Which linux system call is used by ls command in linux to display the folder/file name?

c linux kernel system-calls ls
footy picture footy · Oct 16, 2012 · Viewed 19.7k times · Source

I wanted to know which system call is used in linux by the ls command to display the folder's (or file's name)? Especially the files/folders starting with "." (dot)

I executed the strace ls -a command to look at the system calls.There is a lot of fstat calls which occur for the all the other attributes (inode to permisisons). Which one actually gives it's name?

execve("/bin/ls", ["ls", "-a"], [/* 37 vars */]) = 0
brk(0)                                  = 0x1762000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1c14025000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=109464, ...}) = 0
mmap(NULL, 109464, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f1c1400a000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\20T\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=121936, ...}) = 0
mmap(NULL, 2221680, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1c13be6000
mprotect(0x7f1c13c03000, 2093056, PROT_NONE) = 0
mmap(0x7f1c13e02000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c000) = 0x7f1c13e02000
mmap(0x7f1c13e04000, 1648, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f1c13e04000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/librt.so.1", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340!\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=31752, ...}) = 0
mmap(NULL, 2128984, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1c139de000
mprotect(0x7f1c139e5000, 2093056, PROT_NONE) = 0
mmap(0x7f1c13be4000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x6000) = 0x7f1c13be4000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libacl.so.1", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P\33\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=31096, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1c14009000
mmap(NULL, 2126312, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1c137d6000
mprotect(0x7f1c137dd000, 2093056, PROT_NONE) = 0
mmap(0x7f1c139dc000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x6000) = 0x7f1c139dc000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\200\30\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1811128, ...}) = 0
mmap(NULL, 3925208, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1c13417000
mprotect(0x7f1c135cc000, 2093056, PROT_NONE) = 0
mmap(0x7f1c137cb000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b4000) = 0x7f1c137cb000
mmap(0x7f1c137d1000, 17624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f1c137d1000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340\r\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=14768, ...}) = 0
mmap(NULL, 2109704, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1c13213000
mprotect(0x7f1c13215000, 2097152, PROT_NONE) = 0
mmap(0x7f1c13415000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x2000) = 0x7f1c13415000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\200l\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=135366, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1c14008000
mmap(NULL, 2212904, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1c12ff6000
mprotect(0x7f1c1300e000, 2093056, PROT_NONE) = 0
mmap(0x7f1c1320d000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x17000) = 0x7f1c1320d000
mmap(0x7f1c1320f000, 13352, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f1c1320f000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libattr.so.1", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340\17\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=18552, ...}) = 0
mmap(NULL, 2113736, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f1c12df1000
mprotect(0x7f1c12df5000, 2093056, PROT_NONE) = 0
mmap(0x7f1c12ff4000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x3000) = 0x7f1c12ff4000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1c14007000
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1c14005000
arch_prctl(ARCH_SET_FS, 0x7f1c140057c0) = 0
mprotect(0x7f1c137cb000, 16384, PROT_READ) = 0
mprotect(0x7f1c12ff4000, 4096, PROT_READ) = 0
mprotect(0x7f1c1320d000, 4096, PROT_READ) = 0
mprotect(0x7f1c13415000, 4096, PROT_READ) = 0
mprotect(0x7f1c139dc000, 4096, PROT_READ) = 0
mprotect(0x7f1c13be4000, 4096, PROT_READ) = 0
mprotect(0x7f1c13e02000, 4096, PROT_READ) = 0
mprotect(0x618000, 4096, PROT_READ)     = 0
mprotect(0x7f1c14027000, 4096, PROT_READ) = 0
munmap(0x7f1c1400a000, 109464)          = 0
set_tid_address(0x7f1c14005a90)         = 4490
set_robust_list(0x7f1c14005aa0, 0x18)   = 0
futex(0x7fff2e09d99c, FUTEX_WAIT_BITSET_PRIVATE|FUTEX_CLOCK_REALTIME, 1, NULL, 7f1c140057c0) = -1 EAGAIN (Resource temporarily unavailable)
rt_sigaction(SIGRTMIN, {0x7f1c12ffc750, [], SA_RESTORER|SA_SIGINFO, 0x7f1c13005cb0}, NULL, 8) = 0
rt_sigaction(SIGRT_1, {0x7f1c12ffc7e0, [], SA_RESTORER|SA_RESTART|SA_SIGINFO, 0x7f1c13005cb0}, NULL, 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [RTMIN RT_1], NULL, 8) = 0
getrlimit(RLIMIT_STACK, {rlim_cur=8192*1024, rlim_max=RLIM_INFINITY}) = 0
statfs("/selinux", {f_type="EXT2_SUPER_MAGIC", f_bsize=4096, f_blocks=238304997, f_bfree=232078639, f_bavail=219973436, f_files=60530688, f_ffree=60120220, f_fsid={-1173666966, -474985328}, f_namelen=255, f_frsize=4096}) = 0
brk(0)                                  = 0x1762000
brk(0x1783000)                          = 0x1783000
open("/proc/filesystems", O_RDONLY)     = 3
fstat(3, {st_mode=S_IFREG|0444, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1c14024000
read(3, "nodev\tsysfs\nnodev\trootfs\nnodev\tb"..., 1024) = 328
read(3, "", 1024)                       = 0
close(3)                                = 0
munmap(0x7f1c14024000, 4096)            = 0
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=7220736, ...}) = 0
mmap(NULL, 7220736, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f1c1270e000
close(3)                                = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(1, TIOCGWINSZ, {ws_row=41, ws_col=144, ws_xpixel=0, ws_ypixel=0}) = 0
openat(AT_FDCWD, ".", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
getdents(3, /* 2 entries */, 32768)     = 48
getdents(3, /* 0 entries */, 32768)     = 0
close(3)                                = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1c14024000
write(1, ".  ..\n", 6.  ..
)                  = 6
close(1)                                = 0
munmap(0x7f1c14024000, 4096)            = 0
close(2)                                = 0
exit_group(0)                           = ?

Answer

Adam Rosenfield picture Adam Rosenfield · Oct 16, 2012

Most of the system calls there are noise from loading shared libraries at startup. The interesting things happen here:

openat(AT_FDCWD, ".", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
getdents(3, /* 2 entries */, 32768)     = 48
getdents(3, /* 0 entries */, 32768)     = 0
close(3)

The openat(2) system call is used to open the current directory (".") relative to the current working directory (the AT_FDCWD flag). The O_DIRECTORY flag indicates that it wants to open the directory and read the directory's contents.

The actual directory data is read using the getdents(2) system call. In this case, it called it twice, since until it returns 0, it's not sure if there's more data or not. Finally, the file descriptor is closed after it's done.

If you were to write your own program, however, you wouldn't call these directly -- instead you'd use opendir(3), readdir(3), and closedir(3) to read a directory. They're portable (POSIX-compliant), and they insulate you from the details of the underlying system calls. They're also easier to use, IMO.

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