I think it’ll be quite accurate to say that an average programmer like me is daunted when first faced with the concept of pointers and memory management in C. During the initial programming years (much of which inevitably had to be in C), I tried my best to avoid using pointers in my code. Whether it be using character arrays with pre-defined size (who cares if it takes much more memory than is needed) or some other “nifty” trick… I thought I could get away as long as I can compile and run just “this” program. But I had to face it during a networking class in school when I worked on a peer-to-peer file sharing project and one of my classmates convinced me to “do it right” and pay heed to the requirement of the program being able to work with other students’ code.
So after much head-scratching and soul-searching, I begrudgingly revisited the concepts and began (or so I thought) to grasp the idea of address spaces. l-values vs. r-values and pointers. It all seemed to make sense and fit in place like a jigsaw puzzle finally coming together. And not before long, with some encouragement and confidence building, I rewrote the program using pointers instead of pre-allocated char arrays and making it as compatible as possible. But if it was supposed to make me happy and content, it didn’t last long and whatever feeling of competence I felt was shattered to bits when I compiled the program for the first time and got compile time errors that didn’t make any sense at all. It was even worse and torturous when after a few cycles of modifying and debugging, it finally compiled beautifully (oh the elation…) and then the first time I ran it, it threw a slew of memory violation errors and crashed as beautifully as it compiled.
Years have passed since then…some of which were spent in writing code in other languages (it doesn’t have pointers? I’ll take 10). And some others in trying to come to terms with how even after understanding the concepts if I now have to write a C program and I decide to (or have to) use pointers, the error messages still baffle the heck out of me. Brief moments of competence have existed…when after writing, re-writing, debugging, debugging again…debugging a few more times, I was finally able to turn out a decent piece of code that could accomplish what it was supposed to in a relatively efficient manner.
Background:
So how does all this relate to the subject of this post? A few days ago, I was writing a LoadRunner script for a web application which had an inquiry page that submitted a request. Depending on the data submitted, the request returned either a response page with the final result or a set of intermediate questions that needed to be answered. After submitting the answers, it again returned either another set of questions or the final result. Once the answers were submitted the second time, it returned the final result page. The number of questions returned was not constant, however 8 questions was the maximum. So a part of scenario logic was this:
a. Submit the initial inquiry
b. If Question Set A is returned, determine the number of questions, construct an answer string and submit
c. If Question Set B is returned, determine the number of questions, construct an answer string and submit
d. Final response
Problem:
Since the questions returned were in a select box, it was easy to find the left and right boundaries and use web_reg_save_param with "Ord=All". In this case, since the questions were in the form:
<SELECT NAME="Answer2" SIZE="5">
<OPTION value="0"> ABC</OPTION><br>
<OPTION value="1"> DEF</OPTION><br>
<OPTION value="2"> MTG</OPTION><br>
<OPTION value="3"> SVG</OPTION><br>
<OPTION value="4">NONE</OPTION><br>
</SELECT>
it will be:
web_reg_save_param ("suffix", "LB=<SELECT NAME=\"Answer", "RB=\" SIZE",
"Ord=All","NOTFOUND=Warning", LAST); However, it wasn't as easy as determining the number of matches from {suffix_count} and looping to create a custom answer string. Each question had a relative order from 1 – 8 like the one above (the numeral after “Answer”) and the answer string had to be constructed based on that. The challenge was that the number of questions was variable and the order didn’t always start with 1 and go 1, 2, 3…and so on. So if 4 questions were returned, they could be labeled Answer2, Answer3, Answer5, Answer6 and based on this, the answer string would be answer2=2&answer3=2&answer5=2&answer6=2 (it didn’t matter if the questions were answered correctly, so a constant 2 would do). If I submitted answer1=2… here, it would return an error saying “Invalid Response” or something like that.
Solution:
So what I had to do was to save that suffix number in a temp variable and construct the answer string by concatenating it together. So, something like:
c = atoi(lr_eval_string("{suffix_count}"));
if(c>0){
for(i=1;i<=c;i++){
strcat(answerString, "Answer");
sprintf(sfx, "{suffix_%d}", i);
strcat(answerString, lr_eval_string(sfx));
strcat(answerString, "=2&");
}
lr_save_string(answerString, "aString");
... And then I would create a web_custom_request and submit it:
web_custom_request("AnswerSetA",
…
…
…
"Body={aString}submit1.x=61&submit1.y=27",
"LAST"); My first instinct, as I’ve mentioned earlier was to use character arrays for both answerString and sfx.
char answerString[256], sfx[10];
I figured that the chances of the string being longer than 256 chars was remote so I was safe. And it worked fine when I ran it in VuGen. But when I ran the load scenario, all the users belonging to this script’s group failed exactly after 4th or 5th iteration. The error was something I hadn’t seen before:
Action.c(162): Error (-17991): Failed to add item to mfifo data structure.
on the line with lr_eval_string. I searched online and came across this link (http://www.sqaforums.com/showflat.php?Number=420958) which suggested using lr_eval_string_ext instead of lr_eval_string to free memory earlier. The help on lr_eval_string also mentions:
Note: lr_eval_string allocates memory internally. The memory is freed at the end of each iteration. If you evaluate a parameter or parameters in a loop, conserve memory by not using lr_eval_string . Instead, use lr_eval_string_ext and free the memory in each loop iteration with lr_eval_string_ext_free.
I changed the code to:
c = atoi(lr_eval_string("{suffix_count}"));
if(c>0){
for(i=1;i<=c;i++){
strcat(answerString, "Answer");
sprintf(sfx, "{suffix_%d}", i);
lr_eval_string_ext(sfx,strlen(sfx), &sfx1, &prmLen, 0, 0, -1);
strcat(answerString, sfx1);
strcat(answerString, "=2&");
lr_eval_string_ext_free(&sfx1);
}
lr_save_string(answerString, "aString");
... but to my disappointment, it still threw an error when executing in Controller after 4th iteration. The good part was that the error message was familiar, the bad part was that it was a memory violation exception:
Action.c(168): Error: C interpreter run time error: Action.c (168): Error -- memory violation : Exception ACCESS_VIOLATION
For some reason, I felt that all the years of avoiding or trying to avoid using pointers obligated me to get to the root of the issue this time and fix it instead of coming up with a workaround. The actual issue however turned out to be something else and I’ll come back to it later. First order of the day…use char pointers instead of static arrays to manage the strings.
To start:
char *answerString; //instead of char answerString[256]
char sfx[10]; //this still can be an array
Next step was to figure out how much memory will I need to allocate based on the number of questions returned and then allocate it. This was done using malloc:
//we need AnswerX=2& times the number of questions, + 1 for '\0'
if((answerString = (char *)malloc(c * 10 * sizeof(char) + 1)) == NULL){
lr_output_message("Insufficient Memory!!");
return -1;
} Also, we have to initialize it because there may be some garbage in the allocated space that may hinder the proper functioning of strcat:
Now we have something very similar to a brand new char array, but only of the exact size that we need. Next, we create the string exactly as above, and I used lr_eval_string instead of lr_eval_string_ext because I honestly didn’t think that was the issue. After creating the string, I null-terminated it.
for(i=1;i<=c;i++){
strcat(answerString, "Answer");
sprintf(sfx, "{suffix_%d}", i);
strcat(answerString, lr_eval_string(sfx));
strcat(answerString, "=2&");
}
answerString[c * 10 * sizeof(char)] = '\0';
lr_save_string(answerString, "aString");
free(answerString); And the best part, after saving the string in a parameter, I free the associated memory and relish my guilt-free existence (at least in terms of this script). The script worked like a charm, not only through VuGen but multiple iterations through the scenario in Controller.
So what was the issue with using character array: the issue was not that LR agents were running out of memory because I had used 256 bytes when I actually only needed less than that. The issue was that I was not emptying the array before using it. I had declared it within the Action itself:
Action()
{
int i,c;
char answerString[256], sfx[10]; and I wrongly assumed that LoadRunner throws away variables from previous iteration and initializes brand new variables in every new iteration. Instead, what happened as in this case when I used strcat was that it was concatenating the new answer string from this iteration to whatever was left from the previous iteration. So after a few iterations, it ran out of the pre-allocated 256 bytes of space and threw the memory violation exception. I could’ve continued to use char array (and I’m glad I didn’t) by just re-initializing it in every iteration.
So lesson learnt. Hopefully all this helps somebody not make the same mistakes I made. I certainly won’t and I will also be less hesitant in using pointers. Even though I’m pretty sure this is not the last I’ve seen of memory violation exceptions, I can say that I’ll be ready to learn something new next time that happens.
By the way…that peer-to-peer file sharing application, I finally was able to compile it and make it work using a mix of pointers and character arrays. It worked great and I felt satisfied when I completed it. But of course when I was demonstrating it to the TA, it didn’t function as expected and I later found out that it was because I had forgotten to null-terminate a string.
