CODEWITHSUNDEEP
c
NONE
NONE

Reputation: 233

Extracting first part of a string

I'm posting this as a vent for my questions (I will have a multitude). I decided it would be rather annoying to keep asking the same person one question at a time (said person is very busy), so I'll be posting questions as I come across them in my project. If you feel like helping, please do, and I would greatly appreciate it!

Note: this means I'll be updating this post frequently. Help is greatly, greatly appreciated as always.

EDIT so you guys want me to just keep posting different questions if I come across them? Of course I always do research before asking you guys, you talented group of men and women only get the most persistent of errors.

My first question:

I keep getting the error:

lvalue required as left operand of assignment

THE PURPOSE of this code is to copy the first n character up to ':'. For instance, if currentline is: "LABEL: .long 100" then GetLabelName would return "LABEL".

NOTE strncpy isn't working for this. It returns the remaining characters after ignoring the first n characters instead of just returning the first n characters...

Here's the code that's causing the error:

char *GetLabelName(char *currentline){
   char *labelname[200];
   while((((*labelname)++)=(*currentline)++)!=':');
   return labelname;
}

Something is fishy about this code I guess, but I can't figure out what. Any ideas?

Upvotes: 1

Views: 910

Answers (2)

AusCBloke
AusCBloke

Reputation: 18522

What I think you're trying to do is extract/copy all of the characters in a string up until a certain point (':' or NUL) and return that buffer. If that's the case, you're going to need to dynamically allocate memory for the new string (you can't return a local buffer allocated on the stack), and you should also take advantage of functions in <string.h> like strchr and memcpy.

Here's an alternative working example:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char *extract_string(char *str, char delim)
{
   size_t len;
   char *new_str;
   char *delim_pos = strchr(str, delim);

   /* new string is the length from the start of the old string to the
    * delimiter, or if it doesn't exist, a copy of the whole string */
   if (delim_pos == NULL)
      return strdup(str);

   len = delim_pos - str;
   new_str = malloc(len + 1);
   memcpy(new_str, str, len);
   new_str[len] = '\0'; /* NUL terminate the new string */

   return new_str;
}

int main(void)
{
   char *extracted1 = extract_string("some:string", ':');
   char *extracted2 = extract_string("no delimiter", ':');

   puts(extracted1);
   puts(extracted2);

   /* free the dynamically allocated buffers */
   free(extracted1);
   free(extracted2);

   return 0;
}

Output:

some
no delimiter

If you don't want to make a copy when the delimiter isn't found, you could alternatively return NULL.

Alternatively, if you don't mind mangling your initial string, you could use strtok to extract tokens.

Upvotes: 4

mwk
mwk

Reputation: 436

The problem is (*labelname)++. You are incrementing the value that's pointed to by labelname and simultaneously assigning to it the value that's pointed to by currentline. If you want to increment the pointers, use *labelname++ and *currentline++

Upvotes: 0

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