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

# explain\_open(3)

explain open(2) errors

## NAME

explain\_open - explain [open(2)](/open/2.md) errors

## SYNOPSIS

\#include \<libexplain/open.h\>\
 const char \*explain\_open(const char \*pathname, int flags, int mode);\
 const char \*explain\_errno\_open(int errnum, const char \*pathname, int flags, int mode);\
 void explain\_message\_open(char \*message, int message\_size, const char \*pathname, int flags, int mode);\
 void explain\_message\_errno\_open(char \*message, int message\_size, int errnum, const char \*pathname, int flags, int mode);

## DESCRIPTION

These functions may be used to obtains explanations for [*open*(2)](/open/2.md) errors.

### explain\_open(const char \*pathname, int flags, int mode);

const char \*explain\_open(const char \*pathname, int flags, int mode);

The explain\_open function is used to obtain an explanation of an error returned by the [*open*(2)](/open/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:

```
int fd = open(pathname, flags, mode);
if (fd < 0)
{
    fprintf(stderr, '%s0, explain_open(pathname, flags, mode));
    exit(EXIT_FAILURE);
}
```

- *pathname*

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

- *flags*

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

- *mode*

  The original mode, exactly as passed to the [*open*(2)](/open/2.md) system call (or zero if the original call didn't need a mode argument).

- 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\_open

const char \*explain\_errno\_open(int errnum, const char \*pathname, int flags, int mode);

The explain\_errno\_open function is used to obtain an explanation of an error returned by the [*open*(2)](/open/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:

```
int fd = open(pathname, flags, mode);
if (fd < 0)
{
    int err = errno;
    fprintf(stderr, '%s0, explain_errno_open(err, pathname,
        flags, mode));
    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*.

- *pathname*

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

- *flags*

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

- *mode*

  The original mode, exactly as passed to the [*open*(2)](/open/2.md) system call (or zero if the original call didn't need a mode argument).

- 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\_open

void explain\_message\_open(char \*message, int message\_size, const char \*pathname, int flags, int mode);

The explain\_message\_open function is used to obtain an explanation of an error returned by the [*open*(2)](/open/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:

```
int fd = open(pathname, flags, mode);
if (fd < 0)
{
    char message[3000];
    explain_message_open(message, sizeof(message), pathname, flags,
        mode);
    fprintf(stderr, '%s0, message);
    exit(EXIT_FAILURE);
}
```

- *message*

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

- *message\_size*

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

- *pathname*

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

- *flags*

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

- *mode*

  The original mode, exactly as passed to the [*open*(2)](/open/2.md) system call (or zero if the original call didn't need a mode argument).

### explain\_message\_errno\_open

void explain\_message\_errno\_open(char \*message, int message\_size, int errnum, const char \*pathname, int flags, int mode);

The explain\_message\_errno\_open function is used to obtain an explanation of an error returned by the [*open*(2)](/open/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 exameple:

```
int fd = open(pathname, flags, mode);
if (fd < 0)
{
    int err = errno;
    char message[3000];
    explain_message_errno_open(message, sizeof(message), err, pathname,
        flags, mode);
    fprintf(stderr, '%s0, message);
    exit(EXIT_FAILURE);
}
```

- *message*

  The location in which to store the returned message.  Because a message return buffer has been 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*.

- *pathname*

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

- *flags*

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

- *mode*

  The original mode, exactly as passed to the [*open*(2)](/open/2.md) system call (or zero if the original call didn't need a mode argument).

## COPYRIGHT

libexplain version 1.4\
 Copyright (C) 2008 Peter Miller

## AUTHOR

Written by Peter Miller \<pmiller@opensource.org.au\>