Struggling with libraries

[Tony Burrows]

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)+s trlen(source)+1]; strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
strcpy(source, temp);
}

In a simple file, it all works, but in a static library, when I call it I
get a segnmentation fault. The villain is the final strcpy.

I tried returning temp instead and that doesn't work either, I don't get
quite the string that exists in the function (and the compiler warns about
returning a local variable).

I know there's a solution, I just have no idea what it is. Any help would
be great.

Tony

[Nils O. Selåsdal]

Tony Burrows wrote:

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)+s trlen(source)+1]; strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
strcpy(source, temp);
}

In a simple file, it all works, but in a static library, when I call it I
get a segnmentation fault. The villain is the final strcpy.

I tried returning temp instead and that doesn't work either, I don't get quite the string that exists in the function (and the compiler warns about
returning a local variable). It's illegal to return pointers to auto objects.

I know there's a solution, I just have no idea what it is. Any help would
be great.

WIthout seeing how it is beeing called, hard to tell.

Be very very sure your 'source' has enough room for
'insrt'.

[Default User]

Tony Burrows wrote:

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)+s trlen(source)+1]; strncpy(temp, source,
place); temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
strcpy(source, temp);
}

In a simple file, it all works, but in a static library, when I call
it I get a segnmentation fault. The villain is the final strcpy.

I tried returning temp instead and that doesn't work either, I don't
get quite the string that exists in the function (and the compiler
warns about returning a local variable).

I know there's a solution, I just have no idea what it is. Any help
would be great.

You need to help us. Post a COMPLETE minimal program that demonstrates
the problem. I all likelihood, there is not sufficient room in source
to contain the expanded string, but how can we tell?


Brian

[Tony Burrows]

On Mon, 17 Apr 2006 18:46:44 +0200, Nils O. Selåsdal wrote:

Tony Burrows wrote:

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)+s trlen(source)+1]; strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
strcpy(source, temp);
}

In a simple file, it all works, but in a static library, when I call it I
get a segnmentation fault. The villain is the final strcpy.

I tried returning temp instead and that doesn't work either, I don't get

quite the string that exists in the function (and the compiler warns about
returning a local variable).

It's illegal to return pointers to auto objects.

I know there's a solution, I just have no idea what it is. Any help would
be great.

WIthout seeing how it is beeing called, hard to tell.

Be very very sure your 'source' has enough room for
'insrt'.

I've called it from a simple little program:

#include <stdio.h>
#include "insert.h"

int main(void){
char *str1 = "You are a nutcase!";
char *str2 = "big ";
puts(str1);
puts(str2);
insert(str2, str1, 10);
return 0;
}

insert.h is
#include <string.h>
void insert(char* insrt, char* source, int place);
void insertn(char* insrt, char* source, int place, int size);

Compiled with gcc -o testInsert testInsert.c insert.c

If I put print statements in, then the segmentation fault happens in the
insert function when strcpy(source, temp) is executed.

If I created a string on the heap with (I think) malloc and returned a
pointer to that, would that deal with the problem? If so, is it the right
way? The C programming books don't seem to touch on this.

Tony

[Fred Kleinschmidt]

"Tony Burrows" <to**@tonyburro ws.com> wrote in message
news:pa******** *************** *****@tonyburro ws.com...

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)+s trlen(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.
You need to allocate or reallocate space. What you need is:

char * insert(char* insrt, char* source, int place){
char *temp=NULL;
size_t nch = strlen(insrt)+s trlen(source)+1 ;
temp = malloc(nch);
if ( temp ) {
strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
}
return temp;
}

Or you could reallocate source

void insert(char* insrt, char** source, int place){
char *temp=NULL;
size_t nch = strlen(insrt)+s trlen(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 */
...
}
}

In a simple file, it all works, but in a static library, when I call it I
get a segnmentation fault. The villain is the final strcpy.

I tried returning temp instead and that doesn't work either, I don't get
quite the string that exists in the function (and the compiler warns about
returning a local variable).

I know there's a solution, I just have no idea what it is. Any help would
be great.

Tony

--
Fred L. Kleinschmidt
Boeing Associate Technical Fellow
Technical Architect, Software Reuse Project

[Default User]

Tony Burrows wrote:

I've called it from a simple little program:

#include <stdio.h>
#include "insert.h"

int main(void){
char *str1 = "You are a nutcase!";
char *str2 = "big ";
puts(str1);
puts(str2);
insert(str2, str1, 10);
return 0;
}

insert.h is
#include <string.h>
void insert(char* insrt, char* source, int place);
void insertn(char* insrt, char* source, int place, int size);

Compiled with gcc -o testInsert testInsert.c insert.c

If I put print statements in, then the segmentation fault happens in
the insert function when strcpy(source, temp) is executed.
You have two problems. One is that string literals are not modifiable.
Passing str1 to a function that modifies it is undefined behavior. For
you, luckily, that UB was a crash and alerted you to the problem.

The second problem is that even if it was modifiable, str1 is an array
of 18 characters, with no place to put in any extras.
If I created a string on the heap with (I think) malloc and returned a
pointer to that, would that deal with the problem? If so, is it the
right way? The C programming books don't seem to touch on this.

Inside the insert() function? That's one way to do it. Some people
don't like to have functions that allocate memory and leave the
responsibility to deallocate to the caller. The other way is to pass in
a buffer large enough to hold the resulting string.

void insert(const char* insrt, const char* source, char *dest, int
place);

Allocate dest in the calling function, with its size equal to the
string length of insrt + source + 1.

Brian

[Keith Thompson]

Tony Burrows <to**@tonyburro ws.com> writes:

On Mon, 17 Apr 2006 18:46:44 +0200, Nils O. Selåsdal wrote:

Tony Burrows wrote:

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)+s trlen(source)+1]; strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
strcpy(source, temp);
}

[snip] I've called it from a simple little program:

#include <stdio.h>
#include "insert.h"

int main(void){
char *str1 = "You are a nutcase!";
char *str2 = "big ";
puts(str1);
puts(str2);
insert(str2, str1, 10);
return 0;
}

insert.h is
#include <string.h>
void insert(char* insrt, char* source, int place);
void insertn(char* insrt, char* source, int place, int size);

Two problems. First, you're attempting to modify a string literal.
Second, even if the string literal were modifiable (as it might be on
some implementations ), you haven't allocated enough space to hold the
result.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Eric Sosman]

Default User wrote On 04/17/06 15:08,:

Tony Burrows wrote:

I've called it from a simple little program:

#include <stdio.h>
#include "insert.h"

int main(void){
char *str1 = "You are a nutcase!";
char *str2 = "big ";
puts(str1);
puts(str2);
insert(str2, str1, 10);
return 0;
}

insert.h is
#include <string.h>
void insert(char* insrt, char* source, int place);
void insertn(char* insrt, char* source, int place, int size);

Compiled with gcc -o testInsert testInsert.c insert.c

If I put print statements in, then the segmentation fault happens in
the insert function when strcpy(source, temp) is executed.

You have two problems. One is that string literals are not modifiable.
Passing str1 to a function that modifies it is undefined behavior. For
you, luckily, that UB was a crash and alerted you to the problem.

The second problem is that even if it was modifiable, str1 is an array
of 18 characters, with no place to put in any extras.

ITYM "19 characters;" C's most characteristic error strikes
again. Also, since the O.P. is a beginner it's as well to avoid
using shorthand language that is literally untrue: str1 is not an
array, but a pointer. (And it's not a "pointer to an array,"
either.) A review of Section 6 in the FAQ <http://www.c-faq.com/>
is recommended.

--
Er*********@sun .com

[Flash Gordon]

Fred Kleinschmidt wrote:

"Tony Burrows" <to**@tonyburro ws.com> wrote in message
news:pa******** *************** *****@tonyburro ws.com...

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)+s trlen(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)+s trlen(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)+s trlen(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.
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

[Flash Gordon]

Tony Burrows wrote:

On Mon, 17 Apr 2006 18:46:44 +0200, Nils O. Selåsdal wrote:

Tony Burrows wrote:

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)+s trlen(source)+1]; strncpy(temp, source, place);
temp[place]='\0';
strcat(temp, insrt);
strcat(temp, source+place);
strcpy(source, temp);
}

In a simple file, it all works, but in a static library, when I call it I
get a segnmentation fault. The villain is the final strcpy.

I tried returning temp instead and that doesn't work either, I don't get
quite the string that exists in the function (and the compiler warns about
returning a local variable).

It's illegal to return pointers to auto objects.

I know there's a solution, I just have no idea what it is. Any help would
be great.

WIthout seeing how it is beeing called, hard to tell.

Be very very sure your 'source' has enough room for
'insrt'.

I've called it from a simple little program:

#include <stdio.h>
#include "insert.h"

int main(void){
char *str1 = "You are a nutcase!";
char *str2 = "big ";
puts(str1);
puts(str2);
insert(str2, str1, 10);
return 0;
}

insert.h is
#include <string.h>
void insert(char* insrt, char* source, int place);
void insertn(char* insrt, char* source, int place, int size);

Compiled with gcc -o testInsert testInsert.c insert.c

If I put print statements in, then the segmentation fault happens in the
insert function when strcpy(source, temp) is executed.

If I created a string on the heap with (I think) malloc and returned a
pointer to that, would that deal with the problem? If so, is it the right
way? The C programming books don't seem to touch on this.

I'm sure they do. The comp.lang.c FAQ certainly does with a much simpler
example here http://c-faq.com/strangeprob/strlitnomod.html which tells
you that string literals are not modifiable.

In addition, even if string literals were modifiable, you are trying to
make the string longer, so this would not work either:
#include <stdio.h>
#include "insert.h"

int main(void){
char str1[] = "You are a nutcase!";
char *str2 = "big ";
puts(str1);
puts(str2);
insert(str2, str1, 10);
return 0;
}

Since it would go off the end of str1.

If I was writing a function like your insert I would probably make is
malloc enough space for a new string and return the resultant string in
that rather than modifying one of the input strings.
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc