Squelch some (valid) compiler warnings.

[libpcap] / sf-pcap-ng.c

/*
 * Copyright (c) 1993, 1994, 1995, 1996, 1997
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that: (1) source code distributions
 * retain the above copyright notice and this paragraph in its entirety, (2)
 * distributions including binary code include the above copyright notice and
 * this paragraph in its entirety in the documentation or other materials
 * provided with the distribution, and (3) all advertising materials mentioning
 * features or use of this software display the following acknowledgement:
 * ``This product includes software developed by the University of California,
 * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
 * the University nor the names of its contributors may be used to endorse
 * or promote products derived from this software without specific prior
 * written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 *
 * sf-pcap-ng.c - pcap-ng-file-format-specific code from savefile.c
 */

#ifndef lint
static const char rcsid[] _U_ =
    "@(#) $Header$ (LBL)";
#endif

#ifdef HAVE_CONFIG_H
#include "config.h"
#endif

#ifdef WIN32
#include <pcap-stdinc.h>
#else /* WIN32 */
#if HAVE_INTTYPES_H
#include <inttypes.h>
#elif HAVE_STDINT_H
#include <stdint.h>
#endif
#ifdef HAVE_SYS_BITYPES_H
#include <sys/bitypes.h>
#endif
#include <sys/types.h>
#endif /* WIN32 */

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

#include "pcap-int.h"

#include "pcap-common.h"

#ifdef HAVE_OS_PROTO_H
#include "os-proto.h"
#endif

#include "sf-pcap-ng.h"

/*
 * Block types.
 */

/*
 * Common part at the beginning of all blocks.
 */
struct block_header {
        bpf_u_int32     block_type;
        bpf_u_int32     total_length;
};

/*
 * Common trailer at the end of all blocks.
 */
struct block_trailer {
        bpf_u_int32     total_length;
};

/*
 * Common options.
 */
#define OPT_ENDOFOPT    0       /* end of options */
#define OPT_COMMENT     1       /* comment string */

/*
 * Option header.
 */
struct option_header {
        u_short         option_code;
        u_short         option_length;
};

/*
 * Structures for the part of each block type following the common
 * part.
 */

/*
 * Section Header Block.
 */
#define BT_SHB                  0x0A0D0D0A

struct section_header_block {
        bpf_u_int32     byte_order_magic;
        u_short         major_version;
        u_short         minor_version;
        u_int64_t       section_length;
        /* followed by options and trailer */
};

/*
 * Byte-order magic value.
 */
#define BYTE_ORDER_MAGIC        0x1A2B3C4D

/*
 * Current version number.  If major_version isn't PCAP_NG_VERSION_MAJOR,
 * that means that this code can't read the file.
 */
#define PCAP_NG_VERSION_MAJOR   1

/*
 * Interface Description Block.
 */
#define BT_IDB                  0x00000001

struct interface_description_block {
        u_short         linktype;
        u_short         reserved;
        bpf_u_int32     snaplen;
        /* followed by options and trailer */
};

/*
 * Options in the IDB.
 */
#define IF_NAME         2       /* interface name string */
#define IF_DESCRIPTION  3       /* interface description string */
#define IF_IPV4ADDR     4       /* interface's IPv4 address and netmask */
#define IF_IPV6ADDR     5       /* interface's IPv6 address and prefix length */
#define IF_MACADDR      6       /* interface's MAC address */
#define IF_EUIADDR      7       /* interface's EUI address */
#define IF_SPEED        8       /* interface's speed, in bits/s */
#define IF_TSRESOL      9       /* interface's time stamp resolution */
#define IF_TZONE        10      /* interface's time zone */
#define IF_FILTER       11      /* filter used when capturing on interface */
#define IF_OS           12      /* string OS on which capture on this interface was done */
#define IF_FCSLEN       13      /* FCS length for this interface */
#define IF_TSOFFSET     14      /* time stamp offset for this interface */

/*
 * Enhanced Packet Block.
 */
#define BT_EPB                  0x00000006

struct enhanced_packet_block {
        bpf_u_int32     interface_id;
        bpf_u_int32     timestamp_high;
        bpf_u_int32     timestamp_low;
        bpf_u_int32     caplen;
        bpf_u_int32     len;
        /* followed by packet data, options, and trailer */
};

/*
 * Simple Packet Block.
 */
#define BT_SPB                  0x00000003

struct simple_packet_block {
        bpf_u_int32     len;
        /* followed by packet data and trailer */
};

/*
 * Packet Block.
 */
#define BT_PB                   0x00000002

struct packet_block {
        u_short         interface_id;
        u_short         drops_count;
        bpf_u_int32     timestamp_high;
        bpf_u_int32     timestamp_low;
        bpf_u_int32     caplen;
        bpf_u_int32     len;
        /* followed by packet data, options, and trailer */
};

/*
 * Block cursor - used when processing the contents of a block.
 * Contains a pointer into the data being processed and a count
 * of bytes remaining in the block.
 */
struct block_cursor {
        u_char          *data;
        size_t          data_remaining;
        bpf_u_int32     block_type;
};

static int pcap_ng_next_packet(pcap_t *p, struct pcap_pkthdr *hdr,
    u_char **data);

static int
read_bytes(FILE *fp, void *buf, size_t bytes_to_read, int fail_on_eof,
    char *errbuf)
{
        size_t amt_read;

        amt_read = fread(buf, 1, bytes_to_read, fp);
        if (amt_read != bytes_to_read) {
                if (ferror(fp)) {
                        snprintf(errbuf, PCAP_ERRBUF_SIZE,
                            "error reading dump file: %s",
                            pcap_strerror(errno));
                } else {
                        if (amt_read == 0 && !fail_on_eof)
                                return (0);     /* EOF */
                        snprintf(errbuf, PCAP_ERRBUF_SIZE,
                            "truncated dump file; tried to read %lu bytes, only got %lu",
                            (unsigned long)bytes_to_read,
                            (unsigned long)amt_read);
                }
                return (-1);
        }
        return (1);
}

static int
read_block(FILE *fp, pcap_t *p, struct block_cursor *cursor, char *errbuf)
{
        int status;
        struct block_header bhdr;

        status = read_bytes(fp, &bhdr, sizeof(bhdr), 0, errbuf);
        if (status <= 0)
                return (status);        /* error or EOF */

        if (p->sf.swapped) {
                bhdr.block_type = SWAPLONG(bhdr.block_type);
                bhdr.total_length = SWAPLONG(bhdr.total_length);
        }

        /*
         * Is this block "too big"?
         *
         * We choose 16MB as "too big", for now, so that we handle
         * "reasonably" large buffers but don't chew up all the
         * memory if we read a malformed file.
         */
        if (bhdr.total_length > 16*1024*1024) {
                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                    "pcap-ng block size %u > maximum %u",
                    bhdr.total_length, 16*1024*1024);
                    return (-1);
        }

        /*
         * Is this block "too small" - i.e., is it shorter than a block
         * header plus a block trailer?
         */
        if (bhdr.total_length < sizeof(struct block_header) +
            sizeof(struct block_trailer)) {
                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                    "block in pcap-ng dump file has a length of %u < %lu",
                    bhdr.total_length,
                    (unsigned long)(sizeof(struct block_header) + sizeof(struct block_trailer)));
                return (-1);
        }

        /*
         * Is the buffer big enough?
         */
        if (p->bufsize < bhdr.total_length) {
                /*
                 * No - make it big enough.
                 */
                p->buffer = realloc(p->buffer, bhdr.total_length);
                if (p->buffer == NULL) {
                        snprintf(errbuf, PCAP_ERRBUF_SIZE, "out of memory");
                        return (-1);
                }
        }

        /*
         * Copy the stuff we've read to the buffer, and read the rest
         * of the block.
         */
        memcpy(p->buffer, &bhdr, sizeof(bhdr));
        if (read_bytes(fp, p->buffer + sizeof(bhdr),
            bhdr.total_length - sizeof(bhdr), 1, errbuf) == -1)
                return (-1);

        /*
         * Initialize the cursor.
         */
        cursor->data = p->buffer + sizeof(bhdr);
        cursor->data_remaining = bhdr.total_length - sizeof(bhdr) -
            sizeof(struct block_trailer);
        cursor->block_type = bhdr.block_type;
        return (1);
}

static void *
get_from_block_data(struct block_cursor *cursor, size_t chunk_size,
    char *errbuf)
{
        void *data;

        /*
         * Make sure we have the specified amount of data remaining in
         * the block data.
         */
        if (cursor->data_remaining < chunk_size) {
                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                    "block of type %u in pcap-ng dump file is too short",
                    cursor->block_type);
                return (NULL);
        }

        /*
         * Return the current pointer, and skip past the chunk.
         */
        data = cursor->data;
        cursor->data += chunk_size;
        cursor->data_remaining -= chunk_size;
        return (data);
}

static struct option_header *
get_opthdr_from_block_data(pcap_t *p, struct block_cursor *cursor, char *errbuf)
{
        struct option_header *opthdr;

        opthdr = get_from_block_data(cursor, sizeof(*opthdr), errbuf);
        if (opthdr == NULL) {
                /*
                 * Option header is cut short.
                 */
                return (NULL);
        }

        /*
         * Byte-swap it if necessary.
         */
        if (p->sf.swapped) {
                opthdr->option_code = SWAPSHORT(opthdr->option_code);
                opthdr->option_length = SWAPSHORT(opthdr->option_length);
        }

        return (opthdr);
}

static void *
get_optvalue_from_block_data(struct block_cursor *cursor,
    struct option_header *opthdr, char *errbuf)
{
        size_t padded_option_len;
        void *optvalue;

        /* Pad option length to 4-byte boundary */
        padded_option_len = opthdr->option_length;
        padded_option_len = ((padded_option_len + 3)/4)*4;

        optvalue = get_from_block_data(cursor, padded_option_len, errbuf);
        if (optvalue == NULL) {
                /*
                 * Option value is cut short.
                 */
                return (NULL);
        }

        return (optvalue);
}

static int
process_idb_options(pcap_t *p, struct block_cursor *cursor, u_int *tsresol,
    u_int64_t *tsoffset, char *errbuf)
{
        struct option_header *opthdr;
        void *optvalue;
        int saw_tsresol, saw_tsoffset;
        u_char tsresol_opt;
        u_int i;

        saw_tsresol = 0;
        saw_tsoffset = 0;
        while (cursor->data_remaining != 0) {
                /*
                 * Get the option header.
                 */
                opthdr = get_opthdr_from_block_data(p, cursor, errbuf);
                if (opthdr == NULL) {
                        /*
                         * Option header is cut short.
                         */
                        return (-1);
                }

                /*
                 * Get option value.
                 */
                optvalue = get_optvalue_from_block_data(cursor, opthdr,
                    errbuf);
                if (optvalue == NULL) {
                        /*
                         * Option value is cut short.
                         */
                        return (-1);
                }

                switch (opthdr->option_code) {

                case OPT_ENDOFOPT:
                        if (opthdr->option_length != 0) {
                                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                                    "Interface Description Block has opt_endofopt option with length %u != 0",
                                    opthdr->option_length);
                                return (-1);
                        }
                        goto done;

                case IF_TSRESOL:
                        if (opthdr->option_length != 1) {
                                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                                    "Interface Description Block has if_tsresol option with length %u != 1",
                                    opthdr->option_length);
                                return (-1);
                        }
                        if (saw_tsresol) {
                                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                                    "Interface Description Block has more than one if_tsresol option");
                                return (-1);
                        }
                        saw_tsresol = 1;
                        tsresol_opt = *(u_int *)optvalue;
                        if (tsresol_opt & 0x80) {
                                /*
                                 * Resolution is negative power of 2.
                                 */
                                *tsresol = 1 << (tsresol_opt & 0x7F);
                        } else {
                                /*
                                 * Resolution is negative power of 10.
                                 */
                                *tsresol = 1;
                                for (i = 0; i < tsresol_opt; i++)
                                        *tsresol *= 10;
                        }
                        if (*tsresol == 0) {
                                /*
                                 * Resolution is too high.
                                 */
                                if (tsresol_opt & 0x80) {
                                        snprintf(errbuf, PCAP_ERRBUF_SIZE,
                                            "Interface Description Block if_tsresol option resolution 2^-%u is too high",
                                            tsresol_opt & 0x7F);
                                } else {
                                        snprintf(errbuf, PCAP_ERRBUF_SIZE,
                                            "Interface Description Block if_tsresol option resolution 10^-%u is too high",
                                            tsresol_opt);
                                }
                                return (-1);
                        }
                        break;

                case IF_TSOFFSET:
                        if (opthdr->option_length != 8) {
                                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                                    "Interface Description Block has if_tsoffset option with length %u != 8",
                                    opthdr->option_length);
                                return (-1);
                        }
                        if (saw_tsoffset) {
                                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                                    "Interface Description Block has more than one if_tsoffset option");
                                return (-1);
                        }
                        saw_tsoffset = 1;
                        memcpy(tsoffset, optvalue, sizeof(*tsoffset));
                        if (p->sf.swapped)
                                *tsoffset = SWAPLL(*tsoffset);
                        break;

                default:
                        break;
                }
        }

done:
        return (0);
}

/*
 * Check whether this is a pcap-ng savefile and, if it is, extract the
 * relevant information from the header.
 */
int
pcap_ng_check_header(pcap_t *p, bpf_u_int32 magic, FILE *fp, char *errbuf)
{
        size_t amt_read;
        bpf_u_int32 total_length;
        bpf_u_int32 byte_order_magic;
        struct block_header *bhdrp;
        struct section_header_block *shbp;
        int status;
        struct block_cursor cursor;
        struct interface_description_block *idbp;

        /*
         * Check whether the first 4 bytes of the file are the block
         * type for a pcap-ng savefile. 
         */
        if (magic != BT_SHB) {
                /*
                 * XXX - check whether this looks like what the block
                 * type would be after being munged by mapping between
                 * UN*X and DOS/Windows text file format and, if it
                 * does, look for the byte-order magic number in
                 * the appropriate place and, if we find it, report
                 * this as possibly being a pcap-ng file transferred
                 * between UN*X and Windows in text file format?
                 */
                return (0);     /* nope */
        }

        /*
         * OK, they are.  However, that's just \n\r\r\n, so it could,
         * conceivably, be an ordinary text file.
         *
         * It could not, however, conceivably be any other type of
         * capture file, so we can read the rest of the putative
         * Section Header Block; put the block type in the common
         * header, read the rest of the common header and the
         * fixed-length portion of the SHB, and look for the byte-order
         * magic value.
         */
        amt_read = fread(&total_length, 1, sizeof(total_length), fp);
        if (amt_read < sizeof(total_length)) {
                if (ferror(fp)) {
                        snprintf(errbuf, PCAP_ERRBUF_SIZE,
                            "error reading dump file: %s",
                            pcap_strerror(errno));
                        return (-1);    /* fail */
                }

                /*
                 * Possibly a weird short text file, so just say
                 * "not pcap-ng".
                 */
                return (0);
        }
        amt_read = fread(&byte_order_magic, 1, sizeof(byte_order_magic), fp);
        if (amt_read < sizeof(byte_order_magic)) {
                if (ferror(fp)) {
                        snprintf(errbuf, PCAP_ERRBUF_SIZE,
                            "error reading dump file: %s",
                            pcap_strerror(errno));
                        return (-1);    /* fail */
                }

                /*
                 * Possibly a weird short text file, so just say
                 * "not pcap-ng".
                 */
                return (0);
        }
        if (byte_order_magic != BYTE_ORDER_MAGIC) {
                byte_order_magic = SWAPLONG(byte_order_magic);
                if (byte_order_magic != BYTE_ORDER_MAGIC) {
                        /*
                         * Not a pcap-ng file.
                         */
                        return (0);
                }
                p->sf.swapped = 1;
                total_length = SWAPLONG(total_length);
        }

        /*
         * Check the sanity of the total length.
         */
        if (total_length < sizeof(*bhdrp) + sizeof(*shbp) + sizeof(struct block_trailer)) {
                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                    "Section Header Block in pcap-ng dump file has a length of %u < %lu",
                    total_length,
                    (unsigned long)(sizeof(*bhdrp) + sizeof(*shbp) + sizeof(struct block_trailer)));
                return (-1);
        }

        /*
         * Allocate a buffer into which to read blocks.  We default to
         * the maximum of:
         *
         *      the total length of the SHB for which we read the header;
         *
         *      2K, which should be more than large enough for an Enhanced
         *      Packet Block containing a full-size Ethernet frame, and
         *      leaving room for some options.
         *
         * If we find a bigger block, we reallocate the buffer.
         */
        p->bufsize = 2048;
        if (p->bufsize < total_length)
                p->bufsize = total_length;
        p->buffer = malloc(p->bufsize);
        if (p->buffer == NULL) {
                snprintf(errbuf, PCAP_ERRBUF_SIZE, "out of memory");
                return (-1);
        }

        /*
         * Copy the stuff we've read to the buffer, and read the rest
         * of the SHB.
         */
        bhdrp = (struct block_header *)p->buffer;
        shbp = (struct section_header_block *)(p->buffer + sizeof(struct block_header));
        bhdrp->block_type = magic;
        bhdrp->total_length = total_length;
        shbp->byte_order_magic = byte_order_magic;
        if (read_bytes(fp,
            p->buffer + (sizeof(magic) + sizeof(total_length) + sizeof(byte_order_magic)),
            total_length - (sizeof(magic) + sizeof(total_length) + sizeof(byte_order_magic)),
            1, errbuf) == -1)
                goto fail;

        if (p->sf.swapped) {
                /*
                 * Byte-swap the fields we've read.
                 */
                shbp->major_version = SWAPSHORT(shbp->major_version);
                shbp->minor_version = SWAPSHORT(shbp->minor_version);

                /*
                 * XXX - we don't care about the section length.
                 */
        }
        if (shbp->major_version != PCAP_NG_VERSION_MAJOR) {
                snprintf(errbuf, PCAP_ERRBUF_SIZE,
                    "unknown pcap-ng savefile major version number %u",
                    shbp->major_version);
                goto fail;
        }
        p->sf.version_major = shbp->major_version;
        p->sf.version_minor = shbp->minor_version;

        /*
         * Set the default time stamp resolution and offset.
         */
        p->sf.tsresol = 1000000;        /* microsecond resolution */
        p->sf.tsscale = 1;              /* multiply by 1 to scale to microseconds */
        p->sf.tsoffset = 0;             /* absolute timestamps */

        /*
         * Now start looking for an Interface Description Block.
         */
        for (;;) {
                /*
                 * Read the next block.
                 */
                status = read_block(fp, p, &cursor, errbuf);
                if (status == 0) {
                        /* EOF - no IDB in this file */
                        snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                            "the capture file has no Interface Description Blocks");
                        goto fail;
                }
                if (status == -1)
                        goto fail;      /* error */
                switch (cursor.block_type) {

                case BT_IDB:
                        /*
                         * Get a pointer to the fixed-length portion of the
                         * IDB.
                         */
                        idbp = get_from_block_data(&cursor, sizeof(*idbp),
                            errbuf);
                        if (idbp == NULL)
                                goto fail;      /* error */

                        /*
                         * Byte-swap it if necessary.
                         */
                        if (p->sf.swapped) {
                                idbp->linktype = SWAPSHORT(idbp->linktype);
                                idbp->snaplen = SWAPLONG(idbp->snaplen);
                        }

                        /*
                         * Count this interface.
                         */
                        p->sf.ifcount++;

                        /*
                         * Now look for various time stamp options, so
                         * we know how to interpret the time stamps.
                         */
                        if (process_idb_options(p, &cursor, &p->sf.tsresol,
                            &p->sf.tsoffset, errbuf) == -1)
                                goto fail;

                        /*
                         * Compute the scaling factor to convert the
                         * sub-second part of the time stamp to
                         * microseconds.
                         */
                        if (p->sf.tsresol > 1000000) {
                                /*
                                 * Higher than microsecond resolution;
                                 * scale down to microseconds.
                                 */
                                p->sf.tsscale = (p->sf.tsresol / 1000000);
                        } else {
                                /*
                                 * Lower than microsecond resolution;
                                 * scale up to microseconds.
                                 */
                                p->sf.tsscale = (1000000 / p->sf.tsresol);
                        }
                        goto done;

                case BT_EPB:
                case BT_SPB:
                case BT_PB:
                        /*
                         * Saw a packet before we saw any IDBs.  That's
                         * not valid, as we don't know what link-layer
                         * encapsulation the packet has.
                         */
                        snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                            "the capture file has a packet block before any Interface Description Blocks");
                        goto fail;

                default:
                        /*
                         * Just ignore it.
                         */
                        break;
                }
        }

done:
        p->tzoff = 0;   /* XXX - not used in pcap */
        p->snapshot = idbp->snaplen;
        p->linktype = idbp->linktype;
        p->linktype_ext = 0;

        p->sf.next_packet_op = pcap_ng_next_packet;

        return (1);

fail:
        free(p->buffer);
        return (-1);
}

/*
 * Read and return the next packet from the savefile.  Return the header
 * in hdr and a pointer to the contents in data.  Return 0 on success, 1
 * if there were no more packets, and -1 on an error.
 */
static int
pcap_ng_next_packet(pcap_t *p, struct pcap_pkthdr *hdr, u_char **data)
{
        struct block_cursor cursor;
        int status;
        struct enhanced_packet_block *epbp;
        struct simple_packet_block *spbp;
        struct packet_block *pbp;
        bpf_u_int32 interface_id = 0xFFFFFFFF;
        size_t pblock_len;
        struct interface_description_block *idbp;
        struct section_header_block *shbp;
        FILE *fp = p->sf.rfile;
        u_int tsresol;
        u_int64_t tsoffset;
        u_int64_t t, sec, frac;

        /*
         * Look for an Enhanced Packet Block, a Simple Packet Block,
         * or a Packet Block.
         */
        for (;;) {
                /*
                 * Read the block type and length; those are common
                 * to all blocks.
                 */
                status = read_block(fp, p, &cursor, p->errbuf);
                if (status == 0)
                        return (1);     /* EOF */
                if (status == -1)
                        return (-1);    /* error */
                switch (cursor.block_type) {

                case BT_EPB:
                        /*
                         * Get a pointer to the fixed-length portion of the
                         * EPB.
                         */
                        epbp = get_from_block_data(&cursor, sizeof(*epbp),
                            p->errbuf);
                        if (epbp == NULL)
                                return (-1);    /* error */

                        /*
                         * Byte-swap it if necessary.
                         */
                        if (p->sf.swapped) {
                                /* these were written in opposite byte order */
                                interface_id = SWAPLONG(epbp->interface_id);
                                hdr->caplen = SWAPLONG(epbp->caplen);
                                hdr->len = SWAPLONG(epbp->len);
                                t = ((u_int64_t)SWAPLONG(epbp->timestamp_high)) << 32 |
                                    SWAPLONG(epbp->timestamp_low);
                        } else {
                                interface_id = epbp->interface_id;
                                hdr->caplen = epbp->caplen;
                                hdr->len = epbp->len;
                                t = ((u_int64_t)epbp->timestamp_high) << 32 |
                                    epbp->timestamp_low;
                        }
                        pblock_len = sizeof(*epbp);
                        goto found;
                        
                case BT_SPB:
                        /*
                         * Get a pointer to the fixed-length portion of the
                         * SPB.
                         */
                        spbp = get_from_block_data(&cursor, sizeof(*spbp),
                            p->errbuf);
                        if (spbp == NULL)
                                return (-1);    /* error */

                        /*
                         * SPB packets are assumed to have arrived on
                         * the first interface.
                         */
                        interface_id = 0;

                        /*
                         * Byte-swap it if necessary.
                         */
                        if (p->sf.swapped) {
                                /* these were written in opposite byte order */
                                hdr->len = SWAPLONG(spbp->len);
                        } else
                                hdr->len = spbp->len;

                        /*
                         * The SPB doesn't give the captured length;
                         * it's the minimum of the snapshot length
                         * and the packet length.
                         */
                        hdr->caplen = hdr->len;
                        if (hdr->caplen > p->snapshot)
                                hdr->caplen = p->snapshot;
                        t = 0;  /* no time stamps */
                        pblock_len = sizeof(*spbp);
                        goto found;

                case BT_PB:
                        /*
                         * Get a pointer to the fixed-length portion of the
                         * PB.
                         */
                        pbp = get_from_block_data(&cursor, sizeof(*pbp),
                            p->errbuf);
                        if (pbp == NULL)
                                return (-1);    /* error */

                        /*
                         * Byte-swap it if necessary.
                         */
                        if (p->sf.swapped) {
                                /* these were written in opposite byte order */
                                interface_id = SWAPSHORT(pbp->interface_id);
                                hdr->caplen = SWAPLONG(pbp->caplen);
                                hdr->len = SWAPLONG(pbp->len);
                                t = ((u_int64_t)SWAPLONG(pbp->timestamp_high)) << 32 |
                                    SWAPLONG(pbp->timestamp_low);
                        } else {
                                interface_id = pbp->interface_id;
                                hdr->caplen = pbp->caplen;
                                hdr->len = pbp->len;
                                t = ((u_int64_t)pbp->timestamp_high) << 32 |
                                    pbp->timestamp_low;
                        }
                        pblock_len = sizeof(*pbp);
                        goto found;

                case BT_IDB:
                        /*
                         * Interface Description Block.  Get a pointer
                         * to its fixed-length portion.
                         */
                        idbp = get_from_block_data(&cursor, sizeof(*idbp),
                            p->errbuf);
                        if (idbp == NULL)
                                return (-1);    /* error */

                        /*
                         * Byte-swap it if necessary.
                         */
                        if (p->sf.swapped) {
                                idbp->linktype = SWAPSHORT(idbp->linktype);
                                idbp->snaplen = SWAPLONG(idbp->snaplen);
                        }

                        /*
                         * If the link-layer type or snapshot length
                         * differ from the ones for the first IDB we
                         * saw, quit.
                         *
                         * XXX - just discard packets from those
                         * interfaces?
                         */
                        if (p->linktype != idbp->linktype) {
                                snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                                    "an interface has a type %u different from the type of the first interface",
                                    idbp->linktype);
                                return (-1);
                        }
                        if (p->snapshot != idbp->snaplen) {
                                snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                                    "an interface has a snapshot length %u different from the type of the first interface",
                                    idbp->snaplen);
                                return (-1);
                        }

                        /*
                         * Count this interface.
                         */
                        p->sf.ifcount++;

                        /*
                         * Set the default time stamp resolution and offset.
                         */
                        tsresol = 1000000;      /* microsecond resolution */
                        tsoffset = 0;           /* absolute timestamps */

                        /*
                         * Now look for various time stamp options, to
                         * make sure they're the same.
                         *
                         * XXX - we could, in theory, handle multiple
                         * different resolutions and offsets, but we
                         * don't do so for now.
                         */
                        if (process_idb_options(p, &cursor, &tsresol, &tsoffset,
                            p->errbuf) == -1)
                                return (-1);
                        if (tsresol != p->sf.tsresol) {
                                snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                                    "an interface has a time stamp resolution different from the time stamp resolution of the first interface");
                                return (-1);
                        }
                        if (tsoffset != p->sf.tsoffset) {
                                snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                                    "an interface has a time stamp offset different from the time stamp offset of the first interface");
                                return (-1);
                        }
                        break;

                case BT_SHB:
                        /*
                         * Section Header Block.  Get a pointer
                         * to its fixed-length portion.
                         */
                        shbp = get_from_block_data(&cursor, sizeof(*shbp),
                            p->errbuf);
                        if (shbp == NULL)
                                return (-1);    /* error */

                        /*
                         * Assume the byte order of this section is
                         * the same as that of the previous section.
                         * We'll check for that later.
                         */
                        if (p->sf.swapped) {
                                shbp->byte_order_magic =
                                    SWAPLONG(shbp->byte_order_magic);
                                shbp->major_version =
                                    SWAPSHORT(shbp->major_version);
                        }

                        /*
                         * Make sure the byte order doesn't change;
                         * pcap_is_swapped() shouldn't change its
                         * return value in the middle of reading a capture.
                         */
                        switch (shbp->byte_order_magic) {

                        case BYTE_ORDER_MAGIC:
                                /*
                                 * OK.
                                 */
                                break;

                        case SWAPLONG(BYTE_ORDER_MAGIC):
                                /*
                                 * Byte order changes.
                                 */
                                snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                                    "the file has sections with different byte orders");
                                return (-1);

                        default:
                                /*
                                 * Not a valid SHB.
                                 */
                                snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                                    "the file has a section with a bad byte order magic field");
                                return (-1);
                        }

                        /*
                         * Make sure the major version is the version
                         * we handle.
                         */
                        if (shbp->major_version != PCAP_NG_VERSION_MAJOR) {
                                snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                                    "unknown pcap-ng savefile major version number %u",
                                    shbp->major_version);
                                return (-1);
                        }

                        /*
                         * Reset the interface count; this section should
                         * have its own set of IDBs.  If any of them
                         * don't have the same interface type, snapshot
                         * length, or resolution as the first interface
                         * we saw, we'll fail.  (And if we don't see
                         * any IDBs, we'll fail when we see a packet
                         * block.)
                         */
                        p->sf.ifcount = 0;
                        break;

                default:
                        /*
                         * Not a packet block, IDB, or SHB; ignore it.
                         */
                        break;
                }                
        }

found:
        /*
         * Is the interface ID an interface we know?
         */
        if (interface_id > p->sf.ifcount) {
                /*
                 * Yes.  Fail.
                 */
                snprintf(p->errbuf, PCAP_ERRBUF_SIZE,
                    "a packet arrived on interface %u, but there's no Interface Description Block for that interface",
                    interface_id);
                return (-1);
        }

        /*
         * Convert the time stamp to a struct timeval.
         */
        sec = t / p->sf.tsresol + p->sf.tsoffset;
        frac = t % p->sf.tsresol;
        if (p->sf.tsresol > 1000000) {
                /*
                 * Higher than microsecond resolution; scale down to
                 * microseconds.
                 */
                frac /= p->sf.tsscale;
        } else {
                /*
                 * Lower than microsecond resolution; scale up to
                 * microseconds.
                 */
                frac *= p->sf.tsscale;
        }
        hdr->ts.tv_sec = sec;
        hdr->ts.tv_usec = frac;

        /*
         * Get a pointer to the packet data.
         */
        *data = get_from_block_data(&cursor, hdr->caplen, p->errbuf);
        if (*data == NULL)
                return (-1);

        if (p->sf.swapped) {
                /*
                 * Convert pseudo-headers from the byte order of
                 * the host on which the file was saved to our
                 * byte order, as necessary.
                 */
                switch (p->linktype) {

                case DLT_USB_LINUX:
                case DLT_USB_LINUX_MMAPPED:
                        swap_linux_usb_header(hdr, *data);
                        break;
                }
        }

        return (0);
}