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

# explain\_vsnprintf(3)

explain vsnprintf(3) errors

## NAME

explain\_vsnprintf - explain [*vsnprintf*(3)](/vsnprintf/3.md) errors

## SYNOPSIS

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

const char \*explain\_vsnprintf(char \*data, size\_t data\_size, const char \*format, va\_list ap);\
 const char \*explain\_errno\_vsnprintf(int errnum, char \*data, size\_t data\_size, const char \*format, va\_list ap);\
 void explain\_message\_vsnprintf(char \*message, int message\_size, char \*data, size\_t data\_size, const char \*format, va\_list ap);\
 void explain\_message\_errno\_vsnprintf(char \*message, int message\_size, int errnum, char \*data, size\_t data\_size, const char \*format, va\_list ap);

## DESCRIPTION

These functions may be used to obtain explanations for errors returned by the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

### explain\_vsnprintf

const char \*explain\_vsnprintf(char \*data, size\_t data\_size, const char \*format, va\_list ap);

The **explain\_vsnprintf** function is used to obtain an explanation of an error returned by the [*vsnprintf*(3)](/vsnprintf/3.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.

- *data*

  The original data, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *data\_size*

  The original data\_size, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *format*

  The original format, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *ap*

  The original ap, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.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.

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

`errno = 0;` `int result = vsnprintf(data, data_size, format, ap);`\
 `if (result < 0 && errno != 0)`

```
{
```

`fprintf(stderr, "%s\n", explain_vsnprintf(data, data_size, format, ap));`

```
    exit(EXIT_FAILURE);
}
```

The above code example is available pre‐packaged as the [*explain\_vsnprintf\_or\_die*(3)](/explain_vsnprintf_or_die/3.md) function.

### explain\_errno\_vsnprintf

const char \*explain\_errno\_vsnprintf(int errnum, char \*data, size\_t data\_size, const char \*format, va\_list ap);

The **explain\_errno\_vsnprintf** function is used to obtain an explanation of an error returned by the [*vsnprintf*(3)](/vsnprintf/3.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.

- *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*.

- *data*

  The original data, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *data\_size*

  The original data\_size, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *format*

  The original format, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *ap*

  The original ap, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.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.

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

`errno = 0;` `int result = vsnprintf(data, data_size, format, ap);`\
 `if (result < 0 && errno != 0)`

```
{
    int err = errno;
```

`fprintf(stderr, "%s\n", explain_errno_vsnprintf(err, data, data_size,` `format, ap));`

```
    exit(EXIT_FAILURE);
}
```

The above code example is available pre‐packaged as the [*explain\_vsnprintf\_or\_die*(3)](/explain_vsnprintf_or_die/3.md) function.

### explain\_message\_vsnprintf

void explain\_message\_vsnprintf(char \*message, int message\_size, char \*data, size\_t data\_size, const char \*format, va\_list ap);

The **explain\_message\_vsnprintf** function is used to obtain an explanation of an error returned by the [*vsnprintf*(3)](/vsnprintf/3.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.

- *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.

- *data*

  The original data, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *data\_size*

  The original data\_size, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *format*

  The original format, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *ap*

  The original ap, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

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

`errno = 0;` `int result = vsnprintf(data, data_size, format, ap);`\
 `if (result < 0 && errno != 0)`

```
{
    char message[3000];
```

`explain_message_vsnprintf(message, sizeof(message), data, data_size,` `format, ap);`

```
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}
```

The above code example is available pre‐packaged as the [*explain\_vsnprintf\_or\_die*(3)](/explain_vsnprintf_or_die/3.md) function.

### explain\_message\_errno\_vsnprintf

void explain\_message\_errno\_vsnprintf(char \*message, int message\_size, int errnum, char \*data, size\_t data\_size, const char \*format, va\_list ap);

The **explain\_message\_errno\_vsnprintf** function is used to obtain an explanation of an error returned by the [*vsnprintf*(3)](/vsnprintf/3.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.

- *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*.

- *data*

  The original data, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *data\_size*

  The original data\_size, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *format*

  The original format, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

- *ap*

  The original ap, exactly as passed to the [*vsnprintf*(3)](/vsnprintf/3.md) system call.

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

`errno = 0;` `int result = vsnprintf(data, data_size, format, ap);`\
 `if (result < 0 && errno != 0)`

```
{
    int err = errno;
    char message[3000];
```

`explain_message_errno_vsnprintf(message, sizeof(message), err, data,` `data_size, format, ap);`

```
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}
```

The above code example is available pre‐packaged as the [*explain\_vsnprintf\_or\_die*(3)](/explain_vsnprintf_or_die/3.md) function.

## SEE ALSO

- [*vsnprintf*(3)](/vsnprintf/3.md)

  formatted output conversion

- [*explain\_vsnprintf\_or\_die*(3)](/explain_vsnprintf_or_die/3.md)

  formatted output conversion and report errors

## COPYRIGHT

libexplain version 1.4\
 Copyright (C) 2010 Peter Miller