darshan-logutils.c

/*
 * Copyright (C) 2015 University of Chicago.
 * See COPYRIGHT notice in top-level directory.
 *
 */

#define _GNU_SOURCE
#include "darshan-util-config.h"
#include <stdio.h>
#include <string.h>
#include <assert.h>
#include <stdlib.h>
#include <unistd.h>
#include <inttypes.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

#include "darshan-logutils.h"

/* default input buffer size for decompression algorithm */
#define DARSHAN_DEF_COMP_BUF_SZ (1024*1024) /* 1 MiB */

/* special identifers for referring to header, job, and
 * record map regions of the darshan log file
 */
#define DARSHAN_HEADER_REGION_ID    (-3)
#define DARSHAN_JOB_REGION_ID       (-2)
#define DARSHAN_NAME_MAP_REGION_ID  (-1)

struct darshan_dz_state
{
    /* pointer to arbitrary data structure used for managing
     * compression/decompression state (e.g., z_stream
     * structure needed for libz)
     */
    void *comp_dat;
    /* buffer for staging compressed data to/from log file */
    unsigned char *buf;
    /* size of staging buffer */
    unsigned int size;
    /* for reading logs, flag indicating end of log file region */
    int eor;
    /* the region id we last tried reading/writing */
    int prev_reg_id;
};

/* internal fd data structure */
struct darshan_fd_int_state
{
    /* posix file descriptor for the log file */
    int fildes;
    /* file pointer position */
    int64_t pos;
    /* flag indicating whether log file was created (and written) */
    int creat_flag;
    /* log file path name */
    char logfile_path[PATH_MAX];
    /* pointer to exe & mount data in darshan job data structure */
    char *exe_mnt_data;
    /* whether previous file operations have failed */
    int err;

    /* compression/decompression stream read/write state */
    struct darshan_dz_state dz;
};

static int darshan_log_get_header(darshan_fd fd);
static int darshan_log_put_header(darshan_fd fd);
static int darshan_log_seek(darshan_fd fd, off_t offset);
static int darshan_log_read(darshan_fd fd, void *buf, int len);
static int darshan_log_write(darshan_fd fd, void *buf, int len);
static int darshan_log_dzinit(darshan_fd fd);
static void darshan_log_dzdestroy(darshan_fd fd);
static int darshan_log_dzread(darshan_fd fd, int region_id, void *buf, int len);
static int darshan_log_dzwrite(darshan_fd fd, int region_id, void *buf, int len);
static int darshan_log_libz_read(darshan_fd fd, struct darshan_log_map map, 
    void *buf, int len, int reset_strm_flag);
static int darshan_log_libz_write(darshan_fd fd, struct darshan_log_map *map_p,
    void *buf, int len, int flush_strm_flag);
static int darshan_log_libz_flush(darshan_fd fd, int region_id);
#ifdef HAVE_LIBBZ2
static int darshan_log_bzip2_read(darshan_fd fd, struct darshan_log_map map, 
    void *buf, int len, int reset_strm_flag);
static int darshan_log_bzip2_write(darshan_fd fd, struct darshan_log_map *map_p,
    void *buf, int len, int flush_strm_flag);
static int darshan_log_bzip2_flush(darshan_fd fd, int region_id);
#endif
static int darshan_log_dzload(darshan_fd fd, struct darshan_log_map map);
static int darshan_log_dzunload(darshan_fd fd, struct darshan_log_map *map_p);
static int darshan_log_noz_read(darshan_fd fd, struct darshan_log_map map,
    void *buf, int len, int reset_strm_flag);

/* each module's implementation of the darshan logutil functions */
#define X(a, b, c, d) d,
struct darshan_mod_logutil_funcs *mod_logutils[DARSHAN_MAX_MODS] =
{
    DARSHAN_MODULE_IDS
};
#undef X

/* darshan_log_open()
 *
 * open an existing darshan log file for reading only
 *
 * returns file descriptor on success, NULL on failure
 */
darshan_fd darshan_log_open(const char *name)
{
    darshan_fd tmp_fd;
    int ret;

    /* allocate a darshan file descriptor */
    tmp_fd = malloc(sizeof(*tmp_fd));
    if(!tmp_fd)
        return(NULL);
    memset(tmp_fd, 0, sizeof(*tmp_fd));
    tmp_fd->state = malloc(sizeof(struct darshan_fd_int_state));
    if(!tmp_fd->state)
    {
        free(tmp_fd->state);
        return(NULL);
    }
    memset(tmp_fd->state, 0, sizeof(struct darshan_fd_int_state));

    /* open the log file in read mode */
    tmp_fd->state->fildes = open(name, O_RDONLY);
    if(tmp_fd->state->fildes < 0)
    {
        fprintf(stderr, "Error: failed to open darshan log file %s.\n", name);
        free(tmp_fd->state);
        free(tmp_fd);
        return(NULL);
    }
    strncpy(tmp_fd->state->logfile_path, name, PATH_MAX);

    /* read the header from the log file to init fd data structures */
    ret = darshan_log_get_header(tmp_fd);
    if(ret < 0)
    {
        fprintf(stderr, "Error: failed to read darshan log file header.\n");
        close(tmp_fd->state->fildes);
        free(tmp_fd->state);
        free(tmp_fd);
        return(NULL);
    }

    /* initialize compression data structures */
    ret = darshan_log_dzinit(tmp_fd);
    if(ret < 0)
    {
        fprintf(stderr, "Error: failed to initialize decompression data structures.\n");
        close(tmp_fd->state->fildes);
        free(tmp_fd->state);
        free(tmp_fd);
        return(NULL);
    }

    return(tmp_fd);
}

/* darshan_log_create()
 *
 * create a darshan log file for writing with the given compression method
 *
 * returns file descriptor on success, NULL on failure
 */
darshan_fd darshan_log_create(const char *name, enum darshan_comp_type comp_type,
    int partial_flag)
{
    darshan_fd tmp_fd;
    int ret;

    /* allocate a darshan file descriptor */
    tmp_fd = malloc(sizeof(*tmp_fd));
    if(!tmp_fd)
        return(NULL);
    memset(tmp_fd, 0, sizeof(*tmp_fd));
    tmp_fd->state = malloc(sizeof(struct darshan_fd_int_state));
    if(!tmp_fd->state)
    {
        free(tmp_fd);
        return(NULL);
    }
    memset(tmp_fd->state, 0, sizeof(struct darshan_fd_int_state));
    tmp_fd->comp_type = comp_type;

    /* create the log for writing, making sure to not overwrite existing log */
    tmp_fd->state->fildes = creat(name, 0400);
    if(tmp_fd->state->fildes < 0)
    {
        fprintf(stderr, "Error: failed to open darshan log file %s.\n", name);
        free(tmp_fd->state);
        free(tmp_fd);
        return(NULL);
    }
    tmp_fd->state->creat_flag = 1;
    tmp_fd->partial_flag = partial_flag;
    strncpy(tmp_fd->state->logfile_path, name, PATH_MAX);

    /* position file pointer to prealloc space for the log file header
     * NOTE: the header is written at close time, after all internal data
     * structures have been properly set
     */
    ret = darshan_log_seek(tmp_fd, sizeof(struct darshan_header));
    if(ret < 0)
    {
        fprintf(stderr, "Error: unable to seek in darshan log file.\n");
        close(tmp_fd->state->fildes);
        free(tmp_fd->state);
        free(tmp_fd);
        unlink(name);
        return(NULL);
    }

    /* initialize compression data structures */
    ret = darshan_log_dzinit(tmp_fd);
    if(ret < 0)
    {
        fprintf(stderr, "Error: failed to initialize compression data structures.\n");
        close(tmp_fd->state->fildes);
        free(tmp_fd->state);
        free(tmp_fd);
        unlink(name);
        return(NULL);
    }

    return(tmp_fd);
}

/* darshan_log_get_job()
 *
 * read job level metadata from the darshan log file
 *
 * returns 0 on success, -1 on failure
 */
int darshan_log_get_job(darshan_fd fd, struct darshan_job *job)
{
    struct darshan_fd_int_state *state = fd->state;
    char job_buf[DARSHAN_JOB_RECORD_SIZE] = {0};
    int job_buf_sz = DARSHAN_JOB_RECORD_SIZE;
    int ret;

    assert(state);
    assert(fd->job_map.len > 0 && fd->job_map.off > 0);

    /* read the compressed job data from the log file */
    ret = darshan_log_dzread(fd, DARSHAN_JOB_REGION_ID, job_buf, job_buf_sz);
    if(ret <= (int)sizeof(*job))
    {
        fprintf(stderr, "Error: failed to read darshan log file job data.\n");
        return(-1);
    }

    memcpy(job, job_buf, sizeof(*job));

    if(fd->swap_flag)
    {
        /* swap bytes if necessary */
        DARSHAN_BSWAP64(&job->uid);
        DARSHAN_BSWAP64(&job->start_time);
        DARSHAN_BSWAP64(&job->end_time);
        DARSHAN_BSWAP64(&job->nprocs);
        DARSHAN_BSWAP64(&job->jobid);
    }

    /* save trailing exe & mount information, so it can be retrieved later */
    if(!(state->exe_mnt_data))
        state->exe_mnt_data = malloc(DARSHAN_EXE_LEN+1);
    if(!(state->exe_mnt_data))
        return(-1);
    memcpy(state->exe_mnt_data, &job_buf[sizeof(*job)], DARSHAN_EXE_LEN+1);

    return(0);
}

/* darshan_log_put_job()
 *
 * write job level metadata to darshan log file
 *
 * returns 0 on success, -1 on failure
 */
int darshan_log_put_job(darshan_fd fd, struct darshan_job *job)
{
    struct darshan_fd_int_state *state = fd->state;
    struct darshan_job job_copy;
    int len;
    int ret;

    assert(state);

    memset(&job_copy, 0, sizeof(*job));
    memcpy(&job_copy, job, sizeof(*job));

    /* check for newline in existing metadata, add if needed */
    len = strlen(job_copy.metadata);
    if(len > 0 && len < DARSHAN_JOB_METADATA_LEN)
    {
        if(job_copy.metadata[len-1] != '\n')
        {
            job_copy.metadata[len] = '\n';
            job_copy.metadata[len+1] = '\0';
        }
    }

    /* write the compressed job data to log file */
    ret = darshan_log_dzwrite(fd, DARSHAN_JOB_REGION_ID, &job_copy, sizeof(*job));
    if(ret != sizeof(*job))
    {
        state->err = -1;
        fprintf(stderr, "Error: failed to write darshan log file job data.\n");
        return(-1);
    }

    return(0);
}

/* darshan_log_get_exe()
 *
 * reads the application exe name from darshan log file
 * 
 * returns 0 on success, -1 on failure 
 */
int darshan_log_get_exe(darshan_fd fd, char *buf)
{
    struct darshan_fd_int_state *state = fd->state;
    char *newline;
    int ret;

    assert(state);

    /* if the exe/mount data has not been saved yet, read in the job info */
    if(!(state->exe_mnt_data))
    {
        struct darshan_job job;
        ret = darshan_log_get_job(fd, &job);

        if(ret < 0 || !(state->exe_mnt_data))
            return(-1);
    }

    /* exe string is located before the first line break */
    newline = strchr(state->exe_mnt_data, '\n');

    /* copy over the exe string */
    if(newline)
        memcpy(buf, state->exe_mnt_data, (newline - state->exe_mnt_data));

    return (0);
}

/* darshan_log_put_exe()
 *
 * wrties the application exe name to darshan log file
 * NOTE: this needs to be called immediately following put_job as it
 * expects the file pointer to be positioned immediately following
 * the darshan job information
 *
 * returns 0 on success, -1 on failure 
 */
int darshan_log_put_exe(darshan_fd fd, char *buf)
{
    struct darshan_fd_int_state *state = fd->state;
    int len = strlen(buf);
    int ret;

    assert(fd->state);

    ret = darshan_log_dzwrite(fd, DARSHAN_JOB_REGION_ID, buf, len);
    if(ret != len)
    {
        state->err = -1;
        fprintf(stderr, "Error: failed to write exe string to darshan log file.\n");
        return(-1);
    }

    return(0);
}

/* darshan_log_get_mounts()
 * 
 * retrieves mount table information from the log. Note that mnt_pts and
 * fs_types are arrays that will be allocated by the function and must be
 * freed by the caller. count will indicate the size of the arrays
 *
 * returns 0 on success, -1 on failure
 */
int darshan_log_get_mounts(darshan_fd fd, char*** mnt_pts,
    char*** fs_types, int* count)
{
    struct darshan_fd_int_state *state = fd->state;
    char *pos;
    int array_index = 0;
    int ret;

    assert(state);

    /* if the exe/mount data has not been saved yet, read in the job info */
    if(!(state->exe_mnt_data))
    {
        struct darshan_job job;
        ret = darshan_log_get_job(fd, &job);

        if(ret < 0 || !(state->exe_mnt_data))
            return(-1);
    }

    /* count entries */
    *count = 0;
    pos = state->exe_mnt_data;
    while((pos = strchr(pos, '\n')) != NULL)
    {
        pos++;
        (*count)++;
    }

    if(*count == 0)
    {
        /* no mount entries present */
        return(0);
    }

    /* allocate output arrays */
    *mnt_pts = malloc((*count)*sizeof(char*));
    assert(*mnt_pts);
    *fs_types = malloc((*count)*sizeof(char*));
    assert(*fs_types);

    /* work backwards through the table and parse each line (except for
     * first, which holds command line information)
     */
    while((pos = strrchr(state->exe_mnt_data, '\n')) != NULL)
    {
        /* overestimate string lengths */
        (*mnt_pts)[array_index] = malloc(DARSHAN_EXE_LEN);
        assert((*mnt_pts)[array_index]);
        (*fs_types)[array_index] = malloc(DARSHAN_EXE_LEN);
        assert((*fs_types)[array_index]);

        ret = sscanf(++pos, "%s\t%s", (*fs_types)[array_index],
            (*mnt_pts)[array_index]);
        if(ret != 2)
        {
            fprintf(stderr, "Error: poorly formatted mount table in darshan log file.\n");
            return(-1);
        }
        pos--;
        *pos = '\0';
        array_index++;
    }

    return(0);
}

/* darshan_log_put_mounts()
 *
 * writes mount information to the darshan log file
 * NOTE: this function call should follow immediately after the call
 * to darshan_log_putexe(), as it assumes the darshan log file pointer
 * is pointing to the offset immediately following the exe string
 *
 * returns 0 on success, -1 on failure
 */
int darshan_log_put_mounts(darshan_fd fd, char** mnt_pts, char** fs_types, int count)
{
    struct darshan_fd_int_state *state = fd->state;
    int i;
    char line[1024];
    char mnt_dat[DARSHAN_EXE_LEN] = {0};
    int mnt_dat_sz = 0;
    char *tmp;
    int ret;

    assert(state);

    /* write each mount entry to file */
    tmp = mnt_dat;
    for(i=count-1; i>=0; i--)
    {
        sprintf(line, "\n%s\t%s", fs_types[i], mnt_pts[i]);

        memcpy(tmp, line, strlen(line));
        tmp += strlen(line);
        mnt_dat_sz += strlen(line);
    }

    ret = darshan_log_dzwrite(fd, DARSHAN_JOB_REGION_ID, mnt_dat, mnt_dat_sz);
    if (ret != mnt_dat_sz)
    {
        state->err = -1;
        fprintf(stderr, "Error: failed to write darshan log mount data.\n");
        return(-1);
    }

    return(0);
}

/* darshan_log_get_namehash()
 *
 * read the hash of name records from the darshan log file
 *
 * returns 0 on success, -1 on failure
 */
int darshan_log_get_namehash(darshan_fd fd, struct darshan_name_record_ref **hash)
{
    struct darshan_fd_int_state *state = fd->state;
    char *name_rec_buf;
    char *tmp_p;
    int name_rec_buf_sz;
    int read;
    int read_req_sz;
    struct darshan_name_record_ref *ref;
    struct darshan_name_record *name_rec;
    int buf_rem = 0;
    int rec_len;

    assert(state);

    /* just return if there is no name record mapping data */
    if(fd->name_map.len == 0)
    {
        *hash = NULL;
        return(0);
    }

    /* default to buffer twice as big as default compression buf */
    name_rec_buf_sz = DARSHAN_DEF_COMP_BUF_SZ * 2;
    name_rec_buf = malloc(name_rec_buf_sz);
    if(!name_rec_buf)
        return(-1);
    memset(name_rec_buf, 0, name_rec_buf_sz);

    do
    {
        /* read chunks of the darshan record id -> name mapping from log file,
         * constructing a hash table in the process
         */
        read_req_sz = name_rec_buf_sz - buf_rem;
        read = darshan_log_dzread(fd, DARSHAN_NAME_MAP_REGION_ID,
            name_rec_buf + buf_rem, read_req_sz);
        if(read < 0)
        {
            fprintf(stderr, "Error: failed to read name hash from darshan log file.\n");
            free(name_rec_buf);
            return(-1);
        }
        buf_rem += read;

        /* work through the name record buffer -- deserialize the mapping data and
         * add to the output hash table
         * NOTE: these mapping pairs are variable in length, so we have to be able
         * to handle incomplete mappings temporarily here
         */
        name_rec = (struct darshan_name_record *)name_rec_buf;
        while(buf_rem > sizeof(darshan_record_id) + 1)
        {
            if(strnlen(name_rec->name, buf_rem - sizeof(darshan_record_id)) ==
                (buf_rem - sizeof(darshan_record_id)))
            {
                /* if this record name's terminating null character is not
                 * present, we need to read more of the buffer before continuing
                 */
                break;
            }

            if(fd->swap_flag)
            {
                /* we need to sort out endianness issues before deserializing */
                DARSHAN_BSWAP64(&(name_rec->id));
            }

            HASH_FIND(hlink, *hash, &(name_rec->id), sizeof(darshan_record_id), ref);
            if(!ref)
            {
                rec_len = sizeof(darshan_record_id) + strlen(name_rec->name) + 1;
                ref = malloc(sizeof(*ref));
                if(!ref)
                {
                    free(name_rec_buf);
                    return(-1);
                }
                ref->name_record = malloc(rec_len);
                if(!ref->name_record)
                {
                    free(ref);
                    free(name_rec_buf);
                    return(-1);
                }

                /* copy the name record over from the hash buffer */
                memcpy(ref->name_record, name_rec, rec_len);

                /* add this record to the hash */
                HASH_ADD(hlink, *hash, name_record->id, sizeof(darshan_record_id), ref);
            }

            tmp_p = (char *)name_rec + rec_len;
            name_rec = (struct darshan_name_record *)tmp_p;
            buf_rem -= rec_len;
        }

        /* copy any leftover data to beginning of buffer to parse next */
        memcpy(name_rec_buf, name_rec, buf_rem);

        /* we keep reading until we get a short read informing us we have
         * read all of the record hash
         */
    } while(read == read_req_sz);
    assert(buf_rem == 0);

    free(name_rec_buf);
    return(0);
}

/* darshan_log_put_namehash()
 *
 * writes the hash table of name records to the darshan log file
 * NOTE: this function call should follow immediately after the call
 * to darshan_log_putmounts(), as it assumes the darshan log file pointer
 * is pointing to the offset immediately following the mount information
 *
 * returns 0 on success, -1 on failure
 */
int darshan_log_put_namehash(darshan_fd fd, struct darshan_name_record_ref *hash)
{
    struct darshan_fd_int_state *state = fd->state;
    struct darshan_name_record_ref *ref, *tmp;
    struct darshan_name_record_ref *name_rec;
    int name_rec_len;
    int wrote;

    assert(state);

    /* allocate memory for largest possible hash record */
    name_rec = malloc(sizeof(struct darshan_name_record) + PATH_MAX);
    if(!name_rec)
        return(-1);
    memset(name_rec, 0, sizeof(struct darshan_name_record) + PATH_MAX);

    /* individually serialize each hash record and write to log file */
    HASH_ITER(hlink, hash, ref, tmp)
    {
        name_rec_len = sizeof(struct darshan_name_record) + strlen(ref->name_record->name);
        memcpy(name_rec, ref->name_record, name_rec_len);

        /* write this hash entry to log file */
        wrote = darshan_log_dzwrite(fd, DARSHAN_NAME_MAP_REGION_ID,
            name_rec, name_rec_len);
        if(wrote != name_rec_len)
        {
            state->err = -1;
            fprintf(stderr, "Error: failed to write name hash to darshan log file.\n");
            free(name_rec);
            return(-1);
        }
    }

    free(name_rec);
    return(0);
}

/* darshan_log_get_mod()
 *
 * get a chunk of module data from the darshan log file
 *
 * returns number of bytes read on success, -1 on failure
 */
int darshan_log_get_mod(darshan_fd fd, darshan_module_id mod_id,
    void *mod_buf, int mod_buf_sz)
{
    struct darshan_fd_int_state *state = fd->state;
    int ret;

    assert(state);

    if(mod_id < 0 || mod_id >= DARSHAN_MAX_MODS)
    {
        fprintf(stderr, "Error: invalid Darshan module id.\n");
        return(-1);
    }

    if(fd->mod_map[mod_id].len == 0)
        return(0); /* no data corresponding to this mod_id */

    /* read this module's data from the log file */
    ret = darshan_log_dzread(fd, mod_id, mod_buf, mod_buf_sz);
    if(ret < 0)
    {
        fprintf(stderr,
            "Error: failed to read module %s data from darshan log file.\n",
            darshan_module_names[mod_id]);
        return(-1);
    }

    return(ret);
}

/* darshan_log_put_mod()
 *
 * write a chunk of module data to the darshan log file
 * NOTE: this function call should be called directly after the
 * put_hash() function, as it expects the file pointer to be
 * positioned directly past the record hash location. Also,
 * for a set of modules with data to write to file, this function
 * should be called in order of increasing module identifiers,
 * as the darshan log file format expects this ordering.
 *
 * returns number of bytes written on success, -1 on failure
 */
int darshan_log_put_mod(darshan_fd fd, darshan_module_id mod_id,
    void *mod_buf, int mod_buf_sz, int ver)
{
    struct darshan_fd_int_state *state = fd->state;
    int ret;

    assert(state);

    if(mod_id < 0 || mod_id >= DARSHAN_MAX_MODS)
    {
        state->err = -1;
        fprintf(stderr, "Error: invalid Darshan module id.\n");
        return(-1);
    }

    /* write the module chunk to the log file */
    ret = darshan_log_dzwrite(fd, mod_id, mod_buf, mod_buf_sz);
    if(ret != mod_buf_sz)
    {
        state->err = -1;
        fprintf(stderr,
            "Error: failed to write module %s data to darshan log file.\n",
            darshan_module_names[mod_id]);
        return(-1);
    }

    /* set the version number for this module's data */
    fd->mod_ver[mod_id] = ver;

    return(0);
}

/* darshan_log_close()
 *
 * close an open darshan file descriptor, freeing any resources
 *
 */
void darshan_log_close(darshan_fd fd)
{
    struct darshan_fd_int_state *state = fd->state;
    int ret;

    assert(state);

    /* if the file was created for writing */
    if(state->creat_flag)
    {
        /* flush the last region of the log to file */
        switch(fd->comp_type)
        {
            case DARSHAN_ZLIB_COMP:
                ret = darshan_log_libz_flush(fd, state->dz.prev_reg_id);
                if(ret == 0)
                    break;
#ifdef HAVE_LIBBZ2
            case DARSHAN_BZIP2_COMP:
                ret = darshan_log_bzip2_flush(fd, state->dz.prev_reg_id);
                if(ret == 0)
                    break;
#endif 
            default:
                /* if flush fails, remove the output log file */
                state->err = -1;
                fprintf(stderr, "Error: final flush to log file failed.\n");
                break;
        }

        /* if no errors flushing, write the log header before closing */
        if(state->err != -1)
        {
            ret = darshan_log_put_header(fd);
            if(ret < 0)
                state->err = -1;
        }
    }

    close(state->fildes);

    /* remove output log file if error writing to it */
    if((state->creat_flag) && (state->err == -1))
    {
        fprintf(stderr, "Unlinking darshan log file %s ...\n",
            state->logfile_path);
        unlink(state->logfile_path);
    }

    darshan_log_dzdestroy(fd);
    if(state->exe_mnt_data)
        free(state->exe_mnt_data);
    free(state);
    free(fd);

    return;
}

/* **************************************************** */

/* read the header of the darshan log and set internal fd data structures
 * NOTE: this is the only portion of the darshan log that is uncompressed
 *
 * returns 0 on success, -1 on failure
 */
static int darshan_log_get_header(darshan_fd fd)
{
    struct darshan_header header;
    int i;
    int ret;

    ret = darshan_log_seek(fd, 0);
    if(ret < 0)
    {
        fprintf(stderr, "Error: unable to seek in darshan log file.\n");
        return(-1);
    }

    /* read the version number so we know how to process this log */
    ret = darshan_log_read(fd, &fd->version, 8);
    if(ret < 8)
    {
        fprintf(stderr, "Error: invalid log file (failed to read version).\n");
        return(-1);
    }

    /* other log file versions can be detected and handled here */
    if(strcmp(fd->version, "3.00"))
    {
        fprintf(stderr, "Error: incompatible darshan file.\n");
        fprintf(stderr, "Error: expected version %s\n", DARSHAN_LOG_VERSION);
        return(-1);
    }

    /* seek back so we can read the entire header */
    ret = darshan_log_seek(fd, 0);
    if(ret < 0)
    {
        fprintf(stderr, "Error: unable to seek in darshan log file.\n");
        return(-1);
    }

    /* read uncompressed header from log file */
    ret = darshan_log_read(fd, &header, sizeof(header));
    if(ret != (int)sizeof(header))
    {
        fprintf(stderr, "Error: failed to read darshan log file header.\n");
        return(-1);
    }

    if(header.magic_nr == DARSHAN_MAGIC_NR)
    {
        /* no byte swapping needed, this file is in host format already */
        fd->swap_flag = 0;
    }
    else
    {
        /* try byte swapping */
        DARSHAN_BSWAP64(&(header.magic_nr));
        if(header.magic_nr == DARSHAN_MAGIC_NR)
        {
            fd->swap_flag = 1;

            /* swap the log map variables in the header */
            DARSHAN_BSWAP64(&(header.name_map.off));
            DARSHAN_BSWAP64(&(header.name_map.len));
            for(i = 0; i < DARSHAN_MAX_MODS; i++)
            {
                DARSHAN_BSWAP64(&(header.mod_map[i].off));
                DARSHAN_BSWAP64(&(header.mod_map[i].len));
            }
        }
        else
        {
            /* otherwise this file is just broken */
            fprintf(stderr, "Error: bad magic number in darshan log file.\n");
            return(-1);
        }
    }

    /* set some fd fields based on what's stored in the header */
    fd->comp_type = header.comp_type;
    fd->partial_flag = header.partial_flag;
    memcpy(fd->mod_ver, header.mod_ver, DARSHAN_MAX_MODS * sizeof(uint32_t));

    /* save the mapping of data within log file to this file descriptor */
    memcpy(&fd->name_map, &(header.name_map), sizeof(struct darshan_log_map));
    memcpy(&fd->mod_map, &(header.mod_map), DARSHAN_MAX_MODS * sizeof(struct darshan_log_map));

    /* there may be nothing following the job data, so safety check map */
    fd->job_map.off = sizeof(struct darshan_header);
    if(fd->name_map.off == 0)