无法读取jpeg二进制数据，缓冲区仅包含4个字节的数据 [英] Cannot read jpeg binary data, buffer only has 4 bytes of data

问题描述

我正在尝试读取JPEG文件并将其存储在C中的缓冲区中，作为HTTP POST请求的标头的一部分，然后使用HTTP POST上载到服务器.但是似乎它只读取图像数据的前4个字节，因为在服务器端，我只得到图像的前4个十六进制字符.我的代码有什么问题?操作系统是LINUX.

I am trying to read the JPEG file and store it in a buffer in C as part of the header of HTTP POST request, then use HTTP POST to upload to the server. But seems that it only reads the first 4 bytes of image data because on the server side I only get the first 4 hex characters of the image. What is wrong with my code? The OS is LINUX.

我意识到在IMAGE的第五个十六进制上，它的0x00，我认为C会将其解释为终止符，或者只是认为文件就此停止...如何避免此问题?

I realize that on the fifth HEX of the IMAGE, its 0x00, which i think C interprets it as the terminator, or it just thinks the file stops right here... How to avoid this issue?

unsigned long fileLen;
char *buffer;

if ((fp = fopen(filename, "rb")) == NULL){
    printf("File could not be opened\n");
    exit(1);
}else{
     fseek(fp, 0, SEEK_END);
     fileLen=ftell(fp);
     fseek(fp, 0, SEEK_SET);
     buffer=(char *)malloc(fileLen);
     fread(buffer, fileLen, 1, fp);
     fclose(fp);
}

推荐答案

buffer是char *，指向char的指针，因此您获得了计算机上指针的大小(4字节，您正在运行32位，对吧?)

buffer is a char *, pointer to a char, so you get the size of a pointer on your machine (4 bytes, you are running on 32-bit, right?).

fread的返回值是读取的字节数，我建议您使用它.

The return value from fread is the number of bytes read, I suggest you use that.

0x00只是C中字符串的定界符，在这种情况下不相关.

0x00 is only a delimiter in C for strings, it is not relevant in this case.

顺便说一句，您可能希望考虑引入一些错误处理.如果ftell失败，它将返回-1，然后在malloc中使用它.然后，这将失败并返回NULL，您将在fread中将其用作第一个参数.这将使您违反SIGSEGV细分，即使程序崩溃.

By the way, you might wish to consider putting-in some error handling. If ftell failed it would return -1, which you then use in your malloc. This would then fail and return NULL, which you would use in your fread as the first argument. That will give you a SIGSEGV Segmentation Violation, i.e. crash your program.

另一个问题可能是文件很大.您尝试获取文件末尾以获取其大小(请考虑使用fstat)，但是如果文件大小超出最大堆大小怎么办?例如2 GB的文件?这将导致malloc失败并返回NULL.

another issue might be if you have a very large file. You seek to end-of-file to get its size (consider using fstat instead) but what if the file size exceeds your maximum heap size? For example a 2 gigabyte file? That will cause malloc to fail and return NULL.

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