprintf(stderr, "Cannot read from input.txt");
return 1;
}
if ((fout = fopen("output.txt", "w")) == NULL) {
fprintf(stderr, "Cannot write to output.txt");
fclose(fin);
return 1;
}
// Transfer character by character.
while ((c = fgetc(fin)) >= 0) {
fputc (c, fout);
}
// Close both files and exit.
fclose(fin);
fclose(fout);
return 0;
}
回答by Crashworks
It's just an ordinary pointer like any other.
它只是一个普通的指针，就像任何其他指针一样。
FILE *CreateLogFile()
{
return fopen("logfile.txt","w"); // allocates a FILE object and returns a pointer to it
}
void UsefulFunction()
{
FILE *pLog = CreateLogFile(); // it's safe to return a pointer from a func
int resultsOfWork = DoSomeWork();
fprintf( pLog, "Work did %d\n", resultsOfWork ); // you can pass it to other functions
fclose( pLog ); // just be sure to clean it up when you are done with fclose()
pLog = NULL; // and it's a good idea to overwrite the pointer afterwards
// so it's obvious you deleted what it points to
}
回答by Chris Lutz
int main(void)
{
char c;
FILE *read;
read = fopen("myfile", "r"); // opens "myfile" for reading
if(read == NULL)
{
perror("Error: could not open \"myfile\" for reading.\n");
exit(1);
}
c = fgetc(read);
fclose(read);
printf("The first character of myfile is %c.\n", c);
return 0;
}
You're perfectly allowed to declare global filehandles if you like, just like any other variable, but it may not be recommended.
如果愿意，您完全可以声明全局文件句柄，就像任何其他变量一样，但可能不建议这样做。
This is the C way. C++ can use this, but I think there's a more C++ friendly way of doing it. As a note, I hate it when questions are marked C/C++, because C and C++ are notthe same language and do notwork the same. C++ has a lot of different ways to do things that C doesn't have, and they may be easier for you to do in the context of C++ but are not valid C. So while this will work for either language, it's not what you want if you predominantly use C++.
这是C方式。C++ 可以使用它，但我认为有一种更 C++ 友好的方式来做到这一点。作为一个说明，我恨它，当问题被标记为C / C ++，因为C和C ++是不是同一种语言，也没有工作一样。C++ 有很多不同的方法来做 C 没有的事情，在 C++ 的上下文中它们可能更容易让你做，但不是有效的 C。所以虽然这对任何一种语言都有效，但这不是你想要的如果您主要使用 C++，则需要。
EDIT: Added some error checking. Always use error checking in your code.
编辑：添加了一些错误检查。始终在代码中使用错误检查。
回答by Andrew Barrett
Here is the first hit on google for "file io in c"
这是谷歌上第一次点击“c 中的文件 io”
http://www.cs.bu.edu/teaching/c/file-io/intro/
http://www.cs.bu.edu/teaching/c/file-io/intro/
Here is the third hit from gamedev with more of a C++ slant
这是 gamedev 的第三个热门，更多的是 C++ 倾向
http://www.gamedev.net/reference/articles/article1127.asp
http://www.gamedev.net/reference/articles/article1127.asp
You declare the pointer in the scope that you need it.
您在需要它的范围内声明指针。
回答by Tim Post
First, keep in mind that a file pointer (and the associated allocated structure) is based on the lower level open() read() write() calls. The associated file descriptor (obtained by fileno(file_pointer) is the least interesting thing, but something you might want to watch your scope with.
首先，请记住文件指针（以及相关的分配结构）基于较低级别的 open() read() write() 调用。关联的文件描述符（由 fileno(file_pointer) 获得）是最不有趣的东西，但你可能想用它来观察你的作用域。
If your going to declare a file pointer as global in a module, its usually a very good idea to keep it static (contained within that module / object file). Sometimes this is a little easier than storing it in a structure that is passed from function to function if you need to write something in a hurry.
如果要在模块中将文件指针声明为全局文件指针，通常最好将其保持为静态（包含在该模块/目标文件中）。有时，如果您需要匆忙写一些东西，这比将它存储在从一个函数传递到另一个函数的结构中更容易一些。
For instance, (bad)
例如，（坏）
#include <stdio.h>
#include ...
#define MY_LOG_FILE "file.txt"
FILE *logfile
Better done as:
最好这样做：
#include <stdio.h>
#define MY_LOG_FILE "file.txt"
static FILE *logfile;
int main(void)
{
UNLESS, you need several modules to have access to that pointer, in which case you're better off putting it in a structure that can be passed around.
除非，您需要多个模块才能访问该指针，在这种情况下，最好将其放在可以传递的结构中。
If its needed only in one module, consider declaring it in main() and letting other functions accept a file pointer as an argument. So, unless your functions within the module have so many arguments that another would be unbearable .. there's (usually) no reason to declare a file pointer globally.
如果仅在一个模块中需要它，请考虑在 main() 中声明它并让其他函数接受文件指针作为参数。所以，除非你在模块中的函数有太多的参数，另一个是无法忍受的......（通常）没有理由全局声明文件指针。
Some logging libraries do it, which I don't care for ... especially when dealing with re-entrant functions. Nevermind C's monolithic namespace :)
一些日志库会这样做，我不关心……尤其是在处理可重入函数时。没关系 C 的整体命名空间 :)
