This simple program is a Hello World example that does not use the main() function. This is certainly possible in a C program.
#define syscall(a, D, S, d) __asm__ __volatile__("syscall" : : "a"(a), "D"(D), "S"(S), "d"(d)) void _start(void) { syscall(1, 1, "Hello, World\n", 14); syscall(60, 0, 0, 0); } |
This is how to compile this program. Then the Hello World text will print to the terminal. The -nostartfiles parameter to gcc tells it that we are not using the main() function that is called by the operating system to run the program code.
jason@jason-desktop:~/Documents$ gcc syscall.c -nostartfiles -o sycall |
And it works perfectly.
jason@jason-desktop:~/Documents$ ./sycall Hello, World |
Notice in this strace output, it is still using the write() system call to print the text.
jason@jason-desktop:~/Documents$ strace ./sycall execve("./sycall", ["./sycall"], [/* 51 vars */]) = 0 brk(NULL) = 0x564b96c3f000 access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory) mmap(NULL, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f398360c000 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=102631, ...}) = 0 mmap(NULL, 102631, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f39835f2000 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\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\20\5\2\0\0\0\0\0"..., 832) = 832 fstat(3, {st_mode=S_IFREG|0755, st_size=1856752, ...}) = 0 mmap(NULL, 3959200, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f3983023000 mprotect(0x7f39831e1000, 2093056, PROT_NONE) = 0 mmap(0x7f39833e0000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1bd000) = 0x7f39833e0000 mmap(0x7f39833e6000, 14752, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f39833e6000 close(3) = 0 mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f39835f0000 arch_prctl(ARCH_SET_FS, 0x7f39835f0700) = 0 mprotect(0x7f39833e0000, 16384, PROT_READ) = 0 mprotect(0x564b9633f000, 4096, PROT_READ) = 0 mprotect(0x7f398360f000, 4096, PROT_READ) = 0 munmap(0x7f39835f2000, 102631) = 0 write(1, "Hello, World\n\0", 14Hello, World ) = 14 exit(0) = ? +++ exited with 0 +++ |
But this programming trick really works and is worth trying out in your own programs.
Here is how to get the return value of a program on a UNIX or Linux machine.
jason@jason-desktop:~/Documents$ ./sycall && echo $? Hello, World 0 |