Strange g++ behaviour?

james_b · 11-16-2011, 02:51 PM

Hiya,

I'm new to development on Linux/g++. Could anyone explain why this compiles:

$ g++ test.cpp -o test.o /usr/lib64/mysql/libmysqlclient.a

But this doesn't:

$ g++ -L/usr/lib64/mysql -lmysqlclient test.cpp -o test.o
/tmp/ccqgTNhu.o: In function `main':
/root/test.cpp:10: undefined reference to `mysql_init'
collect2: ld returned 1 exit status

And neither does this?

$ g++ -L/usr/lib64/mysql libmysqlclient.a test.cpp -o test.o
g++: libmysqlclient.a: No such file or directory

I'd guess the last one fails because g++ is looking for the .a in the current directory. I must be misunderstanding how linking on the command-line works, but I can't figure out what's wrong.

Strangely, I'm apparently supposed to use '-L/usr/lib64 -lmysqlclient', but there is something strange with the file:

$ cd /usr/lib64
$ ls *mysql*
libmysqlclient.a
$ file libmysqlclient.a
libmysqlclient.a: cannot open `libmysqlclient.a' (No such file or directory)

Very confusing. Any suggestions are much appreciated!

Cheers

johnsfine · 11-16-2011, 03:13 PM

Quote:

Originally Posted by james_b

But this doesn't:

$ g++ -L/usr/lib64/mysql -lmysqlclient test.cpp -o test.o

test.cpp must come earlier on the line than -lmysqlclient

Also, your output selection of test.o is confusing. You are asking for a complete executable, which is not normally given the extension .o

Quote:

And neither does this?

$ g++ -L/usr/lib64/mysql libmysqlclient.a test.cpp -o test.o
g++: libmysqlclient.a: No such file or directory

I'd guess the last one fails because g++ is looking for the .a in the current directory.

Yes, but even with a correct path the .a file must come later on the line than the file that creates the need for that .a file (which is your .cpp file).

Quote:

Originally Posted by james_b

$ cd /usr/lib64
$ ls *mysql*
libmysqlclient.a
$ file libmysqlclient.a
libmysqlclient.a: cannot open `libmysqlclient.a' (No such file or directory)

Very confusing.

I can't explain that one. But since your original command worked, I think the file must be OK.

Quote:

this compiles:

$ g++ test.cpp -o test.o /usr/lib64/mysql/libmysqlclient.a

NevemTeve · 11-16-2011, 03:28 PM

Code:

$ cd /usr/lib64
$ ls -l *mysql*

james_b · 11-16-2011, 04:57 PM

Thanks! Putting the link options after the source file does indeed work.

I'm a little unsure of why using -L/usr/lib64 doesn't work.

Code:

$ cd /usr/lib64
$ ls -l *mysql*
total 22456
-rw-r--r--. 1 root root  3020142 Oct 12 13:16 libmygcc.a
-rw-r--r--. 1 root root 16785382 Oct 12 13:15 libmysqlclient.a
lrwxrwxrwx. 1 root root       16 Nov 16 20:00 libmysqlclient_r.a -> libmysqlclient.a
lrwxrwxrwx. 1 root root       26 Nov 16 18:41 libmysqlclient_r.so.16 -> libmysqlclient_r.so.16.0.0
-rwxr-xr-x. 1 root root  1588520 Jan 27  2011 libmysqlclient_r.so.16.0.0
lrwxrwxrwx. 1 root root       24 Nov 16 18:41 libmysqlclient.so.16 -> libmysqlclient.so.16.0.0
-rwxr-xr-x. 1 root root  1577896 Jan 27  2011 libmysqlclient.so.16.0.0
-rw-r--r--. 1 root root     9904 Oct 12 13:14 libmysqlservices.a
drwxr-xr-x. 3 root root     4096 Nov 16 19:27 plugin

$ cd mysql
$ ls -l
total 22456
-rw-r--r--. 1 root root  3020142 Oct 12 13:16 libmygcc.a
-rw-r--r--. 1 root root 16785382 Oct 12 13:15 libmysqlclient.a
lrwxrwxrwx. 1 root root       16 Nov 16 20:00 libmysqlclient_r.a -> libmysqlclient.a
lrwxrwxrwx. 1 root root       26 Nov 16 18:41 libmysqlclient_r.so.16 -> libmysqlclient_r.so.16.0.0
-rwxr-xr-x. 1 root root  1588520 Jan 27  2011 libmysqlclient_r.so.16.0.0
lrwxrwxrwx. 1 root root       24 Nov 16 18:41 libmysqlclient.so.16 -> libmysqlclient.so.16.0.0
-rwxr-xr-x. 1 root root  1577896 Jan 27  2011 libmysqlclient.so.16.0.0
-rw-r--r--. 1 root root     9904 Oct 12 13:14 libmysqlservices.a
drwxr-xr-x. 3 root root     4096 Nov 16 19:27 plugin

Thanks again

james_b · 11-16-2011, 05:22 PM

I've managed to get it to link properly in the shell and Netbeans.

For some reason, it's OK to have -lpthread, -lm and -ldl before the source file - but -lmysqlclient has to come after? Is this to do with the way they are linked? Statically or dynamically maybe?

Also, libmysqlclient.a does not exist in /usr/lib64, only in /usr/lib64/mysql. I have no idea why ls -l reports it as being in /usr/lib64..

Please let me know if you've seen this before! It's almost like ls -l is looking in subdirectories too.

Thanks.

Sergei Steshenko · 11-16-2011, 07:11 PM

Quote:

Originally Posted by james_b

I've managed to get it to link properly in the shell and Netbeans.

For some reason, it's OK to have -lpthread, -lm and -ldl before the source file - but -lmysqlclient has to come after? Is this to do with the way they are linked? Statically or dynamically maybe?

Also, libmysqlclient.a does not exist in /usr/lib64, only in /usr/lib64/mysql. I have no idea why ls -l reports it as being in /usr/lib64..

Please let me know if you've seen this before! It's almost like ls -l is looking in subdirectories too.

Thanks.

You probably realize that "under the hood" g++ uses ld. I suggest to read http://linux.die.net/man/1/ld - paying attention to '--verbose'. You have to also learn how from g++ you can pass arguments directly to ld - gcc documentation has a special chapter on this.

NevemTeve · 11-17-2011, 02:48 AM

Then it is (anywhere in commandline): -L/usr/lib64/mysql -lmysqlclient

PS: in retrospect, ls should have been: ls -ld /usr/lib64/*mysql*

johnsfine · 11-17-2011, 08:03 AM

Quote:

Originally Posted by james_b

Also, libmysqlclient.a does not exist in /usr/lib64, only in /usr/lib64/mysql. I have no idea why ls -l reports it as being in /usr/lib64..

That's easy now that you provided correct info.

You used the command
ls -l *mysql*

The shell expands the *mysql* into all the names matching that spec in /usr/lib64. It does that before passing anything to the ls command.

The only name matched is mysql, which is a directory, so ls lists the contents of that directory.

For comparison try the command
ls -ld *mysql*
That will list all the files or directories matched by *mysql*
ls -l *mysql* lists files matched by *mysql* plus the contents of directories matched by *mysql*

james_b · 11-17-2011, 08:22 AM

Ah, I see now.. I just misunderstood what it was telling me.

That's great! Thanks!