syscall read acting weird - c

Question

c lang, ubuntu
so i have a task - write a menu with these 3 options:
1. close program
2. show user id
3. show current working directory
i can only use 3 libraries - unistd.h, sys/syscall.h, sys/sysinfo.h.
so no printf/scanf
i need to use an array of a struct im given, that has a function pointer,
to call the function the user wants to use.
problem is on options 2 & 3;
when i pick 2, on the first time it works fine (i think)
second time i pick 2, it works, but then when going to the third iteration,
it doesn't wait for an input, it takes '\n' as an input for some reason, then it says invalid input. (i checked what it takes as input with printf, i printed index after recalculating it and it because -39, so it means selection[0] = 10 = '\n')
that's the first problem, that i just cant find the solution for.
second problem is on the current working directory function;
the SYS_getcwd returns -1 for some reason, which means there's an error, but i cant figure it out.
any explanations for these things?
(also - slen and __itoa are functions i am given - slen returns the length of a string,
__itoa returns a char*, that was the string representation of an integer)
helper.h:
typedef struct func_desc {
char *name;
void (*func)(void);
} fun_desc;
main.c:
#include <unistd.h>
#include "helper.h"
#include <sys/syscall.h>
#include <sys/sysinfo.h>
void exitProgram();
void printID();
void currDir();
int main() {
fun_desc arrFuncs[3];
arrFuncs[0].name = "exitProgram";
arrFuncs[0].func = &exitProgram;
arrFuncs[1].name = "printID";
arrFuncs[1].func = &printID;
arrFuncs[2].name = "currDir";
arrFuncs[2].func = &currDir;
char selection[2];
int index;
const char* menu = "Welcome to the menu. Please pick one of the following actions:\n1.Close the program\n2.Print the current user's id\n3.Print the current directory's id\n";
while(1 == 1) {
syscall(SYS_write, 0, menu, slen(menu));
syscall(SYS_write, 0, "Your selection: ", slen("Your selection: "));
syscall(SYS_read, 1, selection, slen(selection)); //might be a problem
selection[1] = '\0';
index = selection[0] - '0' - 1;
if(index > 2)
syscall(SYS_write, 0, "Invalid input\n", slen("Invalid input\n"));
else
arrFuncs[index].func();
}
return(0);
}
void exitProgram() {
syscall(SYS_write, 0, "The program will close\n", slen("The program will close\n"));
syscall(SYS_exit);
}
void printID() { //problem
char* uid = __itoa(syscall(SYS_getuid));
syscall(SYS_write, 0, uid, slen(uid));
syscall(SYS_write, 0, "\n", slen("\n"));
}
void currDir() { //????
char* buf = __itoa(syscall(SYS_getcwd));
syscall(SYS_write, 0, buf, slen(buf));
syscall(SYS_write, 0, "\n", slen("\n"));
}

Answer 1

You're passing the wrong number of arguments to some of these system calls. In particular:
syscall(SYS_exit);
_exit() takes one argument: the exit code.
char* buf = __itoa(syscall(SYS_getcwd));
getcwd() takes two arguments: a pointer to a buffer to write the string to, and the length of that buffer. In practice, this probably looks something like:
char pathbuf[PATH_MAX];
syscall(SYS_getcwd, pathbuf, sizeof(pathbuf));
If you don't have the header which defines PATH_MAX, define it yourself. 4096 is an appropriate value.
Note that getcwd() writes a string into the buffer passed to it — it does not return a numeric identifier.
As an aside, you may want to save yourself some time by implementing a wrapper to write a string, e.g.
void putstring(const char *str) {
syscall(SYS_write, 0, str, slen(str));
}
since you seem to be doing that a lot.