Strace Hello World节目

Question

所以我试着用下面的"Hello！“来理解系统调用。节目：

#include <stdio.h>

int main(){

   printf("Hello World!\n");
   return 0;

}

然后运行可执行文件的strace，得到以下内容：

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 +++

有很多系统调用试图打开一些不在磁盘中的库。为什么这个程序试图访问这些文件？我怎么才能防止这种情况发生呢？

变量LD_LIBRARY_PATH的内容如下：

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

Answer 1

这一切都很正常。您不应该阻止失败的库查找发生。

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

这是你的节目开始。由于它是动态链接的，所以第一个要执行的代码来自dynamic loader。

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

动态加载程序正在分配一些堆空间。

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

动态加载程序检查是否有要预加载的动态库。根本就没有。

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

动态加载程序正在寻找libc6，这是标准库。它在几个目录中查找:首先在LD_LIBRARY_PATH中指定的目录中，然后在/etc/ld.so.conf中列出的目录中。(完整的故事见手册)。在每个目录中，加载程序首先检查几个子目录:它确定存在哪些硬件特性(P6指令 )，并查找库二进制文件的版本，该版本可以使用这些额外的特性来更有效地运行；当它找不到可能使用所有功能的子目录时，它会查找更通用的特性。最后，库在一个标准的系统目录中，在一个非专门的版本中找到。

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

加载标准库，然后运行其初始化代码。

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

这是执行printf调用的结果。

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

这是您的程序退出，其中包括刷新标准缓冲区。