C client to load binary data to MySQL

Discussion in 'C Programming' started by Me Alone, Nov 4, 2006.

  1. Me Alone

    Me Alone Guest

    Hello:

    I am trying to edit some C code I found in "The definitive guide to
    using, programming, and administering MySQL" by Paul DuBois. This C
    client program connects and then segfaults when the function load_image
    is called. Would anyone be able to point me to what I might be doing
    wrong?

    Thanks in advance,
    C Newbie


    #include <mysql/mysql.h>
    #include <stdio.h>

    int load_image(MYSQL *conn)
    {
    FILE * pic;
    char query[1024*100], buf[1024*10], *p;
    unsigned int from_len;
    int status;

    if ((pic = fopen("./oreilly.gif", "r")) != NULL)
    {
    printf("Open command succeeded\n");
    }
    else
    printf("Open command failed.\n");

    sprintf(query, "INSERT INTO images VALUES (%d,'", 300);
    p = query + strlen(query);
    while ((from_len = fread(buf, 1, sizeof(buf), pic)) > 0)
    {
    /* don't overrun end of query buffer! */
    if (p + (2*from_len) + 3 > query + sizeof(query))
    {
    fprintf(stderr, "image too big");
    return(1);
    }
    p += mysql_escape_string(p, buf, from_len);
    }
    (void) strcpy(p, "')");
    status = mysql_query(conn, query);
    return(status);
    }

    main() {
    MYSQL * mysql;
    MYSQL_RES *res;
    MYSQL_ROW row;
    char *server = "localhost";
    char *user = "user";
    char *password = "password";
    char *database = "db";
    unsigned int i;
    char * query = "SELECT * FROM table";
    int fromImage;

    if ((fromImage=load_image(mysql)) > 0)
    fprintf(stderr, "%d", fromImage);

    if((mysql = mysql_init(NULL)) == NULL)
    {
    fprintf(stderr, "Cannot initialize MySQL");
    exit(1);
    }

    /* Connect to database */
    if (!mysql_real_connect(mysql, server,
    user, password, database, 0, NULL, 0))
    {
    fprintf(stderr, "%s\n", mysql_error(mysql));
    exit(0);
    }

    /* send SQL query */
    if (mysql_real_query(mysql, query, strlen(query)))
    {
    fprintf(stderr, "%s\n", mysql_error(mysql));
    exit(0);
    }

    // Process the result.
    if((res = mysql_store_result(mysql)) == NULL)
    {
    fprintf(stderr, "mysql_store_result() failed");
    exit(0);
    }

    while ((row = mysql_fetch_row(res)) != NULL)
    {
    for (i = 0; i < mysql_num_fields(res); i++)
    {
    if (i > 0)
    fputc ('\t', stdout);
    printf("%s", row != NULL ? row : "NULL");
    }
    fputc ('\n', stdout);
    }

    if (mysql_errno(mysql) != 0)
    fprintf(stderr, "mysql_fetch_row() failed");
    else
    printf("%lu rows returned\n", (unsigned long)
    mysql_num_rows(res));

    /* Release memory used to store results and close connection */
    mysql_free_result(res);
    mysql_close(mysql);
    }
     
    Me Alone, Nov 4, 2006
    #1
    1. Advertising

  2. Me Alone

    Ian Collins Guest

    Me Alone wrote:
    > Hello:
    >
    > I am trying to edit some C code I found in "The definitive guide to
    > using, programming, and administering MySQL" by Paul DuBois. This C
    > client program connects and then segfaults when the function load_image
    > is called. Would anyone be able to point me to what I might be doing
    > wrong?
    >
    > Thanks in advance,
    > C Newbie
    >
    >
    > #include <mysql/mysql.h>
    > #include <stdio.h>
    >
    > int load_image(MYSQL *conn)
    > {
    > FILE * pic;
    > char query[1024*100], buf[1024*10], *p;


    These 110K of data may be too big for your environment's stack.

    Otherwise, where exactly does the fault occur?

    --
    Ian Collins.
     
    Ian Collins, Nov 4, 2006
    #2
    1. Advertising

  3. Me Alone <> wrote:
    > I am trying to edit some C code I found in "The definitive guide to
    > using, programming, and administering MySQL" by Paul DuBois. This C
    > client program connects and then segfaults when the function load_image
    > is called. Would anyone be able to point me to what I might be doing
    > wrong?


    > #include <mysql/mysql.h>
    > #include <stdio.h>


    > int load_image(MYSQL *conn)
    > {
    > FILE * pic;
    > char query[1024*100], buf[1024*10], *p;
    > unsigned int from_len;
    > int status;


    > if ((pic = fopen("./oreilly.gif", "r")) != NULL)
    > {
    > printf("Open command succeeded\n");
    > }
    > else
    > printf("Open command failed.\n");


    Shouldn't you return, indicating failure, when opening the file fails?
    If it failed you will be passing a NULL pointer to fread(). That might
    result in a crash.

    > sprintf(query, "INSERT INTO images VALUES (%d,'", 300);


    Why not simply use

    strcpy( query, "INSERT INTO images VALUES (300,'");

    That should do exactly the same.

    > p = query + strlen(query);
    > while ((from_len = fread(buf, 1, sizeof(buf), pic)) > 0)
    > {
    > /* don't overrun end of query buffer! */
    > if (p + (2*from_len) + 3 > query + sizeof(query))


    From a nit-picky point of view this is not 100% correct - according
    to the C standard you're only allowed to compare pointers pointing
    witin the same object. But in case 'p + 2 * from_len + 3' is too
    large this expression already points outside of 'query'. In order
    not to violate this constraint you would need to e.g. use a counter
    of how much you already have used of 'query' and compare to that.
    But I don't think that this is the real problem..

    > {
    > fprintf(stderr, "image too big");
    > return(1);
    > }
    > p += mysql_escape_string(p, buf, from_len);


    <OT because not related to C but MySQL>
    The dicumentation recommends to use myqsl_real_escape_string() instead.
    </OT>

    > }
    > (void) strcpy(p, "')");


    Unless you want to use lint or a similar tool te '(void)' bit at the
    start of the line is superfluous.

    > status = mysql_query(conn, query);


    This, of course, assumes that 'conn' is a pointer to an open connection
    to the database. I don't know what will happen if you pass it an invalid
    pointer, one thing that could happen is a crash...

    <OT>
    The MySQL documentation explicitely states:
    mysql_query() cannot be used for queries that contain binary data; you
    should use mysql_real_query() instead. (Binary data may contain the \0
    character, which mysql_query() interprets as the end of the query string.)
    You should take that into consideration since it's rather likeley that
    what's in a .gif file is binary data.
    </OT>

    > return(status);
    > }


    > main() {


    main() is suposed to return an int. And since you don't pass it arguments
    make that

    int main(void) {

    > MYSQL * mysql;
    > MYSQL_RES *res;
    > MYSQL_ROW row;
    > char *server = "localhost";
    > char *user = "user";
    > char *password = "password";
    > char *database = "db";
    > unsigned int i;
    > char * query = "SELECT * FROM table";
    > int fromImage;


    > if ((fromImage=load_image(mysql)) > 0)
    > fprintf(stderr, "%d", fromImage);


    And here's definitely a problem: you call your function for putting
    the image into the database (load_image() seems to be a mis-nomer for
    what the function does) before you ever opened a connection to it.
    Depending on how mysql_query() handles that case (it probably can't
    since 'mysql' contains some garbage data, not even necessarily NULL
    which it could check for) that might be the most likely reason for
    the segmentation fault.
    Regards, Jens
    --
    \ Jens Thoms Toerring ___
    \__________________________ http://toerring.de
     
    Jens Thoms Toerring, Nov 4, 2006
    #3
  4. Me Alone

    Flash Gordon Guest

    Jens Thoms Toerring wrote:
    > Me Alone <> wrote:


    <snip>

    > That should do exactly the same.
    >
    >> p = query + strlen(query);
    >> while ((from_len = fread(buf, 1, sizeof(buf), pic)) > 0)
    >> {
    >> /* don't overrun end of query buffer! */
    >> if (p + (2*from_len) + 3 > query + sizeof(query))

    >
    > From a nit-picky point of view this is not 100% correct - according
    > to the C standard you're only allowed to compare pointers pointing
    > witin the same object. But in case 'p + 2 * from_len + 3' is too
    > large this expression already points outside of 'query'.


    However, by applying a little algebra we can get a test that does not
    have this problem. Subtract query from both sides of the expression and
    we get:
    if ((p - query) + (2*from_len) + 3 > sizeof(query))

    This is valid on any implementation as long as both pointers are in to
    the same object, the difference between them can be represented and we
    don't have anything else causing an arithmetic overflow. Since one is
    unlikely to be constructing a query string anywhere even close to 30000
    characters long I would say this makes it safe for all implementations
    without being any harder to read.

    > In order
    > not to violate this constraint you would need to e.g. use a counter
    > of how much you already have used of 'query' and compare to that.
    > But I don't think that this is the real problem..


    I agree with you. However I don't see any good reason to leave this
    unfixed seeing as the fix is so simple.

    <snip>

    >> if ((fromImage=load_image(mysql)) > 0)
    >> fprintf(stderr, "%d", fromImage);

    >
    > And here's definitely a problem: you call your function for putting
    > the image into the database (load_image() seems to be a mis-nomer for


    <snip>

    To the OP, if you need further help with the MySQL parts of this, such
    as the load_image function and connecting to the database, please take
    it to a suitable group, possibly one with mysql or database in its name,
    or the MySQL mailing lists. This group is for discussing C, not the
    myriads of third party libraries which have their own groups and mailing
    lists.
    --
    Flash Gordon
     
    Flash Gordon, Nov 4, 2006
    #4
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Christoph Krammer

    Read binary data from MySQL database

    Christoph Krammer, May 10, 2007, in forum: Python
    Replies:
    2
    Views:
    982
    Stefan Sonnenberg-Carstens
    May 10, 2007
  2. ist
    Replies:
    3
    Views:
    669
    Cowboy \(Gregory A. Beamer\)
    Mar 7, 2008
  3. MacRules
    Replies:
    9
    Views:
    833
    Dennis Lee Bieber
    Sep 5, 2009
  4. Sune

    Saving binary data in MySQL

    Sune, Nov 4, 2005, in forum: ASP General
    Replies:
    2
    Views:
    177
  5. julian
    Replies:
    8
    Views:
    471
    Avatar
    Apr 6, 2006
Loading...

Share This Page