用C语言实现一个BER编解码器。有简单界面，能将SNMP报文编码为实际输出的字节流，并能将实际收到的字节流。应侧重BER编解码中对不同数据类型的处理细节。至少实现几种主流数据类型的编解码，如Integer，OCTET STRING，get，getNext等。

这是一个比较复杂的任务，需要用到C语言中的位运算、指针操作等知识。下面是一个简单的BER编解码器的实现示例：

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

#define MAX_OID_LEN 128

typedef unsigned char u_char;

enum asn_type {
    ASN_INTEGER = 0x02,
    ASN_OCTET_STRING = 0x04,
    ASN_NULL = 0x05,
    ASN_OBJECT_ID = 0x06,
    ASN_SEQUENCE = 0x30,
};

int encode_header(u_char *buf, u_char type, int len)
{
    int header_len = 0;
    if (len > 127) {
        buf[header_len++] = type | 0x80;
        buf[header_len++] = len & 0xff;
    } else {
        buf[header_len++] = type;
        buf[header_len++] = len;
    }
    return header_len;
}

int decode_header(u_char *buf, u_char *type, int *len)
{
    int header_len = 0;
    if (buf[header_len] & 0x80) {
        *type = buf[header_len++] & 0x7f;
        int len_bytes = buf[header_len++] & 0x7f;
        *len = 0;
        for (int i = 0; i < len_bytes; i++) {
            *len = (*len << 8) | buf[header_len++];
        }
    } else {
        *type = buf[header_len++];
        *len = buf[header_len++];
    }
    return header_len;
}

int encode_integer(u_char *buf, int value)
{
    int len = 0;
    buf[len++] = ASN_INTEGER;
    if (value >= 0 && value <= 127) {
        buf[len++] = 1;
        buf[len++] = value;
    } else if (value < 0 && value >= -128) {
        buf[len++] = 1;
        buf[len++] = value & 0xff;
    } else if (value > 127 && value <= 32767) {
        buf[len++] = 2;
        buf[len++] = (value >> 8) & 0xff;
        buf[len++] = value & 0xff;
    } else {
        buf[len++] = 4;
        buf[len++] = (value >> 24) & 0xff;
        buf[len++] = (value >> 16) & 0xff;
        buf[len++] = (value >> 8) & 0xff;
        buf[len++] = value & 0xff;
    }
    return len;
}

int decode_integer(u_char *buf, int *value)
{
    u_char type;
    int len;
    int offset = decode_header(buf, &type, &len);
    if (type != ASN_INTEGER) {
        return -1;
    }
    if (len == 1) {
        *value = (int)buf[offset];
    } else if (len == 2) {
        *value = (int)(buf[offset] << 8 | buf[offset+1]);
    } else if (len == 4) {
        *value = (int)(buf[offset] << 24 | buf[offset+1] << 16 | buf[offset+2] << 8 | buf[offset+3]);
    } else {
        return -1;
    }
    return offset + len;
}

int encode_octet_string(u_char *buf, u_char *data, int len)
{
    int header_len = encode_header(buf, ASN_OCTET_STRING, len);
    memcpy(buf+header_len, data, len);
    return header_len + len;
}

int decode_octet_string(u_char *buf, u_char *data, int *len)
{
    u_char type;
    int offset = decode_header(buf, &type, len);
    if (type != ASN_OCTET_STRING) {
        return -1;
    }
    memcpy(data, buf+offset, *len);
    return offset + *len;
}

int encode_oid(u_char *buf, int *oid, int oid_len)
{
    int len = 0;
    buf[len++] = ASN_OBJECT_ID;
    int subid = oid[0] * 40 + oid[1];
    if (subid < 128) {
        buf[len++] = 1;
        buf[len++] = subid;
    } else if (subid < 16384) {
        buf[len++] = 2;
        buf[len++] = (subid >> 7) | 0x80;
        buf[len++] = subid & 0x7f;
    } else if (subid < 2097152) {
        buf[len++] = 3;
        buf[len++] = (subid >> 14) | 0x80;
        buf[len++] = (subid >> 7) | 0x80;
        buf[len++] = subid & 0x7f;
    } else {
        buf[len++] = 4;
        buf[len++] = (subid >> 21) | 0x80;
        buf[len++] = (subid >> 14) | 0x80;
        buf[len++] = (subid >> 7) | 0x80;
        buf[len++] = subid & 0x7f;
    }
    for (int i = 2; i < oid_len; i++) {
        subid = oid[i];
        if (subid < 128) {
            buf[len++] = subid;
        } else if (subid < 16384) {
            buf[len++] = (subid >> 7) | 0x80;
            buf[len++] = subid & 0x7f;
        } else if (subid < 2097152) {
            buf[len++] = (subid >> 14) | 0x80;
            buf[len++] = (subid >> 7) | 0x80;
            buf[len++] = subid & 0x7f;
        } else {
            buf[len++] = (subid >> 21) | 0x80;
            buf[len++] = (subid >> 14) | 0x80;
            buf[len++] = (subid >> 7) | 0x80;
            buf[len++] = subid & 0x7f;
        }
    }
    return len;
}

int decode_oid(u_char *buf, int *oid, int *oid_len)
{
    u_char type;
    int len;
    int offset = decode_header(buf, &type, &len);
    if (type != ASN_OBJECT_ID) {
        return -1;
    }
    int subid = buf[offset++] & 0x7f;
    oid[0] = subid / 40;
    oid[1] = subid % 40;
    int index = 2;
    while (offset < len) {
        subid = 0;
        while (buf[offset] & 0x80) {
            subid = (subid << 7) | (buf[offset] & 0x7f);
            offset++;
        }
        subid = (subid << 7) | buf[offset++];
        oid[index++] = subid;
    }
    *oid_len = index;
    return offset;
}

int encode_pdu(u_char *buf, int req_id, int error, int error_index, u_char *varbind, int varbind_len)
{
    int len = 0;
    buf[len++] = ASN_SEQUENCE;
    buf[len++] = varbind_len + 16;
    len += encode_integer(buf+len, req_id);
    len += encode_integer(buf+len, 0);
    len += encode_integer(buf+len, 0);
    buf[len++] = ASN_SEQUENCE;
    buf[len++] = varbind_len + 6;
    len += encode_oid(buf+len, (int[]){1, 3, 6, 1, 2, 1, 1, 4, 0}, 9);
    len += encode_header(buf+len, ASN_NULL, 0);
    len += encode_header(buf+len, ASN_SEQUENCE, varbind_len);
    memcpy(buf+len, varbind, varbind_len);
    len += varbind_len;
    return len;
}

int decode_pdu(u_char *buf, int *req_id, int *error, int *error_index, u_char *varbind, int *varbind_len)
{
    u_char type;
    int len;
    int offset = decode_header(buf, &type, &len);
    if (type != ASN_SEQUENCE) {
        return -1;
    }
    offset += decode_integer(buf+offset, req_id);
    offset += decode_integer(buf+offset, error);
    offset += decode_integer(buf+offset, error_index);
    offset += decode_header(buf+offset, &type, &len);
    if (type != ASN_SEQUENCE) {
        return -1;
    }
    int varbind_offset = decode_oid(buf+offset, NULL, NULL);
    offset += varbind_offset;
    offset += decode_header(buf+offset, &type, varbind_len);
    memcpy(varbind, buf+offset, *varbind_len);
    return offset + *varbind_len;
}

void print_oid(int *oid, int oid_len)
{
    for (int i = 0; i < oid_len; i++) {
        printf("%d", oid[i]);
        if (i < oid_len-1) {
            printf(".");
        }
    }
}

int main()
{
    u_char buf[1024];
    int len;

    // Encode an SNMP GetRequest PDU
    u_char varbind[1024];
    int varbind_len = 0;
    varbind_len += encode_oid(varbind+varbind_len, (int[]){1, 3, 6, 1, 2, 1, 1, 1, 0}, 9);
    varbind_len += encode_header(varbind+varbind_len, ASN_NULL, 0);
    len = encode_pdu(buf, 1234, 0, 0, varbind, varbind_len);
    printf("Encoded PDU (%d bytes):\n", len);
    for (int i = 0; i < len; i++) {
        printf("%02x ", buf[i]);
    }
    printf("\n");

    // Decode an SNMP GetRequest PDU
    int req_id, error, error_index;
    u_char varbind_dec[1024];
    int varbind_len_dec;
    int offset = decode_pdu(buf, &req_id, &error, &error_index, varbind_dec, &varbind_len_dec);
    printf("Decoded PDU:\n");
    printf("  Request ID: %d\n", req_id);
    printf("  Error: %d\n", error);
    printf("  Error Index: %d\n", error_index);
    printf("  Varbind:\n");
    int oid[MAX_OID_LEN];
    int oid_len;
    offset += decode_oid(varbind_dec+offset, oid, &oid_len);
    printf("    OID: ");
    print_oid(oid, oid_len);
    printf("\n");
    if (varbind_dec[offset] != ASN_NULL) {
        printf("    Value: (unknown type)\n");
    } else {
        printf("    Value: NULL\n");
    }

    return 0;
}

在这个示例中，我们定义了一个枚举类型 `asn_type` 来表示不同的ASN.1数据类型。`encode_header` 和 `decode_header` 函数分别用于编码和解码BER编码中的头部信息，包括类型和长度。

针对不同的数据类型，我们实现了 `encode_integer`，`decode_integer`，`encode_octet_string`，`decode_octet_string`，`encode_oid` 和 `decode_oid` 函数，分别用于编码和解码整数、字节流和OID等数据类型。

最后，我们使用 `encode_pdu` 和 `decode_pdu` 函数分别对SNMP PDU进行编码和解码。注意，在实际的SNMP实现中，还需要处理多个varbind的情况。