What is parsing in c programming?

bislinux · 12-18-2014, 06:01 AM

what does parse function do??why is it needed in c programming?please explain in simple terms with a simple example...

i have a sample code, but i dont understand what it does..any explaination would be appreciated.thankss

Code:

void parseReply(deviceRepliesType *pReply)
{
    int i;
    char *key = strtok(pReply->binaryReply, ",");
    printf("\nkey:%s\n", key);
    if (strcmp(key,"Hello")!=0) return;
    
    strtok(NULL, ","); //Skip 0
    
    //mac address
    char *macAddrRaw = strtok(NULL, ",");
    //printf("\nmacAddrRaw:%s\n", macAddrRaw);
    pReply->n.macAddr[0]=sHexDec2ToNum(&macAddrRaw[0]);
    pReply->n.macAddr[1]=sHexDec2ToNum(&macAddrRaw[3]);
    pReply->n.macAddr[2]=sHexDec2ToNum(&macAddrRaw[6]);
    pReply->n.macAddr[3]=sHexDec2ToNum(&macAddrRaw[9]);
    pReply->n.macAddr[4]=sHexDec2ToNum(&macAddrRaw[12]);
    pReply->n.macAddr[5]=sHexDec2ToNum(&macAddrRaw[15]);
}

what is going on here???

pan64 · 12-18-2014, 06:39 AM

have you checked the man page of strtok?

johnsfine · 12-18-2014, 07:33 AM

Transforming a stream of text into some other form of information is called "parsing".
It is very common in computer programs for a wide range of purposes.
For example, when the user types a reply to any prompt in your program.

rtmistler · 12-18-2014, 12:59 PM

johnsfine said it well, I was going to jump on their coattails and elaborate a bit; however I'll address your question about "why".

The answer is you are not required to use parsing to write program in C. And you are not required to write the equivalent of a parser when you write a program in C. It's more like a sort algorithm, if you need to do that, you do it, not because there's some purpose why every C program necessitates the need for a sort algorithm.

You can write or use a parser, as someone has done in the example you've cited and it is a very common programming action to perform when you're converting information from one form to another.

Be aware that verbiage use by a programmer does not guarantee that they are doing exactly what they used for names. In this case I do agree that they are parsing a string; however it seems odd that the name of the string is "binaryReply" which would imply to me that the data potentially could be non-printable ASCII. Further where they say //Skip 0, you have to wonder if that's 0x30 which is ASCII printable "0" or if that's 0x00 which is the NULL string terminator. That would cause some problems, so a guess is that they're skipping a field which they know to be there whatever the content of it is. The strtok() function will just return the next string found after a comma.

The programmer is not verifying the return from strtok(), it could be NULL, and therefore a lot of their following actions could end up becoming segment violations. The string is very customized. How many strings would be exactly: "Hello,<something>,00:00:00:00:00:00\0" as an example? Because that's approximately what it appears they're parsing specifically for. And if there's a format screwup anywhere in there, the program goes kaboom!

sundialsvcs · 12-19-2014, 06:29 AM

"Hickory dickory dock the mouse ran up the clock"

That's the string I've got, and I want to "eat it, one blank-delimited word at a time," producing, in a loop, nine words in all.

In compiler-writing jargon, those words are called tokens, and that's where the name strtok() comes from: "str[ing] tok[en]." (Not "string to 'k'.")

bislinux · 12-27-2014, 09:53 AM

Thank u for your kind reply..dat was really helpfull.Cheerss