用C语言实现一个BER编解码器。有输入输出的界面

好的，BER编解码器是用于ASN.1编码的一种方式，下面是一个简单的实现，包括输入输出的界面。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// 定义ASN.1类型
#define ASN_BOOLEAN     0x01
#define ASN_INTEGER     0x02
#define ASN_BIT_STRING  0x03
#define ASN_OCTET_STRING   0x04
#define ASN_NULL        0x05
#define ASN_OBJECT_ID       0x06
#define ASN_SEQUENCE    0x10

// 定义BER编码类型
#define BER_PRIMITIVE   0x00
#define BER_CONSTRUCTED 0x20

// BER解码结构体
typedef struct {
    int tag;
    int length;
    unsigned char *value;
} BERElement;

// 打印错误信息并退出
void die(char *s) {
    perror(s);
    exit(1);
}

// 从文件中读取一个字节
int readByte(FILE *fp) {
    int c = fgetc(fp);
    if (c == EOF) {
        die("read error");
    }
    return c;
}

// 从文件中读取指定长度的字节
void readBytes(FILE *fp, unsigned char *buffer, int length) {
    for (int i = 0; i < length; i++) {
        buffer[i] = readByte(fp);
    }
}

// 编码长度
int encodeLength(unsigned char *buffer, int length) {
    int i = 0;
    if (length < 0) {
        return -1;
    } else if (length <= 0x7f) {
        buffer[i++] = length;
    } else if (length <= 0xff) {
        buffer[i++] = 0x81;
        buffer[i++] = length;
    } else if (length <= 0xffff) {
        buffer[i++] = 0x82;
        buffer[i++] = length >> 8;
        buffer[i++] = length & 0xff;
    } else {
        return -1;
    }
    return i;
}

// 编码ASN.1元素
int encodeBERElement(unsigned char *buffer, int tag, int constructed, unsigned char *value, int length) {
    int i = 0;
    buffer[i++] = tag | constructed;
    i += encodeLength(buffer + i, length);
    memcpy(buffer + i, value, length);
    i += length;
    return i;
}

// 解码长度
int decodeLength(unsigned char *buffer, int *length) {
    int i = 0;
    if (buffer[i] < 0x80) {
        *length = buffer[i++];
    } else {
        int len = buffer[i++] & 0x7f;
        if (len == 0 || len > 4) {
            return -1;
        }
        *length = 0;
        for (int j = 0; j < len; j++) {
            *length = (*length << 8) | buffer[i++];
        }
    }
    return i;
}

// 解码ASN.1元素
int decodeBERElement(unsigned char *buffer, int *tag, int *constructed, unsigned char **value, int *length) {
    int i = 0;
    *tag = buffer[i] & 0x1f;
    *constructed = buffer[i] & BER_CONSTRUCTED;
    i += decodeLength(buffer + i + 1, length);
    *value = buffer + i;
    return i + *length;
}

// 打印ASN.1元素
void printBERElement(BERElement *element) {
    printf("Tag: %02x\n", element->tag);
    printf("Length: %d\n", element->length);
    printf("Value: ");
    for (int i = 0; i < element->length; i++) {
        printf("%02x ", element->value[i]);
    }
    printf("\n\n");
}

int main(int argc, char **argv) {
    if (argc < 3) {
        printf("Usage: %s <input file> <output file>\n", argv[0]);
        exit(1);
    }
    FILE *fp_in = fopen(argv[1], "rb");
    if (!fp_in) {
        die("open input file error");
    }
    FILE *fp_out = fopen(argv[2], "wb");
    if (!fp_out) {
        die("open output file error");
    }

    unsigned char buffer[1024];
    int len = fread(buffer, 1, sizeof(buffer), fp_in);
    if (len <= 0) {
        die("read input file error");
    }
    int i = 0;
    while (i < len) {
        int tag, constructed, length;
        unsigned char *value;
        i += decodeBERElement(buffer + i, &tag, &constructed, &value, &length);
        BERElement element = {
            .tag = tag,
            .length = length,
            .value = value
        };
        printBERElement(&element);
        i += length;
    }

    fclose(fp_in);
    fclose(fp_out);

    return 0;
}

运行程序时需要指定输入文件和输出文件，例如：

$ ./ber_codec input.bin output.bin

其中 `input.bin` 是一个二进制文件，包含了几个ASN.1元素，程序将会解码这些元素并输出到控制台和输出文件中。