Efficiently Checking File Size in C Without Opening the File

Whether you are working on a project that needs to handle files efficiently or just curious about the size of a file on your system, understanding how to check file size without opening the file can save you time and resources. This article will guide you through the process using modern C features and explain the underlying mechanisms.

Introduction to Checking File Size

In C, traditionally, one might need to open a file to check its size. However, with the introduction of the filesystem library in C17, you can now retrieve file properties, including size, without opening the file. This approach is particularly useful in scenarios where opening files may not be feasible or necessary.

Using the filesystem Header

The filesystem header is a powerful tool introduced in C17 that allows you to interact with the file system without opening files. This section will demonstrate how to use this functionality to check the size of a file.

Example Code Using filesystem

Here’s a simple example of how to check a file size:

include iostreaminclude filesystemint main() {    std::string filePath  /path/to/your/file;    try {        std::filesystem::path path(filePath);        if (std::filesystem::exists(path)  std::filesystem::is_regular_file(path)) {            auto fileSize  std::filesystem::file_size(path);            std::cout  File size:   fileSize   bytes  std::endl;        } else {            std::cout  File does not exist or is not a regular file.  std::endl;        }    } catch (const std::filesystem::filesystem_error e) {        std::cerr  Error:   e.what()  std::endl;    }    return 0;}

In this example, we check if the file exists and if it is a regular file, then retrieve its size using std::filesystem::file_size.

Using stat64 and fstat64

For more advanced use cases, especially when the file is already open, there are system-specific functions that do not require opening the file. For instance, the stat64 and fstat64 functions can be used. These functions provide access to file properties without opening the file.

Using stat64

The stat64 function is used to retrieve the status of a file, including its size, without needing to open the file. Here is an example of how to use it:

include sys/types.hinclude sys/stat.hinclude stdio.hint main() {    char *path  /path/to/your/file;    struct stat sb;    if (stat64(path, sb)  0) {        int fileSize  _size;        printf(File size:   strong%d bytes/strong, fileSize);    } else {        perror(stat64);    }    return 0;}

In this example, the stat64 function is called with the file path, and the file size is stored in the st_size member of the struct stat.

Using fstat64

The fstat64 function is similar but takes a file descriptor instead of a file path. It is useful when you already have a file descriptor, such as after opening the file with open.

include sys/types.hinclude sys/stat.hinclude stdio.hint main() {    int fd  open(/path/to/your/file, O_RDONLY);    if (fd  -1) {        perror(open);        return -1;    }    struct stat sb;    if (fstat64(fd, sb)  0) {        int fileSize  _size;        printf(File size:   strong%d bytes/strong, fileSize);    } else {        perror(fstat64);    }    close(fd);    return 0;}

The fstat64 function retrieves the file status and size using the file descriptor obtained from the open call.

Conclusion

By utilizing modern C features like the filesystem library or system-specific functions like stat64 and fstat64, you can efficiently check file sizes without opening the files. This technique is particularly useful in scenarios where file opening might not be feasible or necessary, such as in networked environments or security-sensitive applications.