如何将 STDIN 流更改为二进制

Question

我正在通过 CGI 界面制作上传表单。我正在用 C 编写它并且不想使用任何外部库（即 cgic）。

我认为程序已经完成，因为第一个测试文件已正确上传。但它们是 ASCII 文件。当我使用二进制文件 (JPG) 进行测试时。似乎 STDIN 正在尝试将二进制数据读取为 ASCII，这会为 [=11=] 这样的字符带来问题，它出现在 ASCII 文件的末尾，但在二进制文件中是常见字符。上传一个 1.9MB 文件的结果最终是一个 38kB 文件。

在搜索如何将 STDIN 流更改为二进制文件时，我参考了命令 freopen 并被告知使用 NULL 作为文件的参数。 example 1

它说：

If filename is a null pointer, the freopen() function shall attempt to change the mode of the stream to that specified by mode, as if the name of the file currently associated with the stream had been used. In this case, the file descriptor associated with the stream need not be closed if the call to freopen() succeeds. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances.

但是当我用 man 3 freopen 查看我系统的手册页时，它没有说任何这一点。此外，阅读手册页，我发现了二进制选项（将 'b' 添加到模式）不再被识别并且只存在于古老的合规性：

The mode string can also include the letter 'b' either as a last character or as a character between the characters in any of the two-character strings described above. This is strictly for compatibility with C89 and has no effect; the 'b' is ignored on all POSIX conforming systems, including Linux.

所以现在我完全迷路了。如何更改 STDIN 流以读取二进制输入？

代码如下：

#include <stdio.h>
#include <stdlib.h>
#include <libgen.h>
#include <string.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>

// Declare constants.
#define BUF_SIZE                 4096
#define FILENAME_SIZE             500
#define MARKER_SIZE               100
#define RETURN_FAILURE              0
#define RETURN_SUCCESS              1
#define SEARCH_STRING_1             "filename=\""
#define SEARCH_STRING_2             "\r\n\r\n"

// Declare global variables.
char filename[FILENAME_SIZE + 1];
char *program_name;

// Declare function prototype.
void print_footer (void);
void print_header (void);
void process_input (char *data);

int main (int argc, char *argv[])
{
// Declare variables.
    long long ret;
    char buf[BUF_SIZE + 1];

// Get program name for error reporting.
    program_name = basename(argv[0]);

// Prepare output for browser.
    print_header();

// Protect variable against buffer overflow.
    buf[BUF_SIZE] = '[=10=]';

// Loop through all the file data.
    while(1)
    {
// Read in the next block of data.
        if((ret = (long long) fread(buf, 1, BUF_SIZE, stdin)) != BUF_SIZE)
        {
// Check for error.
            if(ferror(stdin) != 0)
            {
                printf("%s: An error occurred while reading the input file.<br>\n", program_name);
                process_input(NULL);
                exit(EXIT_FAILURE);
            }
// Check for EOF.
            else if(feof(stdin) != 0)
                break;
        }

// Terminate and process uploaded data.
        buf[ret] = '[=10=]';
        process_input(buf);
    }

// Terminate and process uploaded data.
    buf[ret] = '[=10=]';
    process_input(buf);

// Finish user output, close output file and exit.
    print_footer();
    process_input(NULL);
    exit(EXIT_SUCCESS);
}

void process_input (char *data)
{
// Declare variables.
    char *ptr1= NULL;
    char *ptr2;
    int x = 0;
    static FILE *fp;
    static int flag = 0;
    static char marker[MARKER_SIZE + 1];

// If data is NULL, close output file.
    if(data == NULL)
    {
        if(fclose(fp) == EOF)
        {
            printf("%s: process_input: close failed (%s)<br>\n", program_name, strerror(errno));
            exit(EXIT_FAILURE);
        }

        return;
    }

// Check if this is the first time through.
    if(flag == 0)
    {
// Get marker.
        if((ptr1 = strchr(data, '\n')) == NULL)
        {
            printf("%s: process_input: strchr(1) failed (\n)<br>\n", program_name);
            exit(EXIT_FAILURE);
        }

        ptr1[0] = '[=10=]';
        strcpy(marker, data);
        ptr1[0] = '\n';

// Get filename.
        if((ptr1 = strstr(data, SEARCH_STRING_1)) == NULL)
        {
            printf("%s: process_input: strstr(1) failed (%s)<br>\n", program_name, SEARCH_STRING_1);
            exit(EXIT_FAILURE);
        }

// Advance pointer to start of filename.
        ptr1 += 10;

// Find end of filename.
        if((ptr2 = strchr(ptr1, '"')) == NULL)
        {
            printf("%s: process_input: strchr(2) failed (\")<br>\n", program_name);
            exit(EXIT_FAILURE);
        }

// Terminate and store filename.
        ptr2[0] = '[=10=]';
        strcpy(filename, ptr1);
        ptr2[0] = '"';

// Remove spaces from filename.
        while(filename[x] != '[=10=]')
        {
            if(filename[x] == ' ')
                filename[x] = '.';

            x++;
        }

// Open output file.
        if((fp = fopen(filename, "wb")) == NULL)
        {
            printf("%s: process_input: fopen failed (%s) (%s)<br>\n", program_name, strerror(errno), filename);
            exit(EXIT_FAILURE);
        }

// Find start of file data.
        if((ptr1 = strstr(data, SEARCH_STRING_2)) == NULL)
        {
            printf("%s: process_input: strstr(2) failed (%s)<br>\n", program_name, SEARCH_STRING_2);
            fclose(fp);
            exit(EXIT_FAILURE);
        }

// Set flag.
        flag++;
// Advance pointer to start of file data.
        ptr1 += 4;

// Change STDIN stream to binary.
        if(freopen(NULL, "rb", stdin) == NULL)
        {
            printf("%s: process_input: freopen failed (%s)<br>\n", program_name, strerror(errno));
            fclose(fp);
            exit(EXIT_FAILURE);
        }
    }
// Catch everything else.
    else
    {
        ptr1 = data;

        if((ptr2 = strstr(ptr1, marker)) != NULL)
            ptr2[0 - 2] = '[=10=]';
    }

// Write file data.
    if(fwrite(ptr1, 1, strlen(ptr1), fp) != strlen(ptr1))
    {
        printf("%s: process_input: write failed (%s)<br>\n", program_name, strerror(errno));
        fclose(fp);
        exit(EXIT_FAILURE);
    }
}

void print_footer (void)
{
    printf("\nMade it!\n");
}

void print_header (void)
{
    printf("Content-type: text/plain\r\n\r\n");
}

Answer 1

'stdin' 是 STDIN_FILENO 的一个宏，等于 0。另见 'unistd.h'。你没有显示你的代码，但我认为当你遇到 '\0' 或非 ascii 字符时你会停止，因为你说你正在使用 'fread()'.

当 fread() 函数 returns 0 时必须停止，这意味着它停止读取：遇到 EOF。

Answer 2

好的，看来@NominalAnimal 说的是对的。您可以将二进制数据存储在字符串中，但是当您使用 string.h 库中的任何函数时，它几乎总是会更改存储在该字符串中的内容（如果数据是二进制的）。

简单的解决方案是制作一个单独的函数，该函数采用指向二进制数据的指针并在该函数中进行字符串搜索，返回所需的相关信息。这样，原始数据永远不会改变。

如何将 STDIN 流更改为二进制

How to change STDIN stream to binary

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