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```{index} single: call (Peta::FFI)
```
```{index} single: Peta::FFI::call
```

# call

Invoke a C function from an opened library and return its result.

## Synopsis

```perl
my $n = Peta::FFI::call($lib, "strlen",   "(p)L", "hello");      # 5
my $r = Peta::FFI::call($lib, "sqrt",     "(d)d", 2.0);          # 1.4142...
my $s = Peta::FFI::call($lib, "getenv",   "(p)p", "PATH");       # /usr/bin:...
Peta::FFI::call($lib, "srand", "(i)v", 42);                      # void return
```

## Arguments

- `$lib` - handle returned by `dlopen`.
- `$func` - symbol name to look up in the library.
- `$sig` - signature string `(args)ret`; see the module-level
  type-code table.
- the remaining arguments - one value per type code in `args`,
  coerced to the matching C type before the call.

## What you get back

A single scalar whose shape depends on the return code:

- `i` / `l` / `L` - integer.
- `d` / `f` - float.
- `p` - the returned `const char *` copied into a Perl string;
  `undef` if the C function returned `NULL`.
- `v` - `1`, so the call chains naturally in boolean contexts.
- `P` - pointer returned as a string (same rules as `p`).

## Output buffers (`P`)

When an argument position is `P`, `call` allocates a zeroed buffer
sized from the scalar you pass in. If the scalar is a string, its
byte length (minimum 16) is used; otherwise the numeric value of
the scalar (minimum 16) is taken as the byte size. The callee
receives a pointer to that buffer. The buffer is freed after the
call returns - if you need to read bytes the C function wrote into
it, wrap the call in a helper module such as `Peta::FFI::UUID` that
knows how to recover the buffer contents.

## Edge cases

- **Wrong argument count** - croaks with
  `Peta::FFI::call FNAME: expected N args, got M`.
- **Bad signature string** - croaks with
  `FFI signature error: ...` describing the offending character or
  missing `)`.
- **Symbol not found** - croaks with
  `Peta::FFI::call: symbol not found: FNAME` or the message reported
  by `dlerror`.
- **Incorrect type codes** - calling a function with a signature
  that does not match its real prototype is undefined behaviour.
  Check the C header before writing the signature.
- **String argument contains a null byte** - croaks with
  `Peta::FFI::call: CString error: ...`.