Basic file truncate program (GLibc)

1_truncate.c

#include <fcntl.h>

int main(int argc, char* argv[])
{
    if (argc < 2) {
        return(2);
    }

    if (open(argv[1], O_TRUNC | O_WRONLY) < 0) {
        return(1);
    }

    return(0);
}

2_example.md

Versions

$ gcc --version;

gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Copyright (C) 2021 Free Software Foundation, Inc.

Compile

$ gcc -o truncate ./truncate.c &&
ls -lA &&
ldd ./truncate;

total 20
-rwxrwxr-x 1 user user 15960 Dec 28 01:37 truncate
-rw-rw-r-- 1 user user   193 Dec 28 01:31 truncate.c
        linux-vdso.so.1 (0x00007ffef053c000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f4fe0edd000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f4fe1116000)

Execute

$ printf '1234\n' > data &&
cat ./data &&
strace -o ./strace.log -- \
    ./truncate ./data &&
stat ./data;

1234
  File: ./data
  Size: 5               Blocks: 8          IO Block: 4096   regular file
Device: 802h/2050d      Inode: 790958      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/ user)   Gid: ( 1000/ user)
Access: 2024-12-28 01:48:56.816627734 +0200
Modify: 2024-12-28 01:48:56.816627734 +0200
Change: 2024-12-28 01:48:56.816627734 +0200
 Birth: 2024-12-28 01:38:20.051208301 +0200
  File: ./data
  Size: 0               Blocks: 0          IO Block: 4096   regular empty file
Device: 802h/2050d      Inode: 790958      Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/ user)   Gid: ( 1000/ user)
Access: 2024-12-28 01:48:56.816627734 +0200
Modify: 2024-12-28 01:48:56.820627793 +0200
Change: 2024-12-28 01:48:56.820627793 +0200
 Birth: 2024-12-28 01:38:20.051208301 +0200

3_strace.log

execve("./truncate", ["./truncate", "./data"], 0x7ffed8d35bc8 /* 38 vars */) = 0
brk(NULL)                               = 0x56115b782000
arch_prctl(0x3001 /* ARCH_??? */, 0x7ffc316e4b20) = -1 EINVAL (Invalid argument)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f25c760c000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
newfstatat(3, "", {st_mode=S_IFREG|0644, st_size=34991, ...}, AT_EMPTY_PATH) = 0
mmap(NULL, 34991, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f25c7603000
close(3)                                = 0
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P\237\2\0\0\0\0\0"..., 832) = 832
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
pread64(3, "\4\0\0\0 \0\0\0\5\0\0\0GNU\0\2\0\0\300\4\0\0\0\3\0\0\0\0\0\0\0"..., 48, 848) = 48
pread64(3, "\4\0\0\0\24\0\0\0\3\0\0\0GNU\0I\17\357\204\3$\f\221\2039x\324\224\323\236S"..., 68, 896) = 68
newfstatat(3, "", {st_mode=S_IFREG|0755, st_size=2220400, ...}, AT_EMPTY_PATH) = 0
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784
mmap(NULL, 2264656, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f25c73da000
mprotect(0x7f25c7402000, 2023424, PROT_NONE) = 0
mmap(0x7f25c7402000, 1658880, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x28000) = 0x7f25c7402000
mmap(0x7f25c7597000, 360448, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1bd000) = 0x7f25c7597000
mmap(0x7f25c75f0000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x215000) = 0x7f25c75f0000
mmap(0x7f25c75f6000, 52816, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f25c75f6000
close(3)                                = 0
mmap(NULL, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f25c73d7000
arch_prctl(ARCH_SET_FS, 0x7f25c73d7740) = 0
set_tid_address(0x7f25c73d7a10)         = 2740651
set_robust_list(0x7f25c73d7a20, 24)     = 0
rseq(0x7f25c73d80e0, 0x20, 0, 0x53053053) = 0
mprotect(0x7f25c75f0000, 16384, PROT_READ) = 0
mprotect(0x56112cea6000, 4096, PROT_READ) = 0
mprotect(0x7f25c7646000, 8192, PROT_READ) = 0
prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0
munmap(0x7f25c7603000, 34991)           = 0
openat(AT_FDCWD, "./data", O_WRONLY|O_TRUNC) = 3
exit_group(0)                           = ?
+++ exited with 0 +++

4_references.md

- https://man7.org/linux/man-pages/man2/open.2.html