Strace Hello World program

Question

So I am trying to understand systemcalls with the following "Hello World!" program:

#include <stdio.h> int main(){ printf("Hello World!\n"); return 0; }

and then I run strace on the executable and I get the following:

execve("./hello", ["./hello"], [/* 62 vars */]) = 0 brk(0) = 0x85a5000 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb774f000 access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/tls/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/tls/i686/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/tls/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/tls/i686", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/tls/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/tls/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/tls", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/i686/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/i686", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/tls/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/tls/i686/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/tls/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/tls/i686", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/tls/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/tls/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/tls", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/i686/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/i686", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 read(3, "\177ELF\1\1\1\3\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\300\177\1\0004\0\0\0"..., 512) = 512 fstat64(3, {st_mode=S_IFREG|0755, st_size=2035943, ...}) = 0 mmap2(NULL, 1801892, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7573000 mmap2(0xb7724000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b0000) = 0xb7724000 mmap2(0xb7729000, 7844, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb7729000 close(3) = 0 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7572000 set_thread_area({entry_number:-1, base_addr:0xb7572700, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0 (entry_number:6) mprotect(0xb7724000, 12288, PROT_READ) = 0 mprotect(0xb7750000, 4096, PROT_READ) = 0 fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7571000 write(1, "Hello World!\n", 13) = 13 exit_group(0) = ? +++ exited with 0 +++

There are a lot of system calls trying to open some libraries that aren't in the disk. Why is this program trying to access those files? How can I prevent that for happening?

The contents of the variable LD_LIBRARY_PATH are:

/home/miguel/GNUstep/Library/Libraries:/usr/lib

I suggest putting the contents of your paste directly into the body of your question. This is preferable as long as the paste isn't too large. The system is probably just looking at the standard path locations for the libraries. So it is going to have some misses. Don't you see it look for something and find it eventually? — Faheem Mitha
– Faheem Mitha, Commented Oct 11, 2014 at 17:22
Ok so is looking for the file libc.so.6 in the LD_LIBRARY_PATH. Thanks! — Msegade
– Msegade, Commented Oct 11, 2014 at 18:24

Gilles 'SO- stop being evil' · Accepted Answer · 2014-10-12 01:48:49Z

This is all perfectly normal. You aren't supposed to prevent the failing library lookups from happening.

execve("./hello", ["./hello"], [/* 62 vars */]) = 0

This is your program starting. Since it is dynamically linked, the first code to execute is from the dynamic loader.

brk(0) = 0x85a5000 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb774f000

The dynamic loader is allocating some heap space.

access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)

The dynamic loader checks whether there are dynamic libraries to preload. There aren't any.

open("/home/miguel/GNUstep/Library/Libraries/tls/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/tls/i686/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/tls/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/tls/i686", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/tls/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/tls/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/tls", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/i686/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/i686", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/home/miguel/GNUstep/Library/Libraries/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/home/miguel/GNUstep/Library/Libraries", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/tls/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/tls/i686/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/tls/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/tls/i686", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/tls/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/tls/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/tls/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/tls", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/i686/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/i686/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/i686/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/i686", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/sse2/libc.so.6", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory) stat64("/usr/lib/sse2", 0xbf8df160) = -1 ENOENT (No such file or directory) open("/usr/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3

The dynamic loader is looking for libc6, which is the standard library. It looks in several directories: first in the directories specified in the LD_LIBRARY_PATH, then in the directories listed in /etc/ld.so.conf. (See the manual for the full story.). In each directory, the loader checks several subdirectories first: it determines which hardware features are present (P6 instructions, SSE2), and looks for a version of the library binary which may use these extra features to run more efficiently; when it fails to find one that may use all the features, it looks for a more generic one. In the end, the library is found in a standard system directory, in a non-specialized version.

read(3, "\177ELF\1\1\1\3\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\300\177\1\0004\0\0\0"..., 512) = 512 fstat64(3, {st_mode=S_IFREG|0755, st_size=2035943, ...}) = 0 mmap2(NULL, 1801892, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7573000 mmap2(0xb7724000, 20480, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b0000) = 0xb7724000 mmap2(0xb7729000, 7844, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb7729000 close(3) = 0 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7572000 set_thread_area({entry_number:-1, base_addr:0xb7572700, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0 (entry_number:6) mprotect(0xb7724000, 12288, PROT_READ) = 0 mprotect(0xb7750000, 4096, PROT_READ) = 0

The standard library is loaded, then its initialization code runs.

fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0 mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7571000

This is the result of executing the printf call.

write(1, "Hello World!\n", 13) = 13 exit_group(0) = ?

This is your program exiting, which includes flushing the stdout buffer.

"The dynamic loader is allocating some heap space." How much space is it allocating? It doesn't say. — Faheem Mitha
– Faheem Mitha, Commented Oct 12, 2014 at 7:31

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