memset() not setting memory in c

Question

I apologize if my formatting is incorrect as this is my first post, I couldn't find a post on the site that dealt with the same issue I am running into. I'm using plain C on ubuntu 12.04 server. I'm trying to concatenate several strings together into a single string, separated by Ns. The string sizes and space between strings may vary, however. A struct was made to store the positional data as several integers that can be passed to multiple functions:

typedef struct pseuInts {  
    int pseuStartPos;  
    int pseuPos;  
    int posDiff;  
    int scafStartPos;  
} pseuInts;  

As well as a string struct:

typedef struct string {  
    char *str;  
    int len;  
} myString;

Since there are break conditions for the concatenated string multiple nodes of a dynamically linked list were assembled containing an identifier and the concatenated string:

typedef struct entry {  
    myString title;  
    myString seq;  
    struct entry *next;  
} entry;  

The memset call is as follows:

} else if ((*pseuInts)->pseuPos != (*pseuInts)->scafStartPos) {  
    (*pseuEntry)->seq.str = realloc ((*pseuEntry)->seq.str, (((*pseuEntry)->seq.len) + (((*pseuInts)->scafStartPos) - ((*pseuInts)->pseuPos)))); //realloc the string being extended to account for the Ns  
    memset (((*pseuEntry)->seq.str + ((*pseuEntry)->seq.len)), 'N', (((*pseuInts)->scafStartPos) - ((*pseuInts)->pseuPos))); //insert the correct number of Ns
    (*pseuEntry)->seq.len += (((*pseuInts)->scafStartPos) - ((*pseuInts)->pseuPos)); //Update the length of the now extended string  
    (*pseuInts)->pseuPos += (((*pseuInts)->scafStartPos) - ((*pseuInts)->pseuPos)); //update the position values  
}  

These are all being dereferenced as this else if decision is in a function being called by a function called from main, but the changes to the pseuEntry struct need to be updated in main so as to be passed to another function for further processing.
I've double checked the numbers being used in pseuInts by inserting some printf commands and they are correct in the positioning of how many Ns need to be added, even as they change between different short strings. However, when the program is run the memset only inserts Ns the first time it's called. IE:

GATTGT and TAATTTGACT are separated by 4 spaces and they become:
GATTGTNNNNTAATTTGACT

The second time it is called on the same concatenated string it doesn't work though. IE:
TAATTTGACT and TCTCC are separated by 6 spaces so the long string should become:

GATTGTNNNNTAATTTGACTNNNNNNTCTCC
but it only shows:
GATTGTNNNNTAATTTGACTTCTCC

I've added printfs to display the concatenated string immediately before and after the memset and the they are identical in output.
Sometimes the insertion is adding extra character spaces, but not initializing them so they print nonsense (as would be expected). IE:

GAATAAANNNNNNNNNNNNNNNNN¬GCTAATG
should be
GAATAAANNNNNNNNNNNNNNNNNGCTAATG

I've switched the memset with a for or a while loop and I get the same result. I used an intermediate char * to realloc and still get the same result. I'm looking for for suggestions as to where I should look to try and detect the error.

Answer 1

If you are okay with considering a completely different approach, I would like to offer this:

I understand your intent to be: Replace existing spaces between two strings with an equal number of "N"s. memset() (and associated memory allocations) is the primary method to perform the concatenations.

The problems you have described with your current concatenation attempts are :
1) garbage embedded in resulting string.
2) writing "N" in some unintended memory locations.
3) "N" not being written in other intended memory locations.

Different approach:

First: verify that the memory allocated to the string being modified is sufficient to contain results
second: verify all strings to be concatenated are \0 terminated before attempting concatenation.
third: use strcat(), and a for(;;) loop to append all "N"s, and eventually, subsequent strings.

eg.

for(i=0;i<numNs;i++)//compute numNs with your existing variables 
{
    strcat(firstStr, "N");//Note: "N" is already NULL term. , and strcat() also ensures null term.
}
strcat(firstStr, lastStr); //a null terminated concatenation

I know this approach is vastly different from what you were doing, but it does address at least the issues identified from your problem statement. If this makes no sense, please let me know and I will address questions as I am able to. (currently have other projects going on)

Answer 2

Looking at your memset:

memset (((*pseuEntry)->seq.str + ((*pseuEntry)->seq.len))), ...

That's the destination. Shouldn't it be:

(memset (((*pseuEntry)->seq.str + ((*pseuEntry)->seq.len) + ((*pseuEntry)->seq.pseuStartPos))

Otherwise I'm missing the meaninging of pseuInts .