How to compile a tiny simple library by g++?

Xiangbuilder · 10-19-2003, 03:11 AM

I only know use the command g++ filename.cpp -o filename to compile a exetcuable file so far, recently, I want compile a library, the code is from <<thinking in c++>>

Code:

//: C04:CLib.h
// Header file for a C-like library
// An array-like entity created at runtime

typedef struct CStashTag {
  int size;      // Size of each space
  int quantity;  // Number of storage spaces
  int next;      // Next empty space
  // Dynamically allocated array of bytes:
  unsigned char* storage;
} CStash;

void initialize(CStash* s, int size);
void cleanup(CStash* s);
int add(CStash* s, const void* element);
void* fetch(CStash* s, int index);
int count(CStash* s);
void inflate(CStash* s, int increase);
///:~

another file:

Code:

//: C04:CLib.cpp {O}
// Implementation of example C-like library
// Declare structure and functions:
#include "CLib.h"
#include <iostream>
#include <cassert> 
using namespace std;
// Quantity of elements to add
// when increasing storage:
const int increment = 100;

void initialize(CStash* s, int sz) {
  s->size = sz;
  s->quantity = 0;
  s->storage = 0;
  s->next = 0;
}

int add(CStash* s, const void* element) {
  if(s->next >= s->quantity) //Enough space left?
    inflate(s, increment);
  // Copy element into storage,
  // starting at next empty space:
  int startBytes = s->next * s->size;
  unsigned char* e = (unsigned char*)element;
  for(int i = 0; i < s->size; i++)
    s->storage[startBytes + i] = e[i];
  s->next++;
  return(s->next - 1); // Index number
}

void* fetch(CStash* s, int index) {
  // Check index boundaries:
  assert(0 <= index);
  if(index >= s->next)
    return 0; // To indicate the end
  // Produce pointer to desired element:
  return &(s->storage[index * s->size]);
}

int count(CStash* s) {
  return s->next;  // Elements in CStash
}

void inflate(CStash* s, int increase) {
  assert(increase > 0);
  int newQuantity = s->quantity + increase;
  int newBytes = newQuantity * s->size;
  int oldBytes = s->quantity * s->size;
  unsigned char* b = new unsigned char[newBytes];
  for(int i = 0; i < oldBytes; i++)
    b[i] = s->storage[i]; // Copy old to new
  delete [](s->storage); // Old storage
  s->storage = b; // Point to new memory
  s->quantity = newQuantity;
}

void cleanup(CStash* s) {
  if(s->storage != 0) {
   cout << "freeing storage" << endl;
   delete []s->storage;
  }
} ///

What command I should use?
Thank you.

SaTaN · 10-19-2003, 04:17 AM

I think

Quote:

g++ CLib.cpp -o CLib

would do it for you

You don't have to compile the library again as you have included it
in CLib.cpp

Xiangbuilder · 10-19-2003, 04:45 AM

We know a c++ program must have an int main function, but there is no in main function in the file. So I guess the files is used to create a library file such as *.lib, Only is my guess, in fact, I don't if g++ can be used to create a linrary or not.
Here is the error information:

Code:

[root@localhost a_a]# g++ CLib.cpp -o CLib
/usr/lib/gcc-lib/i386-redhat-linux/3.2.2/../../../crt1.o(.text+0x18): In function `_start':
../sysdeps/i386/elf/start.S:77: undefined reference to `main'
collect2: ld returned 1 exit status
[root@localhost a_a]#

Thank you.

SaTaN · 10-19-2003, 06:15 AM

Maybe the author wants you to write the main program by yourself using any of the functions mentioned in both CLib.h and CLib.cpp files.

Xiangbuilder · 10-19-2003, 06:44 AM

Perhaps.
Thank you.

lenlutz · 10-19-2003, 11:26 AM

i didnt see, the "-c" option,
which is: only make an obj.... not an a.out

ALSO, once done, to make a lib, you use ar... like this:

ar ruv CLib.a CLib.o
(you can remove the .o)
now, you have a linkable module

Xiangbuilder · 10-19-2003, 10:41 PM

Code:

[root@localhost a_a]# g++ CLib.cpp -c CLib.o
g++: CLib.o: no such directory or file

However, there is a file Clib.o. After

Code:

ar ruv CLib.a

there is a file Clib.a.
Thank you.

dvogel · 10-19-2003, 11:44 PM

To create a shared library use

g++ -shared -o <lib name> <cpp files>

Xiangbuilder · 10-20-2003, 05:34 AM

Thank you.
I find some sentences below in the book: "To test the library, two CStashes are created. The first holds ints and the second holds arrays of 80 chars:"

Code:

//: C04:CLibTest.cpp
//{L} CLib
// Test the C-like library
#include "CLib.h"
#include <fstream>
#include <iostream>
#include <string>
#include <cassert>
using namespace std;

int main() {
  // Define variables at the beginning
  // of the block, as in C:
  CStash intStash, stringStash;
  int i;
  char* cp;
  ifstream in;
  string line;
  const int bufsize = 80;
  // Now remember to initialize the variables:
  initialize(&intStash, sizeof(int));
  for(i = 0; i < 100; i++)
    add(&intStash, &i);
  for(i = 0; i < count(&intStash); i++)
    cout << "fetch(&intStash, " << i << ") = "
         << *(int*)fetch(&intStash, i)
         << endl;
  // Holds 80-character strings:
  initialize(&stringStash, sizeof(char)*bufsize);
  in.open("CLibTest.cpp");
  assert(in);
  while(getline(in, line))
    add(&stringStash, line.c_str());
  i = 0;
  while((cp = (char*)fetch(&stringStash,i++))!=0)
    cout << "fetch(&stringStash, " << i << ") = "
         << cp << endl;
  cleanup(&intStash);
  cleanup(&stringStash);
} ///

How to compile the file and test the library, in other words, what command I should use?
So far, I can successfully compiled the source code by the command

Code:

[root@localhost a_a]# g++ CLibTest.cpp CLib.cpp -o CLibTest

but I want to test the library, how to do?
Thank you.

dakensta · 10-20-2003, 09:54 AM

1) g++ -c CLib.cpp

2) ar rcs libCLib.a CLib.o

3) g++ CLibTest.cpp -o CLibTest -L. -lCLib

mr_segfault · 10-20-2003, 10:01 AM

To link in your lib:

assuming name and path :

Code:

/usr/local/lib/libMyLib.so

you would use

Code:

g++ CLibTest.cpp -o CLibTest -L/usr/local/lib -lMyLib

Note the -L path is the path to where the libMyLib.so file is, so modify that line to be correct for your lib location.

So if the lib is called libMyLib.so and is located in the same directory as you are compiling in the use

Code:

g++ CLibTest.cpp -o CLibTest -L. -lMyLib

Note: When specifying libraries with the -l flag you leave off the lib and the .so.. ie ( libMyLib.so becomes MyLib ):

Do a:

Code:

man gcc

Hope that helps.

Cheers..

Xiangbuilder · 10-20-2003, 10:50 AM

Thank you.

Code:

[root@localhost a_a]# g++ CLibTest.cpp -o CLibTest -L. -lCLib
/usr/bin/ld: cannot find -lCLib 
collect2: ld returned 1 exit status
[root@localhost a_a]#

but I don't use the -l flag, then can successfully compiled

Code:

[root@localhost a_a]# g++ CLibTest.cpp -o CLibTest -L. CLib.o

I think this is because of my gcc problem perhaps. My guess.
Thank you.
Very helpful to me.

dakensta · 10-20-2003, 11:46 AM

Quote:

[root@localhost a_a]# g++ CLibTest.cpp -o CLibTest -L. -lCLib
/usr/bin/ld: cannot find -lCLib

Can you list (ls) the contents of your working directory and print it here, please.

There should be a file called libCLib.a in your working (i.e. current / compiling / active ) directory, created by the line,

ar rcs libCLib.a CLib.o

I would stick with this for now, then move on to shared libraries (bottom), purely for the sake of keeping things simple and not having to worry about anything like the dynamic linker setup. I am assuming you are clear on the differences between shared (*.so) and static (*.a) libraries.

N.B. As mr_segfault says, you can specify the location of this library file using the -L compiler option, although it's probably just easier to get started by working in one directory.

----------------------------------------------------------------

Quote:

[root@localhost a_a]# g++ CLibTest.cpp -o CLibTest -L. CLib.o

is NOT using the library, you are simply building the raw (unarchived) object file in to your executable.

----------------------------------------------------------------

To create the shared (*.so) library, use these steps

1) g++ -fpic -c CLib.cpp

2) g++ -shared -o libCLib.so CLib.o

3) g++ CLibTest.cpp -o CLibTest -L. -lCLib

Xiangbuilder · 10-20-2003, 09:33 PM

Thank you.

"Can you list (ls) the contents of your working directory and print it here, please."[code]
[root@localhost a_a]# ls
CLib.cpp CLib.h CLibTest.cpp
[root@localhost a_a]# g++ -c CLib.cpp
[root@localhost a_a]# ls
CLib.cpp CLib.h CLib.o CLibTest.cpp
[root@localhost a_a]# ar ruv CLib.a CLib.o
a - CLib.o
[root@localhost a_a]# ls
CLib.a CLib.cpp CLib.h CLib.o CLibTest.cpp
[root@localhost a_a]# g++ CLibTest.cpp -o CLibTest -L. -lCLib
/usr/bin/ld: cannot find -lCLib
collect2: ld returned 1 exit status
[root@localhost a_a]#

Code:

[root@localhost a_a]# ls
CLib.cpp  CLib.h  CLibTest.cpp
[root@localhost a_a]# g++ -c CLib.cpp
[root@localhost a_a]# ls
CLib.cpp  CLib.h  CLib.o  CLibTest.cpp
[root@localhost a_a]# ar rcs libCLib.a CLib.o 
[root@localhost a_a]# ls
CLib.cpp  CLib.h  CLib.o  CLibTest.cpp  libCLib.a
[root@localhost a_a]# g++ CLibTest.cpp -o CLibTest -L. -lCLib
[root@localhost a_a]# ls
CLib.cpp  CLib.h  CLib.o  CLibTest  CLibTest.cpp  libCLib.a
[root@localhost a_a]# ./CLibTest
fetch(&intStash, 0) = 0
... ...
[root@localhost a_a]#

It works fine this time.
When I use

Code:

ar ruv CLib.a CLib.o

, it works not very good. It is because of my poor knowledge instead of lenlutz' command, lenlutz just want to show me the method, but I can't use the method correct.
Sorry, I am not clear on the diffrences between shared (*.so) and static (*.a).
Thank you.