Blog O’ Matty

Using the Linux ltrace utility to trace library calls

While debugging an application on a Linux server this week, I needed to view the library calls (specifically malloc and free) that were being called by a process. This was super easy to do with the Linux ltrace(1) utility:

$ ltrace /bin/cat

__libc_start_main(0x8048f49, 1, 0xbfbec464, 0x804ae98, 0x804aeec 
setlocale(6, "")                                      = "en_US.UTF-8"
bindtextdomain("coreutils", "/usr/share/locale")      = "/usr/share/locale"
textdomain("coreutils")                               = "coreutils"
__cxa_atexit(0x8048f1f, 0, 0, 0x804cb70, 0xbfbec3d8)  = 0
getopt_long(1, 0xbfbec464, "benstuvAET", 0x804c9c0, NULL) = -1
__fxstat64(3, 1, 0xbfbec360)                          = 0
__fxstat64(3, 0, 0xbfbec360)                          = 0
malloc(1024)                                          = 0x8c5f858
read(0,

ltrace(1) also allows you to selectively trace library calls when executed with the “-e” option and a set of calls to trace:

$ ltrace -e malloc /bin/cat

malloc(1024)                                          = 0x8883858

If you need to view the library calls from a live process, you can use ltrace(1)’s “-p” option:

$ ps auxw | grep [g]am_server

root      2644  0.0  0.2   2212  1064 ?        S    12:51   0:00 /usr/libexec/gam_server

$ ltrace -p 2644

--- SIGSTOP (Stopped (signal)) ---
--- SIGSTOP (Stopped (signal)) ---
time(NULL)                                                                      = 1130003202
__xstat64(3, "/etc/xdg/menus/server-settings-m"..., 0xbfe342bc)                 = -1
__errno_location()                                                              = 0xb7f266a0
memcpy(0x84a56e0, "|q\221", 96)                                                 = 0x84a56e0
g_dir_open(0x84a5748, 0, 0, 0x8b59c1, 0x8499fe8)                                = 0
__xstat64(3, "/root/.config/menus/server-setti"..., 0xbfe342bc)                 = -1
__errno_location()                                                              = 0xb7f266a0
memcpy(0x84a55f8, "\330B\343\277_i\256", 96)                                    = 0x84a55f8
g_dir_open(0x84a5660, 0, 0, 0x8b59c1, 0x8499fe8)                                = 0
__xstat64(3, "/etc/xdg/menus/system-settings-m"..., 0xbfe342bc)                 = -1

This is a super cool utility, and is invaluable tool for debugging application problems on Linux systems.