cgic: an ANSI C library for CGI Programming

By Thomas Boutell

Table of Contents

• Credits and license terms
• How to get support
• What's new in version 1.05?
• What's new in version 1.04?
• What's new in version 1.03?
• What is cgic?
• Obtaining cgic
• Building and testing cgic: a sample application
• What to do if it won't compile
• How to write a cgic application
• How can I generate images from my cgic application?
• CGI debugging features: using capture
• cgic function reference
• cgic variable reference
• cgic result code reference
• cgic quick index

Credits and License Terms

Please see the file license.txt for the details of the Basic License and Commercial License, including ordering information for the Commercial License.

How to Get Support

However, priority support is available. Priority support customers receive a special support email address from which a personal response within 24 hours (on working days) is provided. Of course, purchasing priority support also encourages further enhancement of cgic, gd and related free boutell.com software.

Priority support can be purchased at the rate of $50 per month, with up to 12 distinct queries per month (reasonable back-and-forth exchanges to resolve a single query are not charged against this limit). After the twelfth query, each additional query costs $5. Preparations to handle support purchases through First Virtual are being made; support may also be paid for by check or money order.

Alternatively, priority support may be purchased à la carte at a rate of $5 per query.

To purchase priority support, send mail to support-request@boutell.com indicating the time period (or number of queries) for which you wish to purchase support and the address to which boutell.com should send an invoice. Invoices may be sent by email or postal mail, at your option, although boutell.com supports resource conservation efforts and prefers to invoice via email when possible. Once you receive the invoice, boutell.com will expect payment within 30 days. You will begin to receive priority support as soon as you declare your intention to purchase said support.

Currently, priority support is intended only to resolve specific difficulties with the gd and cgic libraries. boutell.com priority support is not a budget consulting service. Other inquiries regarding boutell.com software, documentation and the like should be sent to boutell@boutell.com.

What's new in version 1.05?

What's new in version 1.04?

What's new in version 1.03?

What's new in version 1.02?

• cgiFormDoubleBounded specified its arguments in the wrong order, with surprising results. This bug has been corrected.
• Many small changes have been made to increase compatibility. cgic now compiles with no warnings under the compilers available at boutell.com.

What's new in version 1.01?

• cgiFormInteger, cgiFormIntegerBounded, cgiFormDouble and cgiFormDoubleBounded now accept negative numbers properly. They also accept positive numbers with an explicit + sign.
• Hex values containing the digit 9 are now properly decoded.
• cgiFormString now represents each newline as a single line feed (ascii 10 decimal) as described in the documentation, not a carriage return (ascii 13 decimal) as in version 1.0. The latter approach pleased no one.
• cgiFormString and cgiFormStringNoNewlines no longer erroneously return cgiFormEmpty in place of cgiFormSuccess.
• The main() function of cgic now flushes standard output and sleeps for one second before exiting in order to inhibit problems with the completion of I/O on some platforms. This was not a cgic bug per se, but has been reported as a common problem with CGI when used with the CERN server. This change should improve compatibility.
• The single selection example in the testform.html example now works properly. This was an error in the form itself, not cgic.
• cgiRemoteUser and cgiRemoteIdent are now documented accurately. They were reversed earlier.

What is cgic?

cgic performs the following tasks:

• Parses form data, correcting for defective and/or inconsistent browsers
• Transparently accepts both GET and POST form data
• Handles line breaks in form fields in a consistent manner
• Provides string, integer, floating-point, and single- and multiple-choice functions to retrieve form data
• Provides bounds checking for numeric fields
• Loads CGI environment variables into C strings which are always non-null
• Provides a way to capture CGI situations for replay in a debugging environment
• Provides a somewhat safer form of the system() function

cgic should be compatible with any CGI-compliant server environment.

Obtaining cgic

Important: to use cgic, you will need an ANSI-standard C compiler. The Sun cc distributed with SunOS 4.1.3 is not ANSI-standard. Unix users may wish to obtain gcc, which is free and widely available, or purchase Sun's development package, which also includes a proper compiler. Users of Windows-related operating systems should not have ANSI C-related problems as all of the popular compilers follow the ANSI standard.

Note for Windows Programmers: cgic should work in a 16-bit environment but is not designed to cater to such an environment. Form fields which require more than 64K individually will not work as expected unless the huge memory model is used. Using a 32-bit compiler is strongly recommended.

Your web browser should inquire whether to save the file to disk when you select one of the links below. Under Unix and compatible operating systems, save it, then issue the following commands to unpack it:

uncompress cgic105.tar.Z
tar -xf cgic105.tar

Under Windows and compatible operating systems, save it, open a DOS window, and issue the following commands to unpack it:

pkunzip /d cgic105.zip

cgic is available via the web from www.boutell.com:

• Obtain cgic: compressed tar file
• Obtain cgic: .ZIP file

Building cgic: a sample application

On a Unix system, you can build cgictest simply by typing 'make cgictest'. cgic.c and cgictest.c will be compiled and linked together to produce the cgictest application. Under non-Unix operating systems, you will need to create and compile an appropriate project containing the files cgic.c and cgictest.c.

IMPORTANT: after compiling cgictest, you will need to place it in a location on your server system which is designated by your server administrator as an appropriate location for CGI scripts. Also, the URL of the action of the sample form in testform.html must be changed to correctly indicate the location of cgictest on your web server. The right locations for CGI programs vary greatly from one server to another. Resolving this issue is between you, your web server administrator, and your web server documentation. Before submitting a bug report for cgic, make certain that the CGI example programs which came with your server do work for you. Otherwise it is very likely that you have a server configuration problem.

Once you have moved cgictest to an appropriate cgi directory and edited form.html to properly refer to its location, use the web browser of your choice to access form.html. Fill out the various fields in any manner you wish, then select the SUBMIT button.

If all goes well, cgictest will respond with a page which indicates the various settings you submitted. If not, please see the second paragraph above.

What to do if it won't compile

• Make sure you are using an ANSI C or C++ compiler.
• If your compiler can't find the #include file unistd.h, define the preprocessor macro NO_UNISTD and recompile.
• If your compiler can't find the function system(), define the preprocessor macro NO_SYSTEM and recompile.
• If your compiler can't find the function sleep(), remove the call to that function from the main() function of cgic.c and recompile.

How to write a cgic application

Since all CGI applications must perform certain initial tasks, such as parsing form data and examining environment variables, the cgic library provides its own main() function. When you write applications that use cgic, you will begin your own programs by writing a cgiMain() function, which cgic will invoke when the initial cgi work has been successfully completed. Your program must also be sure to #include the file cgic.h.

Important: if you write your own main() function, your program will not link properly. Your own code should begin with cgiMain(). The library provides main() for you.

Consider the cgiMain function of cgictest.c:

int cgiMain() {
#if DEBUG
        /* Load a saved CGI scenario if we're debugging */
        cgiReadEnvironment("/path/to/capcgi.dat");
#endif
	/* Important: we must indicate the type of document */
        cgiHeaderContentType("text/html");
	/* Now invoke other functions to handle each part of the form */
        fprintf(cgiOut, "<HTML><HEAD>\n");                        
        fprintf(cgiOut, "<TITLE>cgic test</TITLE></HEAD>\n"):
        fprintf(cgiOut, "<BODY><H1>cgic test</H1>\n");
        Name();
        Address();
        Hungry();
        Temperature();
        Frogs();
        Color();
        Flavors();
        NonExButtons();
        RadioButtons();
        fprintf(cgiOut, "</BODY></HTML>\n");
	/* This value will be the exit code of the program; 0 
		generally indicates success among Unix and DOS programs */ 
        return 0;
}

Outputting the Header

Note that cgiHeaderStatus() or cgiHeaderLocation() could have been invoked instead to output an error code or redirect the request to a different URL. Only one of the cgiHeader functions should be called in a single execution of the program.

Important: one of the cgiHeader functions, usually cgiHeaderContentType(), must be invoked before outputting any other response to the user. Otherwise, the result will not be a valid document and the browser's behavior will be unpredictable. You may, of course, output your own ContentType and other header information to cgiOut if you prefer. The cgiHeader functions are provided as a convenience.

Next, cgiMain() invokes various functions to handle individual parts of the form. When the function is finished, it returns 0, the usual return code for a successful program.

Handling Text Input

void Name() {
        char name[81];
        cgiFormStringNoNewlines("name", name, 81);
        fprintf(cgiOut, "Name: %s<BR>\n", name);
}

Handling Output

Important: cgiOut is normally equivalent to stdout, and there is no performance penalty for using it. It is recommended that you write output to cgiOut to ensure compatibility with future versions of the cgic library for special environments that do not provide stdin and stdout for each cgi connection.

Note that, for text input areas in which carriage returns are desired, the function cgiFormString should be used instead. cgiFormString ensures that line breaks are always represented by a single carriage return (ascii decimal 13), making life easier for the programmer. See the source code to the Address() function of cgictest.c for an example.

Handling Single Checkboxes

void Hungry() {
        if (cgiFormCheckboxSingle("hungry") == cgiFormSuccess) {
                fprintf(cgiOut, "I'm Hungry!<BR>\n");
        } else {
                fprintf(cgiOut, "I'm Not Hungry!<BR>\n");
        }
}

Handling Numeric Input

void Temperature() {
        double temperature;
        cgiFormDoubleBounded("temperature", &temperature, 80.0, 120.0, 98.6);
        fprintf(cgiOut, "My temperature is %f.<BR>\n", temperature);
}

This function always retrieves a reasonable value within the specified bounds; values above or below bounds are constrained to fit the bounds. However, the return value of cgiFormDoubleBounded can be checked to make sure the actual user entry was in bounds, not blank, and so forth; see the description of cgiFormDoubleBounded() for more details. If bounds checking is not desired, consider using cgiFormDouble() instead.

Note that, for integer input, the functions cgiFormInteger and cgiFormIntegerBounded are available. The behavior of these functions is similar to that of their floating-point counterparts above.

Handling Single-Choice Input

char *colors[] = {
        "Red",
        "Green",
        "Blue"
};

void Color() {
        int colorChoice;
        cgiFormSelectSingle("colors", colors, 3, &colorChoice, 0);
        fprintf(cgiOut, "I am: %s<BR>\n", colors[colorChoice]);
}

cgiFormSelectSingle() will always indicate a reasonable selection value. However, if the programmer wishes to know for certain that a value was actually submitted, that the value submitted was a legal response, and so on, the return value of cgiFormSelectSingle() can be consulted. See the full description of cgiFormSelectSingle() for more information.

Note that radio button groups and <SELECT> lists can both be handled by this function. If you are processing radio button groups, you may prefer to invoke cgiFormRadio(), which functions identically.

"What if I won't know the acceptable choices at runtime?"

If the acceptable choices aren't known until runtime, one can simply load the choices from disk. But if the acceptable choices aren't fixed at all (consider a list of country names; new names may be added to the form at any time and it is inconvenient to also update program code or a separate list of countries), simply invoke cgiFormStringNoNewlines() instead to retrieve the string directly. Keep in mind that, if you do so, validating the response to make sure it is safe and legitimate becomes a problem for your own program to solve. The advantage of cgiFormSelectSingle() is that invalid responses are never returned.

To handle multiple-selection <SELECT> lists and groups of checkboxes with the same name, see the discussion of the NonExButtons() function of cgictest.c, immediately below.

Handling Multiple-Choice Input

char *votes[] = {
	"A",
	"B",
	"C",
	"D"
};

void NonExButtons() {
	int voteChoices[4];
	int i;
	int result;	
	int invalid;

	char **responses;

	/* Method #1: check for valid votes. This is a good idea,
		since votes for nonexistent candidates should probably
		be discounted... */
	fprintf(cgiOut, "Votes (method 1):<BR>\n");
	result = cgiFormCheckboxMultiple("vote", votes, 4, 
		voteChoices, &invalid);
	if (result == cgiFormNotFound) {
		fprintf(cgiOut, "I hate them all!<p>\n");
	} else {	
		fprintf(cgiOut, "My preferred candidates are:\n");
		fprintf(cgiOut, "<ul>\n");
		for (i=0; (i < 4); i++) {
			if (voteChoices[i]) {
				fprintf(cgiOut, "<li>%s\n", votes[i]);
			}
		}
		fprintf(cgiOut, "</ul>\n");
	}

The first argument to cgiFormCheckboxMultiple() is the name given to all checkbox input fields in the group. The second argument points to an array of legitimate values; these should correspond to the VALUE attributes of the checkboxes (or OPTION tags in a <SELECT> list). The third argument indicates the number of entries in the array of legitimate values. The fourth argument points to an array of integers with the same number of entries as the array of legitimate values; each entry will be set true if that checkbox or option was selected, false otherwise.

The last argument points to an integer which will be set to the number of invalid responses (responses not in the array of valid responses) that were submitted. If this value is not of interest, the last argument may be a null pointer (0).

Note that the return value of cgiFormCheckboxMultiple is inspected to determine whether any choices at all were set. See the full description of cgiFormCheckboxMultiple for other possible return values.

"What if I won't know the acceptable choices at runtime?"

If the acceptable choices aren't known until runtime, one can simply load the choices from disk. But if the acceptable choices aren't fixed at all (consider a list of ice cream flavors; new names may be added to the form at any time and it is inconvenient to also update program code or a separate list of countries), a more dynamic approach is needed. Consider the second half of the NonExButtons() function of cgictest.c:

	/* Method #2: get all the names voted for and trust them.
		This is good if the form will change more often
		than the code and invented responses are not a danger
		or can be checked in some other way. */
	fprintf(cgiOut, "Votes (method 2):<BR>\n");
	result = cgiFormStringMultiple("vote", &responses);
	if (result == cgiFormNotFound) {	
		fprintf(cgiOut, "I hate them all!<p>\n");
	} else {
		int i = 0;
		fprintf(cgiOut, "My preferred candidates are:\n");
		fprintf(cgiOut, "<ul>\n");
		while (responses[i]) {
			fprintf(cgiOut, "<li>%s\n", responses[i]);
			i++;
		}
		fprintf(cgiOut, "</ul>\n");
	}
	/* We must be sure to free the string array or a memory
		leak will occur. Simply calling free() would free
		the array but not the individual strings. The
		function cgiStringArrayFree() does the job completely. */	
	cgiStringArrayFree(responses);
}

The first argument to cgiFormStringMultiple() is the name of the input field or group of input fields in question. The second argument should be the address of a pointer to a pointer to a string, which isn't as bad as it sounds. Consider the following simple call of the function:

/* An array of strings; each C string is an array of characters */
char **responses; 

cgiFormStringMultiple("vote", &responses);

After the call, the last entry in the string array will be a null pointer. Thus the simple loop:

int i = 0;
while (responses[i]) {
	/* Do something with the string responses[i] */
	i++;
}

Important: unlike other functions in the cgic library, the cgiFormStringMultiple function returns a pointer to allocated memory. Your code should not modify the strings in the responses array or the responses array itself; if modification is needed, the strings should be copied. When your code is done examining the responses array, you MUST call cgiStringArrayFree() with the array as an argument to free the memory associated with the array. Otherwise, the memory may be lost permanently (under some operating systems), or will not be available again until the program exists. Don't just call the free() function; if you do, the individual strings will not be freed.

Examining CGI environment variables

Instead of calling getenv() to determine the value of a variable such as HTTP_USER_AGENT (the browser software being used), always use the cgic copies of the environment variables, which are always valid C strings (they are never null, although they may point to an empty string). For instance, the cgic variable containing the name of the browser software is cgiUserAgent.

How can I generate images from my cgic application?

The following short sample program hints at the possibilities:

#include "cgic.h"
#include "gd.h"

char *colors[] = {
	"red", "green", "blue"
};

#define colorsTotal 3

int cgiMain() {
	int colorChosen;
	gdImagePtr im;
	int r, g, b;
	/* Use gd to create an image */
	im = gdImageCreate(64, 64);
	r = gdImageColorAllocate(im, 255, 0, 0);	
	g = gdImageColorAllocate(im, 0, 255, 0);	
	b = gdImageColorAllocate(im, 0, 0, 255);	
	/* Now use cgic to find out what color the user requested */
	cgiFormSelectSingle("color", 3, &colorChosen, 0);	
	/* Now fill with the desired color */
	switch(colorChosen) {
		case 0:
		gdImageFill(im, 32, 32, r);
		break;
		case 1:
		gdImageFill(im, 32, 32, g);
		break;
		case 2:
		gdImageFill(im, 32, 32, b);
		break;
	}	
	/* Now output the image. Note the content type! */
	cgiHeaderContentType("image/gif");
	/* Send the image to cgiOut */
	gdImageGif(im, cgiOut);
	/* Free the gd image */
	gdImageDestroy(im);
	return 0;
}

Debugging CGI applications: using capture

The provided program 'capture.c' can be used to capture CGI environments. Just change the first line of the cgiMain() function of capture.c to save the CGI environment to a filename appropriate on your system and type 'make capture'. Then place capture in your cgi directory and set the form action or other link you want to test to point to it. When the form submission or other link takes place, capture will write the CGI environment active at that time to the filename you specified in the source. The cgiReadEnvironment() function can then be invoked on the same filename at the beginning of the cgiMain() function of the application you want to test in order to restore the captured environment. You can then execute your program in the debugger of your choice, and it should perform exactly as it would have performed had it been launched by the actual web server.

Important: Make sure you specify the full path, as the the current working directory of a CGI script may not be what you think it is.

Even More Important: If you call getenv() yourself in your code, instead of using the provided cgic copies of the CGI environment variables, you will not get the values you expect when running with a saved CGI environment. Always use the cgic variables instead of calling getenv().

cgic function reference

cgiFormResultType cgiFormString( char *name, char *result, int max)

cgiFormString attempts to retrieve the string sent for the specified input field. The text will be copied into the buffer specified by result, up to but not exceeding max-1 bytes; a terminating null is then added to complete the string. Regardless of the newline format submitted by the browser, cgiFormString always encodes each newline as a single line feed (ascii decimal 10); as a result the final string may be slightly shorter than indicated by a call to cgiFormStringSpaceNeeded but will never be longer. cgiFormString returns cgiFormSuccess if the string was successfully retrieved, cgiFormTruncated if the string was retrieved but was truncated to fit the buffer, cgiFormEmpty if the string was retrieved but was empty, cgiFormLong if the string was retrieved but was truncated to fit into the buffer, and cgiFormNotFound if no such input field was submitted. In the last case, an empty string is copied to result.

cgiFormResultType cgiFormStringNoNewlines( char *name, char *result, int max)

cgiFormStringNoNewlines() is exactly equivalent to cgiFormString(), except that any carriage returns or line feeds that occur in the input will be stripped out. The use of this function is recommended for single-line text input fields, as some browsers will submit carriage returns and line feeds when they should not.

cgiFormResultType cgiFormStringSpaceNeeded( char *name, int *length)

cgiFormStringSpaceNeeded() is used to determine the length of the input text buffer needed to receive the contents of the specified input field. This is useful if the programmer wishes to allocate sufficient memory for input of arbitrary length. The actual length of the string retrieved by a subsequent call to cgiFormString() may be slightly shorter but will never be longer than *result. On success, cgiFormStringSpaceNeeded() sets the value pointed to by length to the number of bytes of data, including the terminating null, and returns cgiFormSuccess. If no value was submitted for the specified field, cgiFormStringSpaceNeeded sets the value pointed to by length to 1 and returns cgiFormNotFound. 1 is set to ensure space for an empty string (a single null character) if cgiFormString is called despite the return value.

cgiFormResultType cgiFormStringMultiple( char *name, char ***ptrToStringArray)

cgiFormStringMultiple is useful in the unusual case in which several input elements in the form have the same name and, for whatever reason, the programmer does not wish to use the checkbox, radio button and selection menu functions provided below. This is occasionally needed if the programmer cannot know in advance what values might appear in a multiple-selection list or group of checkboxes on a form. The value pointed to by result will be set to a pointer to an array of strings; the last entry in the array will be a null pointer. This array is allocated by the CGI library. Important: when done working with the array, you must call cgiStringArrayFree() with the array pointer as the argument. cgiFormStringMultiple() returns cgiFormSuccess if at least one occurrence of the name is found, cgiFormNotFound if no occurrences are found, or cgiFormMemory if not enough memory is available to allocate the array to be returned. In all cases except the last, ptrToStringArray is set to point to a valid array of strings, with the last element in the array being a null pointer; in the out-of-memory case ptrToStringArray is set to a null pointer.

void cgiStringArrayFree(char **stringArray)

cgiStringArrayFree() is used to free the memory associated with a string array created by cgiFormStringMultiple().

cgiFormResultType cgiFormInteger( char *name, int *result, int defaultV)

cgiFormInteger() attempts to retrieve the integer sent for the specified input field. The value pointed to by result will be set to the value submitted. cgiFormInteger() returns cgiFormSuccess if the value was successfully retrieved, cgiFormEmpty if the value submitted is an empty string, cgiFormBadType if the value submitted is not an integer, and cgiFormNotFound if no such input field was submitted. In the last three cases, the value pointed to by result is set to the specified default.

cgiFormResultType cgiFormIntegerBounded( char *name, int *result, int min, int max, int defaultV)

cgiFormIntegerBounded() attempts to retrieve the integer sent for the specified input field, and constrains the result to be within the specified bounds. The value pointed to by result will be set to the value submitted. cgiFormIntegerBounded() returns cgiFormSuccess if the value was successfully retrieved, cgiFormConstrained if the value was out of bounds and result was adjusted accordingly, cgiFormEmpty if the value submitted is an empty string, cgiFormBadType if the value submitted is not an integer, and cgiFormNotFound if no such input field was submitted. In the last three cases, the value pointed to by result is set to the specified default.

cgiFormResultType cgiFormDouble( char *name, double *result, double defaultV)

cgiFormDouble attempts to retrieve the floating-point value sent for the specified input field. The value pointed to by result will be set to the value submitted. cgiFormDouble returns cgiFormSuccess if the value was successfully retrieved, cgiFormEmpty if the value submitted is an empty string, cgiFormBadType if the value submitted is not a number, and cgiFormNotFound if no such input field was submitted. In the last three cases, the value pointed to by result is set to the specified default.

cgiFormResultType cgiFormDoubleBounded( char *name, double *result, double min, double max, double defaultV)

cgiFormDoubleBounded() attempts to retrieve the floating-point value sent for the specified input field, and constrains the result to be within the specified bounds. The value pointed to by result will be set to the value submitted. cgiFormDoubleBounded() returns cgiFormSuccess if the value was successfully retrieved, cgiFormConstrained if the value was out of bounds and result was adjusted accordingly, cgiFormEmpty if the value submitted is an empty string, cgiFormBadType if the value submitted is not a number, and cgiFormNotFound if no such input field was submitted. In the last three cases, the value pointed to by result is set to the specified default.

cgiFormResultType cgiFormSelectSingle( char *name, char **choicesText, int choicesTotal, int *result, int defaultV)

cgiFormSelectSingle() retrieves the selection number associated with a <SELECT> element that does not allow multiple selections. name should identify the NAME attribute of the <SELECT> element. choicesText should point to an array of strings identifying each choice; choicesTotal should indicate the total number of choices. The value pointed to by result will be set to the position of the actual choice selected within the choicesText array, if any, or to the value of default, if no selection was submitted or an invalid selection was made. cgiFormSelectSingle() returns cgiFormSuccess if the value was successfully retrieved, cgiFormNotFound if no selection was submitted, and cgiFormNoSuchChoice if the selection does not match any of the possibilities in the choicesText array.

cgiFormResultType cgiFormSelectMultiple( char *name, char **choicesText, int choicesTotal, int *result, int *invalid)

cgiFormSelectMultiple() retrieves the selection numbers associated with a <SELECT> element that does allow multiple selections. name should identify the NAME attribute of the <SELECT> element. choicesText should point to an array of strings identifying each choice; choicesTotal should indicate the total number of choices. result should point to an array of integers with as many elements as there are strings in the choicesText array. For each choice in the choicesText array that is selected, the corresponding integer in the result array will be set to one; other entries in the result array will be set to zero. cgiFormSelectMultiple() returns cgiFormSuccess if at least one valid selection was successfully retrieved or cgiFormNotFound if no valid selections were submitted. The integer pointed to by invalid is set to the number of invalid selections that were submitted, which should be zero unless the form and the choicesText array do not agree.

cgiFormResultType cgiFormCheckboxSingle( char *name)

cgiFormCheckboxSingle determines whether the checkbox with the specified name is checked. cgiFormCheckboxSingle returns cgiFormSuccess if the button is checked, cgiFormNotFound if the checkbox is not checked. cgiFormCheckboxSingle is intended for single checkboxes with a unique name; see below for functions to deal with multiple checkboxes with the same name, and with radio buttons.

cgiFormResultType cgiFormCheckboxMultiple( char *name, char **valuesText, int valuesTotal, int *result, int *invalid)

cgiFormCheckboxMultiple() determines which checkboxes among a group of checkboxes with the same name are checked. This is distinct from radio buttons (see cgiFormRadio). valuesText should point to an array of strings identifying the VALUE attribute of each checkbox; valuesTotal should indicate the total number of checkboxes. result should point to an array of integers with as many elements as there are strings in the valuesText array. For each choice in the valuesText array that is selected, the corresponding integer in the result array will be set to one; other entries in the result array will be set to zero. cgiFormCheckboxMultiple returns cgiFormSuccess if at least one valid checkbox was checked or cgiFormNotFound if no valid checkboxes were checked. The integer pointed to by invalid is set to the number of invalid selections that were submitted, which should be zero unless the form and the valuesText array do not agree.

cgiFormResultType cgiFormRadio( char *name, char **valuesText, int valuesTotal, int *result, int defaultV)

cgiFormRadio() determines which, if any, of a group of radio boxes with the same name was selected. valuesText should point to an array of strings identifying the VALUE attribute of each radio box; valuesTotal should indicate the total number of radio boxes. The value pointed to by result will be set to the position of the actual choice selected within the valuesText array, if any, or to the value of default, if no radio box was checked or an invalid selection was made. cgiFormRadio() returns cgiFormSuccess if a checked radio box was found in the group, cgiFormNotFound if no box was checked, and cgiFormNoSuchChoice if the radio box submitted does not match any of the possibilities in the valuesText array. /* Header output functions. Call the first to specify a new URL if the document request should be redirected. Call the second if you wish to respond to a request with an HTTP error status code and message; see the HTTP documentation for the legal codes. Call the third in the normal case, in order to specify the mime type of the document (such as "text/html"); you may then output the actual document directly to cgiOut. */

void cgiHeaderLocation(char *redirectUrl)

cgiHeaderLocation() should be called if the programmer wishes to redirect the user to a different URL. No futher output is needed in this case. Only one of the cgiHeader functions (cgiHeaderLocation, cgiHeaderStatus and cgiHeaderContentType) should be invoked for each CGI transaction.

void cgiHeaderStatus(int status, char *statusMessage) cgiHeaderStatus() should be called if the programmer wishes to output an HTTP error status code instead of a document. The status code is the first argument; the second argument is the status message to be displayed to the user. Only one of the cgiHeader functions (cgiHeaderLocation, cgiHeaderStatus and cgiHeaderContentType) should be invoked for each CGI transaction.

void cgiHeaderContentType(char *mimeType)

cgiHeaderContentType() should be called if the programmer wishes to output a new document in response to the user's request. This is the normal case. The single argument is the MIME document type of the response; typical values are "text/html" for HTML documents, "text/plain" for plain ASCII without HTML tags, "image/gif" for a GIF image and "audio/basic" for .au-format audio. Only one of the cgiHeader functions (cgiHeaderLocation, cgiHeaderStatus and cgiHeaderContentType) should be invoked for each CGI transaction.

int cgiSaferSystem(char *command)

cgiSaferSystem() is a convenience function used to invoke the system() function less dangerously. That is, cgiSaferSystem() "escapes" the shell metacharacters ";" and "|", which can otherwise cause other programs to be invoked beyond the one intended by the programmer. However, understanding the shell commands you invoke and ensuring that you do not invoke the shell in ways that permit the web user to run arbitrary programs is your own responsibility.

cgiEnvironmentResultType cgiWriteEnvironment(char *filename)

cgiWriteEnvironment() can be used to write the entire CGI environment, including form data, to the specified output file; cgiReadEnvironment() can then be used to restore that environment from the specified input file for debugging. Of course, these will only work as expected if you use the cgic copies of the CGI environment variables and cgiIn and cgiOut rather than stdin and stdout (also see above). These functions are useful in order to capture real CGI situations while the web server is running, then recreate them in a debugging environment. Both functions return cgiEnvironmentSuccess on success, cgiEnvironmentIO on an I/O error, and cgiEnvironmentMemory on an out-of-memory error.

cgiEnvironmentResultType cgiReadEnvironment(char *filename)

cgiReadEnvironment() restores a CGI environment saved to the specified file by cgiWriteEnvironment(). Of course, these will only work as expected if you use the cgic copies of the CGI environment variables and cgiIn and cgiOut rather than stdin and stdout (also see above). These functions are useful in order to capture real CGI situations while the web server is running, then recreate them in a debugging environment. Both functions return cgiEnvironmentSuccess on success, cgiEnvironmentIO on an I/O error, and cgiEnvironmentMemory on an out-of-memory error.

int cgiMain()

The programmer must write this function, which performs the unique task of the program and is invoked by the true main() function, found in the cgic library itself. The return value from cgiMain will be the return value of the program. It is expected that the user will make numerous calls to the cgiForm functions from within this function. See how to write a cgic application for details.

cgic variable reference

Most of these variables are equivalent to various CGI environment variables. The most important difference is that the cgic environment string variables are never null pointers. They will always point to valid C strings of zero or more characters.

char *cgiServerSoftware

Points to the name of the server software, or to an empty string if unknown.

char *cgiServerName

Points to the name of the server, or to an empty string if unknown.

char *cgiGatewayInterface

Points to the name of the gateway interface (usually CGI/1.1), or to an empty string if unknown.

char *cgiServerProtocol

Points to the protocol in use (usually HTTP/1.0), or to an empty string if unknown.

char *cgiServerPort

Points to the port number on which the server is listening for HTTP connections (usually 80), or an empty string if unknown.

char *cgiRequestMethod

Points to the method used in the request (usually GET or POST), or an empty string if unknown (this should not happen).

char *cgiPathInfo

Most web servers recognize any additional path information in the URL of the request beyond the name of the CGI program itself and pass that information on to the program. cgiPathInfo points to this additional path information.

char *cgiPathTranslated

Most web servers recognize any additional path information in the URL of the request beyond the name of the CGI program itself and pass that information on to the program. cgiPathTranslated points to this additional path information, translated by the server into a filesystem path on the local server.

char *cgiScriptName

Points to the name under which the program was invoked.

char *cgiQueryString

Contains any query information submitted by the user as a result of a GET-method form or an <ISINDEX> tag. Note that this information need not be parsed directly unless an <ISINDEX> tag was used; normally it is parsed automatically by the cgic library. Use the cgiForm family of functions to retrieve the values associated with form input fields. See how to write a cgic application for more information.

char *cgiRemoteHost

Points to the fully resolved hostname of the browser, if known, or an empty string if unknown.

char *cgiRemoteAddr

Points to the dotted-decimal IP address of the browser, if known, or an empty string if unknown.

char *cgiAuthType

Points to the type of authorization used for the request, if any, or an empty string if none or unknown.

char *cgiRemoteUser

Points to the user name under which the user has authenticated; an empty string if no authentication has taken place. The certainty of this information depends on the type of authorization in use; see cgiAuthType.

char *cgiRemoteIdent

Points to the user name volunteered by the user via the user identification protocol; an empty string if unknown. This information is not secure. Identification demons can be installed by users on insecure systems such as Windows machines.

char *cgiContentType

Points to the MIME content type of the information submitted by the user, if any; an empty string if no information was submitted. If this string is equal to application/x-www-form-urlencoded, the cgic library will automatically examine the form data submitted. If this string has any other non-empty value, a different type of data has been submitted. This is currently very rare, as most browsers can only submit forms, but if it is of interest to your application, the submitted data can be read from the cgiIn file pointer.

char *cgiAccept

Points to a space-separated list of MIME content types acceptable to the browser (see cgiHeaderContentType() ), or an empty string. Unfortunately, this variable is not supplied by most current browsers. Programmers wishing to make decisions based on the capabilities of the browser are advised to check the cgiUserAgent variable against a list of browsers and capabilities instead.

char *cgiUserAgent

Points to the name of the browser in use, or an empty string if this information is not available.

int cgiContentLength

The number of bytes of form or query data received. Note that if the submission is a form or query submission the library will read and parse all the information directly from cgiIn and/or cgiQueryString. The programmer should not do so, and indeed the cgiIn pointer will be at end-of-file in such cases.

FILE *cgiOut

Pointer to CGI output. The cgiHeader functions, such as cgiHeaderContentType, should be used first to output the mime headers; the output HTML page, GIF image or other web document should then be written to cgiOut by the programmer using standard C I/O functions such as fprintf() and fwrite(). cgiOut is normally equivalent to stdout; however, it is recommended that cgiOut be used to ensure compatibility with future versions of cgic for specialized environments.

FILE *cgiIn

Pointer to CGI input. In 99% of cases, you will not need this. However, in future applications, documents other than form data are posted to the server, in which case this file pointer may be read from in order to retrieve the contents.

cgic result code reference

In most cases, cgic functions are designed to produce reasonable results even when browsers and users do unreasonable things. However, it is sometimes important to know precisely which unreasonable things took place, especially when assigning a default value or bounding a value is an inadequate solution. The following result codes are useful in making this determination.

cgiFormSuccess

Indicates that the function successfully performed at least one action (or retrieved at least one value, where applicable).

cgiFormTruncated

Indicates that a string value retrieved from the user was cut short to avoid overwriting the end of a buffer.

cgiFormBadType

Indicates that a "numeric" value submitted by the user was in fact not a legal number.

cgiFormEmpty

Indicates that a field was retrieved but contained no data.

cgiFormNotFound

Indicates that no value was submitted for a particular field.

cgiFormConstrained

Indicates that a numeric value was beyond the specified bounds and was forced to the lower or upper bound as appropriate.

cgiFormNoSuchChoice

Indicates that the value submitted for a single-choice field (such as a radio-button group) was not one of the acceptable values. This usually indicates a discrepancy between the form and the program.

cgiEnvironmentIO

Indicates that an attempt to read or write the CGI environment to or from a capture file failed due to an I/O error.

cgiEnvironmentMemory

Indicates that an attempt to read or write the CGI environment to or from a capture file failed due to an out-of-memory error.

cgiEnvironmentSuccess

Indicates that an attempt to read or write the CGI environment to or from a capture file was successful.

cgic quick index