A question regarding the gcc compiler ... Post: 302476289

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Top Forums Programming A question regarding the gcc compiler ... Post 302476289 by Corona688 on Wednesday 1st of December 2010 10:30:04 AM

12-01-2010

Registered User

So, you're trawling the entire /usr/lib/ directory? by default gcc uses these and only these:

Code:

# Compiling with -static to make it more obvious which things it uses
$ echo "main() { return(42); }" > 42.c
$ strace -f gcc -static 42.c -o 42 2> 42.log
$  grep "\.[ao]" 42.log | grep open | egrep -v "ENOENT|/tmp"
[pid  7741] open("/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/../../../crt1.o", O_RDONLY|O_LARGEFILE) = 4
[pid  7741] open("/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/../../../crti.o", O_RDONLY|O_LARGEFILE) = 5
[pid  7741] open("/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/crtbeginT.o", O_RDONLY|O_LARGEFILE) = 6
[pid  7741] open("/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/libgcc.a", O_RDONLY|O_LARGEFILE) = 8
[pid  7741] open("/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/libgcc_eh.a", O_RDONLY|O_LARGEFILE) = 9
[pid  7741] open("/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/../../../libc.a", O_RDONLY|O_LARGEFILE) = 10
[pid  7741] open("/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/crtend.o", O_RDONLY|O_LARGEFILE) = 11
[pid  7741] open("/usr/lib/gcc/i686-pc-linux-gnu/4.3.4/../../../crtn.o", O_RDONLY|O_LARGEFILE) = 12

Try a library it'll have to look for, like pthread:

Code:

# Yes, I'm aware the code is nonsense.  Nothing better's needed
$ echo "main() { return(pthread_create(42)); }" > 42.c
$ gcc 42.c
/tmp/cckAe1dw.o: In function `main':
42.c:(.text+0x19): undefined reference to `pthread_create'
collect2: ld returned 1 exit status

...It didn't find it. gcc only looks inside what it's told to look inside -- by default a very small set of libraries encompassing stdio, libc, a few math builtins, that's it. It has to be explicitly told to check in libpthread.so (or libpthread.a when linking statically) like

Code:

$ gcc -lpthread 42.c
$

So gcc never had any special superfast linker finding thing. It's just been told where to look. When you start linking in dozens of libraries, you'll indeed find gcc having to do a lot more work...

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