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

# explain\_fseek(3)

explain fseek(3) errors

## NAME

explain\_fseek - explain [*fseek*(3)](/fseek/3.md) errors

## SYNOPSIS

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

const char \*explain\_fseek(FILE \*fp, long offset, int whence);\
 const char \*explain\_errno\_fseek(int errnum, FILE \*fp, long offset, int whence);\
 void explain\_message\_fseek(char \*message, int message\_size, FILE \*fp, long offset, int whence);\
 void explain\_message\_errno\_fseek(char \*message, int message\_size, int errnum, FILE \*fp, long offset, int whence);

## DESCRIPTION

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

### explain\_fseek

const char \*explain\_fseek(FILE \*fp, long offset, int whence);

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

- *fp*

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

- *offset*

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

- *whence*

  The original whence, exactly as passed to the [*fseek*(3)](/fseek/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:

`if (fseek(fp, offset, whence) < 0)`

```
{
```

`fprintf(stderr, "%s\n", explain_fseek(fp, offset, whence));`

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

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

### explain\_errno\_fseek

const char \*explain\_errno\_fseek(int errnum, FILE \*fp, long offset, int whence);

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

- *fp*

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

- *offset*

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

- *whence*

  The original whence, exactly as passed to the [*fseek*(3)](/fseek/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:

`if (fseek(fp, offset, whence) < 0)`

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

`fprintf(stderr, "%s\n", explain_errno_fseek(err, fp, offset, whence));`

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

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

### explain\_message\_fseek

void explain\_message\_fseek(char \*message, int message\_size, FILE \*fp, long offset, int whence);

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

- *fp*

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

- *offset*

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

- *whence*

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

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

`if (fseek(fp, offset, whence) < 0)`

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

`explain_message_fseek(message, sizeof(message), fp, offset, whence);`

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

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

### explain\_message\_errno\_fseek

void explain\_message\_errno\_fseek(char \*message, int message\_size, int errnum, FILE \*fp, long offset, int whence);

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

- *fp*

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

- *offset*

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

- *whence*

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

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

`if (fseek(fp, offset, whence) < 0)`

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

`explain_message_errno_fseek(message, sizeof(message), err, fp, offset,` `whence);`

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

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

## SEE ALSO

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

  reposition a stream

- [*explain\_fseek\_or\_die*(3)](/explain_fseek_or_die/3.md)

  reposition a stream and report errors

## COPYRIGHT

libexplain version 1.4\
 Copyright (C) 2010 Peter Miller