Using ltrace

ltrace does the same thing, but for library calls instead of system calls

$ ltrace -eopendir,closedir ls -R >/dev/null 
opendir(".")                                      = 0x080567c8
closedir(0x080567c8)                              = 0
qsort(0x08053a50, 16, 116, 0x0804b76c)            = <void>
opendir("./pictures")                             = 0x08057908
closedir(0x08057908)                              = 0
qsort(0x08053a50, 3, 116, 0x0804b76c)             = <void>
opendir("./movies")                               = 0x08056808
closedir(0x08056808)                              = 0
qsort(0x08053a50, 25, 116, 0x0804b76c)            = <void>
opendir("./movies/naughty")                       = 0x080579f0
closedir(0x080579f0)                              = 0
qsort(0x08053a50, 25, 116, 0x0804b76c)            = <void>