Can’t step into string.h function with GDB

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Recompile your code with gcc -fno-builtin -g foo.c and the gdb step command will work. (See -fno-builtin documentation). Otherwise small strcpy(), memcpy() calls would often be translated into open coded data movement instructions, e.g. on x86-64:

4   int main() {
   0x000000000040052c <+0>: push   %rbp
   0x000000000040052d <+1>: mov    %rsp,%rbp
   0x0000000000400530 <+4>: sub    $0x20,%rsp

5       char str1[20];
6       strcpy(str1, "STEP INTO ME\n");
   0x0000000000400534 <+8>: lea    -0x20(%rbp),%rax
   0x0000000000400538 <+12>:    movl   $0x50455453,(%rax)
   0x000000000040053e <+18>:    movl   $0x544e4920,0x4(%rax)
   0x0000000000400545 <+25>:    movl   $0x454d204f,0x8(%rax)
   0x000000000040054c <+32>:    movw   $0xa,0xc(%rax)

7       printf(str1);
   0x0000000000400552 <+38>:    lea    -0x20(%rbp),%rax
   0x0000000000400556 <+42>:    mov    %rax,%rdi
   0x0000000000400559 <+45>:    mov    $0x0,%eax
   0x000000000040055e <+50>:    callq  0x400410 <printf@plt>

8   }
   0x0000000000400563 <+55>:    leaveq 
   0x0000000000400564 <+56>:    retq

You can see the strpcy() call being compiled into multiple MOV instructions.

gcc -fno-builtin compiles the same program into:

4   int main() {
   0x000000000040057c <+0>: push   %rbp
   0x000000000040057d <+1>: mov    %rsp,%rbp
   0x0000000000400580 <+4>: sub    $0x20,%rsp

5       char str1[20];
6       strcpy(str1, "STEP INTO ME\n");
   0x0000000000400584 <+8>: lea    -0x20(%rbp),%rax
   0x0000000000400588 <+12>:    mov    $0x400660,%esi
   0x000000000040058d <+17>:    mov    %rax,%rdi
   0x0000000000400590 <+20>:    callq  0x400450 <strcpy@plt>

7       printf(str1);
   0x0000000000400595 <+25>:    lea    -0x20(%rbp),%rax
   0x0000000000400599 <+29>:    mov    %rax,%rdi
   0x000000000040059c <+32>:    mov    $0x0,%eax
   0x00000000004005a1 <+37>:    callq  0x400460 <printf@plt>

8   }
   0x00000000004005a6 <+42>:    leaveq 
   0x00000000004005a7 <+43>:    retq

and you can see the call to <strcpy@plt>.

Assuming you wanted to step into strcpy() to study its implementation, you’d want to have debug info for libc.so installed. Unfortunately the way to get debug info differs between Linux distros. On Fedora it’s as simple as debuginfo-install glibc. It takes more steps on Ubuntu and Debian. This RPM DPKG Rosetta Stone page have links to instructions for Fedora, Ubuntu and Debian (search for debuginfo).

Since you’re on Ubuntu 12.10 and actually want to see the strcpy() assembly source code:

$ sudo apt-get install libc6-dbg
$ sudo apt-get source libc6-dev
$ gdb ./a.out
(gdb) directory eglibc-2.15/sysdeps
Source directories searched: /home/scottt/eglibc-2.15/sysdeps:$cdir:$cwd
(gdb) break strcpy
Breakpoint 1 at 0x400450
(gdb) run
Starting program: /home/scottt/a.out 

Breakpoint 1, __strcpy_sse2 () at ../sysdeps/x86_64/multiarch/../strcpy.S:32
32      movq %rsi, %rcx     /* Source register. */

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