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  <title>C CGI Library 1.2</title>
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<body style="width:100ex;margin-left:3em">
  <h1 style="text-align:center">C CGI Library 1.2</h1>

  <h2>Contents</h2>

  <ul>
    <li><a href="#intro">Introduction</a></li>

    <li><a href="#types">C CGI Library Data Types</a></li>

    <li><a href="#func">C CGI Library Functions</a></li>

    <li><a href="#using">Using the C CGI Library</a></li>

    <li><a href="#upload">Uploading Files</a></li>

    <li><a href="#crypto">Simple Cryptography Support</a></li>

    <li><a href="#prefork">Pre Forking SCGI Server</a></li>
  </ul>

  <h2><a name="intro" id="intro"></a>Introduction</h2>

  <p><i>C CGI</i> is a C language library for decoding, storing,
  and retrieving CGI data passed by the web server via the CGI
  interface. The library also has several handy data conversion
  functions.</p>

  <p>Author: Stephen C. Losen, University of Virginia</p>

  <h3>C CGI Library Features</h3>

  <ul>
    <li>Decodes and stores CGI variables in <a href=
    "#query">application/x-www-form-urlencoded</a> format, which
    may come from the <i>QUERY_STRING</i> environment variable or
    from standard input.</li>

    <li>Decodes and stores CGI variables in
    <i>multipart/form-data</i> format from standard input.</li>

    <li>Handles <a href="#upload">file uploads</a>.</li>

    <li>Parses and stores HTTP <i>cookies</i> from the
    <i>HTTP_COOKIE</i> environment variable.</li>

    <li>Stores CGI data in lookup tables, which can be accessed
    directly by variable name, or accessed iteratively.</li>

    <li>Allows strings to be stored in lookup tables.</li>

    <li>Encodes/decodes strings in <a href=
    "#query">application/x-www-form-urlencoded</a> format.</li>

    <li>Encodes/decodes data in base64 format.</li>

    <li>Encodes/decodes data in hexadecimal format.</li>

    <li>Encodes text strings using HTML entity encodings such as
    <b>&amp;lt;</b> and <b>&amp;amp;</b>.</li>

    <li>Provides <i>openssl</i> <a href="#crypto">cryptography</a>
    to encrypt/decrypt and verify data.</li>

    <li>Provides a <a href="#prefork">pre forking SCGI
    server</a>.</li>
  </ul>

  <h3><a name="query" id="query"></a>Query Strings and URL
  Encoding</h3>

  <p>A <i>query string</i> is a list of CGI variable names and
  values in <i>application/x-www-form-urlencoded</i> format, which
  looks like this:
  <b>name1=value1&amp;name2=value2&amp;name3=value3</b>&nbsp;...
  Each name and value is <i>URL encoded</i> as follows. Letters and
  digits are not changed. Each <i>space</i> is converted to
  <b>+</b>. Most other characters become <b>%xx</b> (percent
  followed by two hexadecimal digits) where <i>xx</i> is the
  numeric code of the character. For example, <b>Help&nbsp;Me!</b>
  is URL encoded as <b>Help+Me%21</b>. Note that in a query string
  the <b>=</b> and <b>&amp;</b> characters <b>between</b> names and
  values are <b>not</b> URL encoded. However, if a name or value
  itself contains a <b>=</b> or <b>&amp;</b>, then it is URL
  encoded using <b>%xx</b>.</p>

  <p>When decoding query strings, the C CGI Library is tolerant of
  lax <b>%xx</b> encoding. It accepts most literal punctuation
  characters except for <b>+</b> and <b>&amp;</b>, which must be
  encoded. It accepts literal <b>=</b> in variable values and it
  accepts literal <b>%</b> when not followed by two hexadecimal
  digits. For example, <b>var=20%=1/5</b> is treated like
  <b>var=20%25%3D1%2F5</b> and sets the value of <i>var</i> to
  <b>20%=1/5</b>. A string that is not followed by <b>=</b> is a
  variable name whose value is <b>""</b>, a zero length string. For
  example, <b>str1&amp;str2&amp;</b>&nbsp;... is the same as
  <b>str1=&amp;str2=&amp;</b>&nbsp;..., where <i>str1</i> and
  <i>str2</i> are variable names whose value is <b>""</b>.</p>

  <h3>CGI Data Representation and Conversion</h3>

  <p>For simplicity and ease of use, most C CGI Library functions
  accept and/or return null terminated strings. You can easily
  convert a string to a numeric data type with the standard C
  library functions <i>atoi()</i>, <i>atof()</i>, <i>strtol()</i>,
  <i>strtod()</i>, etc. And you can convert a numeric data type to
  a string with <i>sprintf()</i>.</p>

  <p>Unfortunately, null terminated strings are not suitable for
  storing raw binary data, because a null byte in the data is
  mistaken for the string terminator. URL encoded strings
  containing <b>%00</b> do not decode correctly because <b>%00</b>
  results in a null byte. You can still manipulate binary data if
  you encode it beforehand, and the C CGI library has functions for
  encoding/decoding <a href="#CGI_encode_base64">base64</a> and
  <a href="#CGI_encode_hex">hexadecimal</a>.</p>

  <h2><a name="types" id="types"></a>C CGI Library Data Types</h2>

  <p>In your C source you include the <i>ccgi.h</i> header file,
  which declares these data types.</p>

  <dl>
    <dt>CGI_varlist</dt>

    <dd>is a list (lookup table) of CGI variables and/or cookies.
    Each list entry is a name and one or more values, where names
    and values are all null terminated strings. A name may have
    multiple values because 1) some HTML form elements, such as
    <i>checkboxes</i> and <i>selections</i> allow the user to
    choose multiple values and 2) the same name can be given to
    multiple form input elements. A CGI_varlist lists variable
    names and values in the same order that they are stored. In
    practice this ends up being the order of the input tags in the
    HTML form, but there is no requirement that browsers must
    preserve this ordering.</dd>

    <dt>CGI_value</dt>

    <dd>is a read only pointer to a read only null terminated
    string (<i>const char * const</i>). The <a href=
    "#CGI_lookup_all">CGI_lookup_all()</a> function returns a null
    terminated array of these pointers.</dd>
  </dl>

  <h2><a name="func" id="func"></a>C CGI Library Functions</h2>

  <p>The C CGI library provides these functions.</p>

  <ul>
    <li><a href="#CGI_get_query">CGI_get_query()</a> decodes CGI
    variables from the <i>QUERY_STRING</i> environment variable and
    adds them to a CGI_varlist.</li>

    <li><a href="#CGI_get_post">CGI_get_post()</a> decodes CGI
    variables from standard input and adds them to a
    CGI_varlist.</li>

    <li><a href="#CGI_get_cookie">CGI_get_cookie()</a> parses
    cookies from the <i>HTTP_COOKIE</i> environment variable and
    adds them to a CGI_varlist.</li>

    <li><a href="#CGI_get_all">CGI_get_all()</a> decodes all CGI
    variables and cookies and returns them in a CGI_varlist.</li>

    <li><a href="#CGI_decode_query">CGI_decode_query()</a> decodes
    CGI variables in a null terminated <a href="#query">query
    string</a> and adds them to a CGI_varlist.</li>

    <li><a href="#CGI_add_var">CGI_add_var()</a> adds a variable
    name and value to a CGI_varlist.</li>

    <li><a href="#CGI_lookup_all">CGI_lookup_all()</a> looks up a
    name in a CGI_varlist and returns all of its values in an
    array.</li>

    <li><a href="#CGI_lookup">CGI_lookup()</a> looks up a name in a
    CGI_varlist and returns its first (or only) value.</li>

    <li><a href="#CGI_first_name">CGI_first_name()</a> returns the
    first name in a CGI_varlist.</li>

    <li><a href="#CGI_next_name">CGI_next_name()</a> returns the
    next name in a CGI_varlist.</li>

    <li><a href="#CGI_free_varlist">CGI_free_varlist()</a> frees
    memory used by a CGI_varlist.</li>

    <li><a href="#CGI_encode_query">CGI_encode_query()</a> URL
    encodes a list of strings into a <a href="#query">query
    string</a>.</li>

    <li><a href="#CGI_encode_varlist">CGI_encode_varlist()</a> URL
    encodes a CGI_varlist into a <a href="#query">query
    string</a>.</li>

    <li><a href="#CGI_encrypt">CGI_encrypt()</a> encrypts data
    bytes into a secure string.</li>

    <li><a href="#CGI_decrypt">CGI_decrypt()</a> decrypts a secure
    string and verifies the contents.</li>

    <li><a href="#CGI_decode_url">CGI_decode_url()</a> decodes a
    URL encoded string.</li>

    <li><a href="#CGI_encode_url">CGI_encode_url()</a> URL encodes
    a string.</li>

    <li><a href="#CGI_encode_entity">CGI_encode_entity()</a> HTML
    entity encodes a string.</li>

    <li><a href="#CGI_encode_base64">CGI_encode_base64()</a> base64
    encodes data bytes.</li>

    <li><a href="#CGI_decode_base64">CGI_decode_base64()</a>
    decodes a base64 encoded string.</li>

    <li><a href="#CGI_encode_hex">CGI_encode_hex()</a> hexadecimal
    encodes data bytes.</li>

    <li><a href="#CGI_decode_hex">CGI_decode_hex()</a> decodes a
    hexadecimal encoded string.</li>

    <li><a href="#CGI_prefork_server">CGI_prefork_server()</a>
    implements a pre forking SCGI server.</li>
  </ul>

  <p>Except for <i>CGI_prefork_server()</i>, the C CGI library
  functions are reentrant because they do not modify any global
  variables or use any static local variables, so you can use these
  functions with threads.</p>

  <p>Some functions accept null terminated string parameters of
  type <i>const&nbsp;char&nbsp;*</i>. These functions make copies
  of strings as necessary so that after the function returns you
  can safely do anything you want with any string that you have
  passed as a parameter. Some functions return null terminated
  strings of type <i>const&nbsp;char&nbsp;*</i> and you should not
  modify these strings.</p>

  <dl>
    <dt><a name="CGI_get_query" id="CGI_get_query"></a> CGI_varlist
    *CGI_get_query (CGI_varlist *varlist);</dt>

    <dd><i>CGI_get_query()</i> decodes CGI variables in the
    <i>QUERY_STRING</i> environment variable and adds them to
    variable list <i>varlist</i>. <i>QUERY_STRING</i> is presumed
    to be in <a href="#query">application/x-www-form-urlencoded</a>
    format. If <i>varlist</i> is null then a new variable list is
    created and returned, otherwise <i>varlist</i> is returned.
    Null is returned if <i>varlist</i> is null and
    <i>QUERY_STRING</i> does not exist or contains no CGI
    variables.</dd>

    <dt><a name="CGI_get_post" id="CGI_get_post"></a> CGI_varlist
    *CGI_get_post(CGI_varlist *varlist, const char *template);</dt>

    <dd><i>CGI_get_post()</i> reads and decodes CGI variables from
    standard input and adds them to variable list <i>varlist</i>.
    If <i>varlist</i> is null then a new variable list is created
    and returned, otherwise <i>varlist</i> is returned. Null is
    returned if <i>varlist</i> is null and standard input is empty
    or contains no CGI variables. The <i>template</i> parameter
    (which may be null) is a file name template string that is
    passed to the standard C library function <i>mkstemp()</i> when
    uploading a file. (See the <a href="#upload">file upload</a>
    section for more information.) <i>CGI_get_post()</i> checks the
    <i>CONTENT_TYPE</i> environment variable to get the data
    encoding, which is either <a href=
    "#query">application/x-www-form-urlencoded</a> or
    <i>multipart/form-data</i>.</dd>

    <dt><a name="CGI_get_cookie" id="CGI_get_cookie"></a>
    CGI_varlist *CGI_get_cookie(CGI_varlist *varlist);</dt>

    <dd><i>CGI_get_cookie()</i> parses HTTP cookies from the
    <i>HTTP_COOKIE</i> environment variable and adds them to
    variable list <i>varlist</i>. If <i>varlist</i> is null then a
    new variable list is created and returned, otherwise
    <i>varlist</i> is returned. Returns null if <i>varlist</i> is
    null and <i>HTTP_COOKIE</i> does not exist or contains no
    cookies.</dd>

    <dt><a name="CGI_get_all" id="CGI_get_all"></a> CGI_varlist
    *CGI_get_all(const char *template);</dt>

    <dd><i>CGI_get_all()</i> calls <a href=
    "#CGI_get_cookie">CGI_get_cookie()</a>, <a href=
    "#CGI_get_query">CGI_get_query()</a> and <a href=
    "#CGI_get_post">CGI_get_post()</a>, returning all of the CGI
    variables and cookies in one variable list. The <i>template</i>
    parameter (which may be null) is passed on to
    <i>CGI_get_post()</i>.</dd>

    <dt><a name="CGI_decode_query" id="CGI_decode_query"></a>
    CGI_varlist *CGI_decode_query(CGI_varlist *varlist, const char
    *query);</dt>

    <dd><i>CGI_decode_query()</i> decodes CGI variables in null
    terminated <a href="#query">query string</a> <i>query</i>
    (which is in <i>application/x-www-urlencoded</i> format) and
    adds the CGI variables to <i>varlist</i>. If <i>varlist</i> is
    null then a new variable list is created and returned,
    otherwise <i>varlist</i> is returned. Returns null if
    <i>varlist</i> is null and <i>query</i> is null or has no CGI
    variables.</dd>

    <dt><a name="CGI_add_var" id="CGI_add_var"></a> CGI_varlist
    *CGI_add_var(CGI_varlist *varlist, const char *name, const char
    *value);</dt>

    <dd><i>CGI_add_var()</i> adds an entry named <i>name</i> with
    value <i>value</i> to variable list <i>varlist</i>. If
    <i>varlist</i> is null, then a new variable list is created and
    returned, otherwise <i>varlist</i> is returned. If the variable
    list already has an entry named <i>name</i>, then the value is
    added to that entry. This function is provided so that you can
    add data to a variable list by hand, or create a variable list
    for other purposes.</dd>

    <dt><a name="CGI_lookup_all" id="CGI_lookup_all"></a> CGI_value
    *CGI_lookup_all(CGI_varlist *varlist, const char *name);</dt>

    <dd><i>CGI_lookup_all()</i> searches <i>varlist</i> for an
    entry whose name matches <i>name</i> case sensitively and
    returns all the values of the entry, or returns null if no
    entry is found. If <i>name</i> is null, then the values of the
    entry most recently visited by <a href=
    "#CGI_first_name">CGI_first_name()</a> or <a href=
    "#CGI_next_name">CGI_next_name()</a> are returned. The return
    value is a null terminated array of pointers to null terminated
    strings. The array and strings are stored in memory allocated
    to <i>varlist</i>, which you should not modify. The return type
    <i>CGI_value&nbsp;*</i>
    (<i>const&nbsp;char&nbsp;*&nbsp;const&nbsp;*</i>) declares the
    array and strings to be read only to discourage
    modification.</dd>

    <dt><a name="CGI_lookup" id="CGI_lookup"></a> const char
    *CGI_lookup(CGI_varlist *varlist, const char *name);</dt>

    <dd><i>CGI_lookup()</i> searches <i>varlist</i> for an entry
    whose name matches <i>name</i> case sensitively and returns the
    first (or only) value of the entry, or returns null if no entry
    is found. If <i>name</i> is null, then the first value of the
    entry most recently visited by <a href=
    "#CGI_first_name">CGI_first_name()</a> or <a href=
    "#CGI_next_name">CGI_next_name()</a> is returned. If you expect
    an entry to have a single value, then this function is easier
    to use than <i>CGI_lookup_all()</i> and it is more efficient
    because it doesn't construct an array to return multiple
    values. You should not modify the string returned by this
    function.</dd>

    <dt><a name="CGI_first_name" id="CGI_first_name"></a> const
    char *CGI_first_name(CGI_varlist *varlist);</dt>

    <dd><i>CGI_first_name()</i> begins an iteration of
    <i>varlist</i> and returns the name of the first entry, or
    returns null if <i>varlist</i> is null. You can get all the
    values of this entry with
    <i>CGI_lookup_all(varlist,&nbsp;0);</i> You should not modify
    the string returned by this function.</dd>

    <dt><a name="CGI_next_name" id="CGI_next_name"></a> const char
    *CGI_next_name(CGI_varlist *varlist);</dt>

    <dd><i>CGI_next_name()</i> continues an iteration of
    <i>varlist</i> and returns the name of the next entry. You can
    get all the values of this entry with
    <i>CGI_lookup_all(varlist,&nbsp;0);</i> Returns null if 1)
    there are no more entries, or 2) <i>varlist</i> is null, or 3)
    no iteration was started with <i>CGI_first_name()</i>, or 4)
    new data was added to <i>varlist</i> during the iteration. You
    should not modify the string returned by this function.</dd>

    <dt><a name="CGI_free_varlist" id="CGI_free_varlist"></a> void
    CGI_free_varlist(CGI_varlist *varlist);</dt>

    <dd><i>CGI_free_varlist()</i> frees all memory used by variable
    list <i>varlist</i>.</dd>

    <dt><a name="CGI_encode_query" id="CGI_encode_query"></a> char
    *CGI_encode_query(const char *keep, const char *name1, const
    char *value1, ..., (char *)0);</dt>

    <dd><i>CGI_encode_query()</i> returns a query string in
    <a href="#query">application/x-www-form-urlencoded</a> format
    that is built from a null terminated list of null terminated
    string arguments. The first argument <i>keep</i> (which may be
    null) is a null terminated string that specifies characters
    that you do not want to <a href="#query">URL encode</a> with
    <b>%xx</b>. You do not need to specify letters or digits
    because they are never encoded. The first two arguments after
    <i>keep</i> are a name and value pair, the next two are a
    second name and value pair, etc. <b>Be sure to terminate the
    argument list with (char *)0</b>. (When passing a variable
    length argument list, the C compiler does not automatically
    cast <b>0</b> to <b>(char *)0</b>, which is necessary
    on 64 bit platforms.)  In the result the names
    and values are URL encoded and separated with literal
    <b>&amp;</b> and <b>=</b> characters like this:
    <b>name1=value1&amp;name2=value2</b>&nbsp;... Memory is
    allocated with <i>malloc()</i> to hold the result, which you
    should free with <i>free()</i>.</dd>

    <dt><a name="CGI_encode_varlist" id="CGI_encode_varlist"></a>
    char *CGI_encode_varlist(CGI_varlist *varlist, const char
    *keep);</dt>

    <dd><i>CGI_encode_varlist()</i> returns a query string in
    <a href="#query">application/x-www-form-urlencoded</a> format
    that is built from CGI_varlist <i>varlist</i>. The argument
    <i>keep</i> (which may be null) is a null terminated string
    that specifies characters that you do not want to <a href=
    "#query">URL encode</a> with <b>%xx</b>. You do not need to
    specify letters or digits because they are never encoded. The
    names and values in <i>varlist</i> are URL encoded and
    separated with literal <b>&amp;</b> and <b>=</b> characters
    like this: <b>name1=value1&amp;name2=value2</b>&nbsp;... Memory
    is allocated with <i>malloc()</i> to hold the result, which you
    should free with <i>free()</i>. You can use <a href=
    "#CGI_add_var">CGI_add_var()</a> to build <i>varlist</i>.</dd>

    <dt><a name="CGI_encrypt" id="CGI_encrypt"></a> char
    *CGI_encrypt(const void *p, int len, const char
    *password);</dt>

    <dd><i>CGI_encrypt()</i> encrypts input <i>p</i> of length
    <i>len</i> bytes using <i>password</i> to generate the cipher
    key. Also computes a <i>message digest</i> using the input
    data. Returns the encrypted digest and encrypted data in a
    base64 encoded string, which must be decrypted with <a href=
    "#CGI_decrypt">CGI_decrypt()</a> and the same password. Returns
    null if <i>p</i> is null or if <i>len</i> is less than one or
    if <i>password</i> is null or zero length. Memory is allocated
    with <i>malloc()</i> to hold the result, which you should free
    with <i>free()</i>. (See the <a href="#crypto">cryptography</a>
    section for more information.)</dd>

    <dt><a name="CGI_decrypt" id="CGI_decrypt"></a> void
    *CGI_decrypt(const char *p, int *len, const char
    *password);</dt>

    <dd><i>CGI_decrypt()</i> decrypts input <i>p</i>, which was
    encrypted with <a href="#CGI_encrypt">CGI_encrypt()</a>, using
    <i>password</i> to generate the cipher key. The output is a
    <i>message digest</i> and decrypted data bytes. Verifies the
    data using the message digest and returns the data. Returns the
    length of the data in <i>*len</i>. Returns null if <i>p</i>
    cannot be decrypted and verified. Also returns null if <i>p</i>
    or <i>password</i> is null or zero length. Memory is allocated
    with <i>malloc()</i> to hold the result, which you should free
    with <i>free()</i>. (See the <a href="#crypto">cryptography</a>
    section for more information.)</dd>

    <dt><a name="CGI_decode_url" id="CGI_decode_url"></a> char
    *CGI_decode_url(const char *p);</dt>

    <dd><i>CGI_decode_url()</i> returns a <a href="#query">URL
    decoded</a> copy of input string <i>p</i>. Memory is allocated
    with <i>malloc()</i> to hold the result, which you should free
    with <i>free()</i>.</dd>

    <dt><a name="CGI_encode_url" id="CGI_encode_url"></a> char
    *CGI_encode_url(const char *p, const char *keep);</dt>

    <dd><i>CGI_encode_url()</i> returns a <a href="#query">URL
    encoded</a> copy of input string <i>p</i>. Memory is allocated
    with <i>malloc()</i> to hold the result, which you should free
    with <i>free()</i>. The <i>keep</i> argument (which may be
    null) is a null terminated string that specifies characters
    that you do not want to URL encode with <b>%xx</b>. You do not
    need to specify letters or digits because they are never
    encoded.</dd>

    <dt><a name="CGI_encode_entity" id="CGI_encode_entity"></a>
    char *CGI_encode_entity(const char *p);</dt>

    <dd><i>CGI_encode_entity()</i> returns a HTTP entity encoded
    copy of input string <i>p</i>. Memory is allocated with
    <i>malloc()</i> to hold the result, which you should free with
    <i>free()</i>. <i>CGI_encode_entity()</i> makes the following
    conversions: <b>&lt;</b> becomes <b>&amp;lt;</b>, <b>&gt;</b>
    becomes <b>&amp;gt;</b>, <b>&amp;</b> becomes <b>&amp;amp;</b>,
    <b>"</b> becomes <b>&amp;quot;</b>, <b>'</b> becomes
    <b>&amp;#39;</b>, <i>newline</i> becomes <b>&amp;#10;</b>, and
    <i>return</i> becomes <b>&amp;#13;</b>.</dd>

    <dt><a name="CGI_encode_base64" id="CGI_encode_base64"></a>
    char *CGI_encode_base64(const void *p, int len);</dt>

    <dd><i>CGI_encode_base64()</i> encodes input <i>p</i> of length
    <i>len</i> bytes, and returns the result, which is a null
    terminated base64 encoded string. Memory is allocated for the
    result with <i>malloc()</i>, which you should free with
    <i>free()</i>. Base64 is a commonly used encoding that
    represents arbitrary bytes of data using the following
    printable characters: upper case, lower case, digits, <b>+</b>,
    <b>/</b>, and <b>=</b>.</dd>

    <dt><a name="CGI_decode_base64" id="CGI_decode_base64"></a>
    void *CGI_decode_base64(const char *p, int *len);</dt>

    <dd><i>CGI_decode_base64()</i> decodes <i>p</i>, which is a
    null terminated base64 encoded string, and returns the result.
    The length of the result is stored in <i>*len</i> and a null
    byte is written just after the last byte of the result. Memory
    is allocated with <i>malloc()</i> to hold the result, which you
    should free with <i>free()</i>.</dd>

    <dt><a name="CGI_encode_hex" id="CGI_encode_hex"></a> char
    *CGI_encode_hex(const void *p, int len);</dt>

    <dd><i>CGI_encode_hex()</i> encodes input <i>p</i>, of length
    <i>len</i> bytes, and returns the result, which is a null
    terminated hexadecimal encoded string. Memory is allocated for
    the result with <i>malloc()</i>, which you should free with
    <i>free()</i>. Hexadecimal is a commonly used encoding that
    represents arbitrary bytes of data using two hexadecimal digits
    for each byte.</dd>

    <dt><a name="CGI_decode_hex" id="CGI_decode_hex"></a> void
    *CGI_decode_hex(const char *p, int *len);</dt>

    <dd><i>CGI_decode_hex()</i> decodes <i>p</i>, which is a null
    terminated hexadecimal encoded string, and returns the result.
    The length of the result is stored in <i>*len</i> and a null
    byte is written just after the last byte of the result. Memory
    is allocated with <i>malloc()</i> to hold the result, which you
    should free with <i>free()</i>. Returns null if <i>p</i> is
    null, or if the length of <i>p</i> is odd, or if <i>p</i>
    contains characters other than hexadecimal digits.</dd>

    <dt><a name="CGI_prefork_server" id="CGI_prefork_server"></a>
    void CGI_prefork_server(const char *host, int port, const char
    *pidfile, int maxproc, int minidle, int maxidle, int maxreq,
    void (*callback)(void));</dt>

    <dd><i>CGI_prefork_server()</i> implements a <a href=
    "http://www.mems-exchange.org/software/scgi/">SCGI</a> (Simple
    CGI) pre forking server. The <i>host</i> specifies a local
    network address, either by hostname or dotted decimal IP
    address, and <i>port</i> specifies a TCP port number. The SCGI
    server listens for requests on the specified address and port.
    If <i>host</i> is null, then the server listens on all local
    addresses. The <i>pidfile</i> (which may be null) is an
    optional file name where the server writes its process ID. The
    SCGI server forks up to <i>maxproc</i> child processes to
    handle requests. It forks and destroys processes to maintain
    between <i>minidle</i> and <i>maxidle</i> idle processes. Each
    process exits after handling <i>maxreq</i> requests. If
    <i>maxreq</i> is less than one, then it is unlimited. You
    provide the <i>callback</i> function, which the SCGI server
    calls to process each web request. <i>CGI_prefork_server()</i>
    does not return unless it fails. (See the <a href=
    "#prefork">SCGI server</a> section for more information.)</dd>
  </dl>

  <h2><a name="using" id="using"></a>Using the C CGI Library</h2>

  <p>Here is an example program that outputs all of its CGI data.
  In your C source, include <i>ccgi.h</i> and link your program
  with <i>libccgi.a</i>. (If you use <i>CGI_encrypt()</i> or
  <i>CGI_decrypt()</i> then you must also link with the
  <i>openssl</i> library <i>libcrypto</i>.) The simplest way to
  obtain your CGI data is with <i>CGI_get_all()</i>. If you are not
  uploading any files, then just pass it a null argument.</p>
  <pre>
#include &lt;stdio.h&gt;
#include &lt;ccgi.h&gt;

int main(int argc, char **argv) {
    CGI_varlist *varlist;
    const char *name;
    CGI_value  *value;
    int i;

    fputs("Content-type: text/plain\r\n\r\n", stdout);
    if ((varlist = CGI_get_all(0)) == 0) {
        printf("No CGI data received\r\n");
        return 0;
    }

    /* output all values of all variables and cookies */

    for (name = CGI_first_name(varlist); name != 0;
        name = CGI_next_name(varlist))
    {
        value = CGI_lookup_all(varlist, 0);

        /* CGI_lookup_all(varlist, name) could also be used */

        for (i = 0; value[i] != 0; i++) {
            printf("%s [%d] = %s\r\n", name, i, value[i]);
        }
    }
    CGI_free_varlist(varlist);  /* free variable list */
    return 0;
}
</pre>

  <h2><a name="upload" id="upload"></a>File Uploads</h2>

  <p>To upload files to your CGI program, your HTML form must use
  the <i>post</i> method and must specify
  <i>multipart/form-data</i> encoding, so the form tag looks like
  this:</p>
  <pre>
&lt;form method="POST" enctype="multipart/form-data"
    action="url-for-your-CGI"&gt;
</pre>

  <p>Within the HTML form a file upload tag looks like this:</p>
  <pre>
&lt;input type="file" name="uploadfield" /&gt;
</pre>

  <p>Most browsers render this tag with a file browse button and a
  text field to enter and/or display the name of the file being
  uploaded.</p>

  <p>When the user submits the form, the browser sends the file
  data together with any CGI form variables using
  <i>multipart/form-data</i> encoding. To receive uploaded file
  data you must call <a href="#CGI_get_post">CGI_get_post()</a> or
  <a href="#CGI_get_all">CGI_get_all()</a>, and pass a file name
  template string, a copy of which is passed on to standard C
  function <i>mkstemp()</i>. The final six characters of the
  template string must be <b>XXXXXX</b> and <i>mkstemp()</i>
  replaces these with random characters to create a new file with a
  unique name. If you pass a null or invalid template string, then
  uploaded file data is silently discarded.</p>

  <p><i>CGI_get_post()</i> or <i>CGI_get_all()</i> stores two names
  for the uploaded file in the variable list, which you can
  retrieve with</p>
  <pre>
value = CGI_lookup_all(varlist, "uploadfield");
</pre>

  <p>This returns an array of two strings (provided no other form
  input tags are named <i>uploadfield</i>). In <i>value[0]</i> is
  the name of the uploaded file on the web server, which is derived
  from the template string. In <i>value[1]</i> is the name of the
  file specified by the user in the browser. If the user has not
  uploaded a file, then <i>varlist</i> has no entry named
  <i>uploadfield</i> and <i>CGI_lookup_all()</i> returns null.</p>

  <p>We use <i>mkstemp()</i> to guarantee unique file names because
  a form may have multiple file upload fields, resulting in
  multiple files. Furthermore, multiple users can upload files to
  multiple instances of the CGI at the same time. Here is an
  example CGI program that uploads a file.</p>
  <pre>
#include &lt;ccgi.h&gt;
#include &lt;stdio.h&gt;

int main(int argc, char **argv) {
    CGI_varlist *varlist;
    CGI_value *value;

    fputs("Content-type: text/plain\r\n\r\n", stdout);
    varlist = CGI_get_all("/tmp/cgi-upload-XXXXXX");
    value = CGI_lookup_all(varlist, "uploadfield");
    if (value == 0 || value[1] == 0) {
        fputs("No file was uploaded\r\n", stdout);
    }
    else {
        printf("Your file \"%s\" was uploaded to my file \"%s\"\r\n",
            value[1], value[0]);

        /* Do something with the file here */

        unlink(value[0]);
    }
    CGI_free_varlist(varlist);
    return 0;
}
</pre>

  <h2><a name="crypto" id="crypto"></a>Simple Cryptography
  Support</h2>

  <p>HTML and HTTP provide no native support for protecting and
  verifying CGI data. Many web applications pass state data to the
  browser in cookies or form variables. When the browser passes
  this data back, the web application cannot tell if it has been
  tampered with. An attacker can easily handcraft a web request
  that includes forged cookies or forged form data.</p>

  <p>The C CGI Library addresses this problem with <a href=
  "#CGI_encrypt">CGI_encrypt()</a> and <a href=
  "#CGI_decrypt">CGI_decrypt()</a>. <i>CGI_encrypt()</i> computes a
  <i>SHA1 message digest</i> from the input data, encrypts the
  digest and the input data, and returns the result in a base64
  encoded string. (Raw encrypted output is binary.)
  <i>CGI_decrypt()</i> reverses the process. It decrypts the digest
  and the data, and recomputes the digest. If the two digests
  match, then it returns the data. Otherwise it returns null to
  indicate failure. <i>CGI_encrypt()</i> and <i>CGI_decrypt()</i>
  use a password that you provide, which is a null terminated
  string of arbitrary length (the longer the better). It is
  essentially impossible to tamper with the data without knowing
  the password. If the output of <i>CGI_encrypt()</i> is modified
  in any way, then <i>CGI_decrypt()</i> computes a message digest
  that does not match and returns null.</p>

  <p>To protect state data, simply encrypt it with
  <i>CGI_encrypt()</i> and a password before passing it to the
  browser. When the browser passes the encrypted data back, decrypt
  with <i>CGI_decrypt()</i> and the same password. If
  <i>CGI_decrypt()</i> succeeds, then you know that the data has
  not changed. Of course the security of the data depends on the
  security of the password, which should be very difficult to guess
  and very difficult to steal.</p>

  <p><i>CGI_encrypt()</i> uses the <i>openssl</i> library
  <i>libcrypto</i> and has these features:</p>

  <ul>
    <li>Uses the <i>AES-256-CBC</i> cipher.</li>

    <li>Generates the cipher key by feeding the password and a
    random <i>salt</i> to a hash function. One password results in
    a huge number of different cipher keys.</li>

    <li>Uses the <i>SHA1</i> message digest algorithm to verify the
    input data. Feeds the salt, the password, and the input data to
    <i>SHA1</i> to generate the message digest.</li>

    <li>Encrypts the message digest and the input data. (Does not
    encrypt the salt because decryption needs it.)</li>

    <li>Returns a base64 encoded string consisting of the salt,
    encrypted digest and encrypted input data.</li>
  </ul>

  <h2><a name="prefork" id="prefork"></a>Pre Forking SCGI
  Server</h2>

  <p>Usually when a web server receives a request for a CGI
  resource, the web server executes a CGI program, which handles
  the request and exits. This does not perform well under high
  load. <a href=
  "http://www.mems-exchange.org/software/scgi/">SCGI</a> (Simple
  CGI) is a protocol for running a persistent CGI server. When a
  web server receives a request for a SCGI resource, the web server
  connects to a <i>SCGI Server</i> and forwards the request using
  the SCGI protocol. The SCGI server responds to the web server,
  which forwards the response back to the user's browser. This is
  much more efficient than executing a CGI program for each
  request. To configure the Apache httpd web server to use SCGI,
  see the <a href=
  "http://www.mems-exchange.org/software/scgi/">mod_scgi</a>
  module.</p>

  <p>Under high load a SCGI server must be able to handle multiple
  requests concurrently. The SCGI server provided here <i>pre
  forks</i> a specified number of child processes that all wait for
  requests. The parent process monitors how many child processes
  are busy and creates more if necessary. If too many processes are
  idle, then the parent terminates some of them.</p>

  <p>You provide the code that handles web requests in a
  <i>callback</i> function, which is called once for each request.
  The environment, standard input, and standard output are set up
  so that your <i>callback</i> can be written very much like a
  traditional CGI program. All the functions in the C CGI library
  work as specified when using the SCGI server.</p>

  <p>To start the SCGI server, call <a href=
  "#CGI_prefork_server">CGI_prefork_server()</a> and pass it a
  pointer to your <i>callback</i> function. The SCGI server puts
  itself into the background, and forks child processes to handle
  web requests. If <i>CGI_prefork_server()</i> returns, then it has
  failed. The web server does not automatically start the SCGI
  server program, so you must start it. You control which user
  account runs the SCGI server and what privileges it has.</p>

  <p>To terminate the SCGI server, send the <i>SIGTERM</i> signal
  to the parent process and it sends the signal on to its child
  processes and exits. The parent process writes its process ID in
  a file if you pass the file name to <a href=
  "#CGI_prefork_server">CGI_prefork_server()</a> in
  <i>pidfile</i>.</p>

  <p>The <i>callback</i> function operates very much like a
  traditional CGI program, except that it gets called multiple
  times. When writing your <i>callback</i> consider the
  following.</p>

  <ul>
    <li>The <i>callback</i> function should not call <i>exit()</i>
    (unless it encounters a serious error). You defeat the purpose
    of a persistent process if you exit. The parent process
    replaces exited children, so calling <i>exit()</i> does not
    cause the SCGI server to fail.</li>

    <li>Be careful to free memory that you have allocated and close
    files that you have opened inside the <i>callback</i> function.
    Otherwise the process will consume memory and/or file
    descriptors with each call to <i>callback</i>, and eventually
    fail. It may be too difficult to track down all memory leaks.
    You can call <a href=
    "#CGI_prefork_server">CGI_prefork_server()</a> with
    <i>maxreq</i> greater than zero, which causes the child process
    to exit after handling <i>maxreq</i> requests, which releases
    all its resources.</li>

    <li>You may need to handle initialization code differently.
    Initializations made before you call
    <i>CGI_prefork_server()</i> are inherited by all child
    processes. If you open any files or sockets, then the
    corresponding file descriptors are inherited and shared. You
    may not get the behavior you want when multiple processes share
    a file descriptor.</li>

    <li>To initialize each child process individually, place
    initialization code in <i>callback</i> inside an <i>if</i>
    statement that executes the first time <i>callback</i> is
    called. The <i>if</i> statement can test a <i>static</i>
    variable and reset it to prevent executing the body of the
    <i>if</i> statement again.</li>

    <li>Each SCGI request includes a full set of environment
    variables. The SCGI server replaces the entire environment with
    these variables before each call to <i>callback</i>. If you
    need anything from the original environment, then you should
    save it before calling <i>CGI_prefork_server()</i>. If you need
    to manipulate the environment, then the standard C function
    <i>putenv()</i> allows you to add or modify environment
    variables, and the global C variable
    <i>extern&nbsp;char&nbsp;**environ;</i> is a pointer to the
    current environment.</li>

    <li>If you read standard input directly (rather than using
    <i>CGI_get_all()</i> or <i>CGI_get_post()</i>) then use
    <i>stdio</i> library functions. In particular you should not
    use the <i>read()</i> system call because the SCGI server reads
    environment data from standard input using <i>stdio</i> before
    calling <i>callback</i>. Use <i>fread()</i> instead.</li>

    <li>The SCGI server uses <i>syslog()</i> to log error messages.
    If you do not want the default syslog parameters, then
    initialize the logging system with <i>openlog()</i> before
    calling <i>CGI_prefork_server()</i>. Use <i>syslog()</i> inside
    <i>callback</i> to log any errors.</li>
  </ul>

  <p>Here is an example SCGI server.</p>
  <pre>
#include &lt;ccgi.h&gt;
#include &lt;stdio.h&gt;
#include &lt;syslog.h&gt;

static void
cgi_callback() {
    static int first_call = 1;
    CGI_varlist *varlist;

    if (first_call) {
        first_call = 0;

        /* initializations for each child process go here */

    }
    varlist = CGI_get_all(0);

    fputs("Content-type: text/html\r\n\r\n", stdout);

    /* write the rest of the web response to stdout */

    /* free memory and close open files */

    CGI_free_varlist(varlist);
}

int
main(int argc, char **argv) {

    /* initializations before forking child processes go here */

    openlog("my-scgi-server", 0, LOG_DAEMON);

    CGI_prefork_server("localhost", 4000, "/var/run/my-scgi-server.pid",
        /* maxproc */ 100,  /* minidle */ 8,  /* maxidle */ 16,
        /* maxreq */ 1000, cgi_callback);

    /* if CGI_prefork_server() returns, then it failed */

    return 0;
}
</pre>
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</html>