独立于平台获取文件大小 (C)

Question

我正在寻找一些独立于平台的 C 代码来确定给定文件的大小。首先，我阅读了以下答案：

答案将 fseek 与 SEEK_END 和 ftell 结合使用。现在，我的问题是我找到了以下 C 标准引号。

Setting the file position indicator to end-of-file, as with fseek(file, 0, SEEK_END), has undefined behavior for a binary stream (because of possible trailing null characters) or for any stream with state-dependent encoding that does not assuredly end in the initial shift state.

和

A binary stream need not meaningfully support fseek calls with a whence value of SEEK_END.

看来我有问题了。可能，以下计算读取字节数的代码是一种解决方法。

file = fopen(file_path, "rb");
/* ... */
while ( EOF != fgetc(file) ) {
  ret = size_t_inc(&file_size_); /* essentially, this does ++file_size_ */
  /* ... */
}
ret = feof(file);
/* ... */
if (!ret) {
  return 1; /* Error! */
}

（这里的整个函数：https://github.com/630R6/bytelev/blob/8e3d0dd14042f16086f3ca4e9a33d49a0629630e/main.c#L138）

不过，我正在寻找更好的解决方案。

非常感谢您的宝贵时间！

Answer 1

首先，让我指出，如前所述，这个问题有点像傻瓜的差事：在有文件等的平台上，有明确定义的方式以最有效的方式访问此信息.鉴于通常必须满足的环境数量有限，将功能抽象为简单的平台特定功能是可行的方法。

所以，基本上，只要 "file" 的大小有限，读取整个文件并沿途计算字节数是唯一通用的方法。

可以稍微调整一下边缘（比如，对于一个 10GB 的文件，我们真的需要 100 亿次 fgetc 调用吗？）

例如，下面我使用 fread 以最大 64K 的块读取文件：

#include <assert.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>

static const size_t SIZE_T_MAX = (size_t) -1;
static const size_t CHUNK_SIZE = 64 * 1024;

int
calc_file_size_slowly(const char *const filename, size_t *size) {
    FILE *fp;
    int ret = -1;
    size_t bytes_read = 0;
    unsigned char *buffer = malloc(CHUNK_SIZE);
    assert(buffer);

    errno = 0;

    fp = fopen(filename, "rb");
    if (!fp) {
        goto FAIL_FOPEN;
    }

    errno = 0;
    *size = 0;
    while ((bytes_read = fread(buffer, 1, CHUNK_SIZE, fp)) > 0) {
        if (ferror(fp)) {
            goto FAIL_FERROR;
        }
        if ((*size + bytes_read) > *size) {
            (*size) += bytes_read;
        }
        else {
            goto FAIL_OVERFLOW;
        }
        errno = 0;
    }

    if (feof(fp)) {
        ret = 0;
        goto DONE;
    }

FAIL_FOPEN:
    {
        ret = errno;
        goto DONE;
    }
FAIL_FERROR:
    {
        ret = errno;
        fclose(fp);
        goto DONE;
    }
FAIL_OVERFLOW:
    {
        ret = EOVERFLOW;
        fclose(fp);
        goto DONE;
    }
DONE:
    free(buffer);
    return ret;
}

int main(int argc, char *argv[]) {
    int i;
    for (i = 1; i < argc; i += 1) {
        size_t size;
        if (calc_file_size_slowly(argv[i], &size) == 0) {
            printf("%s: %lu bytes\n", argv[i], size);
        }
    }
}

输出：

C:\...\Temp> dir wpi.msi
...
2015-05-15  12:12 PM         1,859,584 wpi.msi
...         
C:\...\Temp> mysizer wpi.msi     
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=65536                                       
Bytes read=24576                                       
wpi.msi: 1859584 bytes

独立于平台获取文件大小 (C)

Getting file size platform-independently (C)

c

file

cross-platform

standards

filesize