Unzip one file with libzip - c

Question

I need to create an application to extract one file from zip archive, after which I want to compile it for Android.
I'm using Ubuntu, with libzip-0.10.1 pre-installed.
I created C project in Eclipse, added include path and found simple script for extracting file. Unfortunately I cannot get the following to build and I could use some advice.
// zip.c file
#include <stdio.h>
#include <stdlib.h>
#include <zip.h>
int main(int argc, char **argv) {
struct zip *zip_file;
struct zip_file *file_in_zip;
int err;
int files_total;
int file_number;
int r;
char buffer[10000];
if (argc < 3) {
fprintf(stderr,"usage: %s <zipfile> <fileindex>\n",argv[0]);
return -1;
};
zip_file = zip_open(argv[1], 0, &err);
if (!zip_file) {
fprintf(stderr,"Error: can't open file %s\n",argv[1]);
return -1;
};
file_number = atoi(argv[2]);
files_total = zip_get_num_files(zip_file);
if (file_number > files_total) {
printf("Error: we have only %d files in ZIP\n",files_total);
return -1;
};
file_in_zip = zip_fopen_index(zip_file, file_number, 0);
if (file_in_zip) {
while ( (r = zip_fread(file_in_zip, buffer, sizeof(buffer))) > 0) {
printf("%s",buffer);
};
zip_fclose(file_in_zip);
} else {
fprintf(stderr,"Error: can't open file %d in zip\n",file_number);
};
zip_close(zip_file);
return 0;
};
Also I added few .h files to include directory in my project and few .c files to directory with zip.c file. After that all dependences was good, but I have an error:
‘struct zip’ has no member named ‘default_password’ in file zip_fopen_index.c
The file zip_fopen_index.c is:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include "zipint.h"
ZIP_EXTERN struct zip_file *
zip_fopen_index(struct zip *za, zip_uint64_t fileno, int flags)
{
return zip_fopen_index_encrypted(za, fileno, flags, za->default_password); // error here
}

Answer 1

First of all allow me some comments:
Your program is not compiled and linked by Eclipse.
Compiling is done by the compiler (gcc using option -c):
make all
Building file: ../zip.c
Invoking: GCC C Compiler
gcc -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"zip.d" -MT"zip.d" -o "zip.o" "../zip.c"
Finished building: ../zip.c
Linking is done by the linker (via the compiler using option -o):
Invoking: GCC C Linker
gcc -o "unzipper" ./zip.o
./main.o: In function `zip':
/home/alk/workspace/unzipper/Debug/../zip.c:20: undefined reference to `zip_open'
/home/alk/workspace/unzipper/Debug/../zip.c:27: undefined reference to `zip_get_num_files'
/home/alk/workspace/unzipper/Debug/../zip.c:33: undefined reference to `zip_fopen_index'
/home/alk/workspace/unzipper/Debug/../zip.c:35: undefined reference to `zip_fread'
/home/alk/workspace/unzipper/Debug/../zip.c:38: undefined reference to `zip_fclose'
/home/alk/workspace/unzipper/Debug/../zip.c:43: undefined reference to `zip_close'
collect2: ld returned 1 exit status
Eclipse provides a framework helping you in managing all sources and their references as also spawing compiler and linker tasks and setting their options.
When the linker told you there where undefined references to the zip_*function during the build of your program, the cause for this was, you were missing to tell the linker (via the compiler, via Eclipse) where those zip_* functions could be found.
Those zip_* functions are located in a library, namely libzip.
So what you as the programmer need to tell the linker (via the compiler, via Eclipse) is to link those functions against what the compiler compiled from your sources.
As the result the linker is able to create a runnable program from your compiled sources together with all libraries needed. Certain libraries are know to Eclipse (and therfore to the linker) by default, for example the one containing the C standard functions, namely libc.
To get things going:
1 Remove the source files you pulled from the libzip librarie's sources from your project. Those sources had been compiled into the library libzip, which you will use in your project.
2 Tell the linker (via Eclipse) to use libzip for your project.
Do so by following the steps below:
open the project's properties
click 'C/C++ General'
click 'Path and Symbols', on the left select the 'Libraries' tab, there click 'Add' and enter zip
finally click 'OK'
3 Then try to build your program:
Building target: unzipper
Invoking: GCC C Linker
gcc -o "unzipper" ./zip.o -lzip
Finished building target: unzipper
(Please note additional option -lzip!)
If the developement version of 'libzip' had been installed properly before, you should be fine.
PS: unzipper was the name I used for the Eclispe project to produce the examples.
PSS: I used Eclipse Juno SR1