Menene system call, kuma menene user mode da kernel mode?

Question

IT Interview Practice · Accepted Answer

**System call (syscall)** hanya ce da shirin user-space ke amfani da ita don neman sabis daga kernel — buɗe file, aika bayanan network, ware memory, ƙirƙirar process. Saboda waɗannan ayyuka suna taɓa hardware ko suna shafar wasu processes, kernel kaɗai ne zai iya yin su, don haka syscall da gangan yana ƙetare iyakar kariya.

## User mode da kernel mode

CPU yana gudana a ɗaya daga cikin (aƙalla) matakan gata biyu:

```text
 user mode   → restricted: no direct hardware access, no privileged instructions
 kernel mode → full access: hardware, all memory, privileged CPU instructions
```

Kiran function na yau da kullum yana zama a user mode. Syscall yana yin **mode switch** mai sarrafawa zuwa kernel mode, yana gudanar da amintaccen kernel code, sa'an nan yana dawowa:

```text
  user code            kernel
  ─────────            ──────
  read(fd, buf, n)
      │  trap (syscall instruction)
      └──────────────▶ enter kernel mode
                       validate args, do the I/O
      ◀────────────── return value, back to user mode
  continue
```

## Yadda yake aiki a aikace

Ba safai kake rubuta trap ɗin da kanka ba — C library ne ke naɗe shi. `fopen`/`fread` a cikin code ɗinka a ƙarshe suna kiran syscalls na `open`/`read`:

```c
// glibc's read() places the syscall number + args in registers,
// then executes a trap instruction (e.g. `syscall` on x86-64).
ssize_t n = read(fd, buffer, 4096);   // → traps into the kernel
```

Trap ɗin (software interrupt / umarni na musamman) shi ne abin da ke canza matakin gata cikin aminci — user code ba zai iya kawai "tsalle zuwa cikin" kernel ba; zai iya shiga ne kawai a wuraren shigar syscall da aka ayyana waɗanda ke tabbatar da arguments ɗinsu.

## Me ya sa yake da muhimmanci

Rarrabuwar user/kernel ita ce tushen tsaro da kwanciyar hankali na OS: shi ne dalilin da ya sa process ɗaya ba zai iya rubuta a kan hardware ko memory na wani process ba. Fahimtar syscalls tana kuma bayyana aiki (performance) — kowanne yana kashe mode switch (ɗaruruwan nanoseconds), shi ya sa haɗa I/O a matsayin batch ko amfani da `mmap`/`io_uring` don rage yawan syscall ke da muhimmanci, kuma shi ya sa `strace` (wanda ke jera syscalls na shiri) yake da ƙarfin gaske a matsayin kayan aikin debug.

User mode da kernel mode

Yadda yake aiki a aikace

Me ya sa yake da muhimmanci

Tambayoyi masu alaƙa