---
name: "explain_fcntl"
section: "3"
platform: "linux"
package: "libexplain-dev"
version: "1.4.D001-16"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/explain_fcntl"
---

# explain\_fcntl(3)

explain fcntl(2) errors

## NAME

explain\_fcntl - explain [fcntl(2)](/fcntl/2.md) errors

## SYNOPSIS

\#include \<libexplain/fcntl.h\>

const char \*explain\_fcntl(int fildes, int command, long arg);\
 const char \*explain\_errno\_fcntl(int errnum, int fildes, int command, long arg);\
 void explain\_message\_fcntl(char \*message, int message\_size, int fildes, int command, long arg);\
 void explain\_message\_errno\_fcntl(char \*message, int message\_size, int errnum, int fildes, int command, long arg);

## DESCRIPTION

These functions may be used to obtain explanations for errors returned by the [*fcntl*(2)](/fcntl/2.md) system call.

### explain\_fcntl

const char \*explain\_fcntl(int fildes, int command, long arg);

The **explain\_fcntl** function is used to obtain an explanation of an error returned by the [*fcntl*(2)](/fcntl/2.md) system call.  The least the message will contain is the value of `strerror(errno)`, but usually it will do much better, and indicate the underlying cause in more detail.

The *errno* global variable will be used to obtain the error value to be decoded.

This function is intended to be used in a fashion similar to the following example:

```
if (fcntl(fildes, command, arg) < 0)
{
    fprintf(stderr, "%s\n", explain_fcntl(fildes, command, arg));
    exit(EXIT_FAILURE);
}
```

- *fildes*

  The original fildes, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- *command*

  The original command, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- *arg*

  The original arg, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- Returns:

  The message explaining the error.  This message buffer is shared by all libexplain functions which do not supply a buffer in their argument list.  This will be overwritten by the next call to any libexplain function which shares this buffer, including other threads.

**Note:** This function is **not** thread safe, because it shares a return buffer across all threads, and many other functions in this library.

### explain\_errno\_fcntl

const char \*explain\_errno\_fcntl(int errnum, int fildes, int command, long arg);

The **explain\_errno\_fcntl** function is used to obtain an explanation of an error returned by the [*fcntl*(2)](/fcntl/2.md) system call. The least the message will contain is the value of `strerror(errnum)`, but usually it will do much better, and indicate the underlying cause in more detail.

This function is intended to be used in a fashion similar to the following example:

```
if (fcntl(fildes, command, arg) < 0)
{
    int err = errno;
    fprintf(stderr, "%s\n", explain_errno_fcntl(err, fildes, command, arg));
    exit(EXIT_FAILURE);
}
```

- *errnum*

  The error value to be decoded, usually obtained from the *errno* global variable just before this function is called.  This is necessary if you need to call **any** code between the system call to be explained and this function, because many libc functions will alter the value of *errno*.

- *fildes*

  The original fildes, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- *command*

  The original command, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- *arg*

  The original arg, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- Returns:

  The message explaining the error.  This message buffer is shared by all libexplain functions which do not supply a buffer in their argument list.  This will be overwritten by the next call to any libexplain function which shares this buffer, including other threads.

**Note:** This function is **not** thread safe, because it shares a return buffer across all threads, and many other functions in this library.

### explain\_message\_fcntl

void explain\_message\_fcntl(char \*message, int message\_size, int fildes, int command, long arg);

The **explain\_message\_fcntl** function may be used to  obtain an explanation of an error returned by the [*fcntl*(2)](/fcntl/2.md) system call. The least the message will contain is the value of `strerror(errno)`, but usually it will do much better, and indicate the underlying cause in more detail.

The *errno* global variable will be used to obtain the error value to be decoded.

This function is intended to be used in a fashion similar to the following example:

```
if (fcntl(fildes, command, arg) < 0)
{
    char message[3000];
    explain_message_fcntl(message, sizeof(message), fildes, command, arg);
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}
```

- *message*

  The location in which to store the returned message. If a suitable message return buffer is supplied, this function is thread safe.

- *message\_size*

  The size in bytes of the location in which to store the returned message.

- *fildes*

  The original fildes, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- *command*

  The original command, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- *arg*

  The original arg, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

### explain\_message\_errno\_fcntl

void explain\_message\_errno\_fcntl(char \*message, int message\_size, int errnum, int fildes, int command, long arg);

The **explain\_message\_errno\_fcntl** function may be used to obtain an explanation of an error returned by the [*fcntl*(2)](/fcntl/2.md) system call. The least the message will contain is the value of `strerror(errnum)`, but usually it will do much better, and indicate the underlying cause in more detail.

This function is intended to be used in a fashion similar to the following example:

```
if (fcntl(fildes, command, arg) < 0)
{
    int err = errno;
    char message[3000];
    explain_message_errno_fcntl(message, sizeof(message), err, fildes,
        command, arg);
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}
```

- *message*

  The location in which to store the returned message. If a suitable message return buffer is supplied, this function is thread safe.

- *message\_size*

  The size in bytes of the location in which to store the returned message.

- *errnum*

  The error value to be decoded, usually obtained from the *errno* global variable just before this function is called.  This is necessary if you need to call **any** code between the system call to be explained and this function, because many libc functions will alter the value of *errno*.

- *fildes*

  The original fildes, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- *command*

  The original command, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

- *arg*

  The original arg, exactly as passed to the [*fcntl*(2)](/fcntl/2.md) system call.

## SEE ALSO

- [*fcntl*(2)](/fcntl/2.md)

  manipulate a file descriptor

- [*explain\_fcntl\_or\_die*(3)](/explain_fcntl_or_die/3.md)

  manipulate a file descriptor and report errors

## COPYRIGHT

libexplain version 1.4\
 Copyright (C) 2008 Peter Miller