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Top Forums Programming Conditions/suggestions to use shared library in C/C++ coding Post 303017431 by jim mcnamara on Wednesday 16th of May 2018 12:11:49 AM
Old 05-16-2018
How to do this with a pretend zlib path -- you need to change it to whatever code you are trying to run in your C/C++ code. C examples:

1. you have to get the header files into your code
Steps:

A. gcc -I /path/to/zlib # when compiling. This adds a directory to the include search path
B. In your C code you have to include the "foreign" header file just like you would do for standard C header files:
Code:
#include <zlib.h>

2. You have to use -L path/to/zlib/libraries also on the gcc command line. To compile:
Code:
gcc -mycode.c -I /path/to/zlib/headers -L /path/to/zlib/shared_libraries

3. Finally you need to have added (or created) an environment variable LD_LIBRARY_PATH which knows where the special shared libraries live on a particular system.
Code:
export LD_LIBRARY_PATH={$LD_LIBRARY_PATH}:/path/to/zlib/shared_libraries

This has to be there (along with the zlib shared libraries in a directory) on every system
you want to run you code one. This is the biggest source of error for this kind of development.
You can find it because the ldd command does not know where zlib libraries are - meaning at runtime the link and run step fails. Which creates all kinds of user complaints and problems. This variable has to be created everywhere for every user who will execute the code. The LD_LIBRARY_PATH definition systemwide .profile file is a possible choice, it may have security issues.

Last edited by jim mcnamara; 05-16-2018 at 01:17 AM..
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ZOPEN(3)						   BSD Library Functions Manual 						  ZOPEN(3)

NAME
zopen -- open a gzip compressed stream LIBRARY
Compression Library (libz, -lz) SYNOPSIS
FILE * zopen(const char *path, const char *mode); DESCRIPTION
The zopen() opens a gzip file whose name is the string pointed to by path and associates a stream with it. It is a wrapper around zlib(3) and standard stream I/O APIs. The argument mode have the same meaning as it does in fopen(3). The zopen function will associate read, write, seek and close functions of zlib(3) after successfully opened a file with funopen(3) so that they will be used to read or write the new stream. RETURN VALUES
Upon successful completion zopen returns a FILE pointer. Otherwise, NULL is returned and the global variable errno is set to indicate the error. ERRORS
In addition to the errors documented for fopen(3), the zopen function may also fail for: [ENOMEM] Insufficient memory is available. COMPATIBILITY
This implementation of zopen function first appeared in NetBSD 1.6 and FreeBSD 4.5. The zopen function may not be portable to systems other than FreeBSD. SEE ALSO
fopen(3), funopen(3), zlib(3) BSD
March 5, 2014 BSD
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