Re: Struggling with libraries

Fred Kleinschmidt wrote:
"Tony Burrows" <tony@xxxxxxxxxxxxxxx> wrote in message news:pan.2006.04.17.16.16.32.615585@xxxxxxxxxxxxxxxxxx
I'm just learning C as another language, and I'm trying to build some
utilities into a library. I have this (crude I know) function:

void insert(char* insrt, char* source, int place){
char temp[strlen(insrt)+strlen(source)+1]; strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
strcpy(source, temp);
Crash! source is not large enough to hold the new characters.

Without the call you don't know that this is the only problem, and in fact given the additional information the OP has now provided it was not the only problem.

You need to allocate or reallocate space. What you need is:

char * insert(char* insrt, char* source, int place){
char *temp=NULL;

That initialisation is pointless. In fact, if you move the definition to after nch you can...

size_t nch = strlen(insrt)+strlen(source)+1;
temp = malloc(nch);

replace the above with
char *temp = malloc(nch);

if ( temp ) {
strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
}
return temp;
}

In general, it is the sort of thing I was thinking of. Mind you, I would not use strcat. You know where you need to copy to (with an additional variable) so you can use strcpy and save having to scan a potentially long string. Also I would use memcpy rather than strncpy since you know you are not copying the entire string. Also one needs to either add error checking or document it's lack, since if place is beyond the end of source you have a problem.

Or you could reallocate source

void insert(char* insrt, char** source, int place){
char *temp=NULL;
size_t nch = strlen(insrt)+strlen(source)+1;
temp = realloc(*source, nch);
if ( temp ) {
strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
}
else {
/* Decide what to do if the realloc fails */
...
}
}

With the additional information now provided this would not work. We now know that source is a pointer to a string literal (which you didn't know when writing this) and so this would still be wrong.

This shows why the OP should have provided the complete program in the first place.
--
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Web site - http://home.flash-gordon.me.uk/
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