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

# explain\_write(3)

explain write(2) errors

## NAME

explain\_write - explain [write(2)](/write/2.md) errors

## SYNOPSIS

\#include \<libexplain/write.h\>\
 const char \*explain\_write(int fildes, const void \*data, long data\_size);\
 const char \*explain\_errno\_write(int errnum, int fildes, const void \*data, long data\_size);\
 void explain\_message\_write(char \*message, int message\_size, int fildes, const void \*data, long data\_size);\
 void explain\_message\_errno\_write(char \*message, int message\_size, int errnum, int fildes, const void \*data, long data\_size);

## DESCRIPTION

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

### explain\_write

const char \*explain\_write(int fildes, const void \*data, long data\_size);

The explain\_write function may be used to obtain a human readable explanation of what went wrong in a [*write*(2)](/write/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 error number will be picked up from the *errno* global variable.

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

```
sszie_t n = write(fd, data, data_size);
if (n < 0)
{
    fprintf(stderr, '%s0, explain_read(fd, data, data_size));
    exit(EXIT_FAILURE);
}
```

- *fildes*

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

- *data*

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

- *data\_size*

  The original data\_size, exactly as passed to the [*write*(2)](/write/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\_write

const char \*explain\_errno\_write(int errnum, int fildes, const void \*data, long data\_size);

The explain\_errno\_write function may be used to obtain a human readable explanation of what went wrong in a [*write*(2)](/write/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:

```
sszie_t n = write(fd, data, data_size);
if (n < 0)
{
    int err = errno;
    fprintf(stderr, '%s0, explain_errno_read(errnum, fd, data,
        data_size));
    exit(EXIT_FAILURE);
}
```

- *errnum*

  The error value to be decoded, usually obtain 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 [*write*(2)](/write/2.md) system call.

- *data*

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

- *data\_size*

  The original data\_size, exactly as passed to the [*write*(2)](/write/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\_write

void explain\_message\_write(char \*message, int message\_size, int fildes, const void \*data, long data\_size);

The explain\_message\_write function may be used to obtain a human readable explanation of what went wrong in a [*write*(2)](/write/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 error number will be picked up from the *errno* global variable.

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

```
sszie_t n = write(fd, data, data_size);
if (n < 0)
{
    char message[3000];
    explain_message_read(message, sizeof(message), fd, data,
        data_size));
    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.

- *fildes*

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

- *data*

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

- *data\_size*

  The original data\_size, exactly as passed to the [*write*(2)](/write/2.md) system call.

**Note:** Given a suitably thread safe buffer, this function is thread safe.

### explain\_message\_errno\_write

void explain\_message\_errno\_write(char \* message, int message\_size, int errnum, int fildes, const void \*data, long data\_size);

The explain\_message\_errno\_write function may be used to obtain a human readable explanation of what went wrong in a [*write*(2)](/write/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:

```
sszie_t n = write(fd, data, data_size);
if (n < 0)
{
    int err = errno;
    char message[3000];
    explain_message_errno_read(message, sizeof(message), errno,
        fd, data, data_size));
    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 obtain 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 [*write*(2)](/write/2.md) system call.

- *data*

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

- *data\_size*

  The original data\_size, exactly as passed to the [*write*(2)](/write/2.md) system call.

**Note:** Given a suitably thread safe buffer, this function is thread safe.

## COPYRIGHT

libexplain version 1.4\
 Copyright (C) 2008 Peter Miller

## AUTHOR

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