I am looking to write a program that will need to do low level work with processes (ie. using the fork system call, among others). This program is to be written in C++ and is to run only on Linux. Ideally, it will be portable across CPU architectures (ie. x86, x86_64, and arm) with nothing more than a recompile, but I only really need x86_64 support.
As each Linux system call takes a number of arguments and returns a number of arguments in cpu registers (often only 1 return value), then a C function wrapper for each system call is likely easy to make. Also, because, AFAIK, system calls, being implemented in the kernel, have identical arguments and return values, if different assembly-level implementations, the same C interface can be exposed.
Does such a thing exist? If so, how can I access it?
Where is its documentation (list of available functions, their arguments with an explanation, and an explanation of exactly what the function does)?
libc already includes the wrapper functions you're looking for. The prototypes for many of them are in #include <unistd.h>, as specified by POSIX.
C is the language of low-level systems program on Unix (and Linux), so this has been a thing since Unix existed. (Providing wrapper functions in libc is easier than teaching compilers the difference between function call and system calls, and allows for setting errno on errors. It also allows for tricks like LD_PRELOAD to intercept system calls in user-space.)
The man pages for system calls are in section 2, vs. section 3 for library functions (which might or might not use system calls as part of their implementation: math.h cos(3), ISO C stdio printf(3) and fwrite(3), vs. POSIX write(2)).
execve(2) is the system call.
See execl(3) and friends are also part of libc, and eventually call execve(2). They are convenience wrappers on top of it for constructing the argv array, doing $PATH lookup, and passing along the current process's environment. Thus they're classed as functions, not system calls.
See syscalls(2) for an overview, and complete list of system Linux calls with links to their man-page wrappers. (I've linked the Linux man pages, but there are also POSIX man pages for all of the standard system calls.)
In the unlikely case that you're not linking libc, you can use macros like MUSL's syscall2 / syscall3 / etc. macros (the number is the arg count) to inline the right asm on whatever platform. You use __NR_write from asm/unistd.h to get system call numbers.
But note that the raw Linux system calls might have small differences from the interface provided by the libc wrappers. For example, they won't check for pthreads cancellation points, and brk / sbrk requires bookkeeping in user-space by libc.
See SYSCALL_INLINE in Android for a portable raw sys_write() inline wrapper using MUSL macros.
But if you are using libc like a normal person for functions like malloc and printf, you should just use its system call wrapper functions.
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