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

# explain\_ftell(3)

explain ftell(3) errors

## NAME

explain\_ftell - explain [*ftell*(3)](/ftell/3.md) errors

## SYNOPSIS

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

const char \*explain\_ftell(FILE \*fp);\
 const char \*explain\_errno\_ftell(int errnum, FILE \*fp);\
 void explain\_message\_ftell(char \*message, int message\_size, FILE \*fp);\
 void explain\_message\_errno\_ftell(char \*message, int message\_size, int errnum, FILE \*fp);

## DESCRIPTION

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

### explain\_ftell

const char \*explain\_ftell(FILE \*fp);

The **explain\_ftell** function is used to obtain an explanation of an error returned by the [*ftell*(3)](/ftell/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 [*ftell*(3)](/ftell/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:

`long result = ftell(fp);`\
 `if (result < 0)`

```
{
```

`fprintf(stderr, "%s\n", explain_ftell(fp));`

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

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

### explain\_errno\_ftell

const char \*explain\_errno\_ftell(int errnum, FILE \*fp);

The **explain\_errno\_ftell** function is used to obtain an explanation of an error returned by the [*ftell*(3)](/ftell/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 [*ftell*(3)](/ftell/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:

`long result = ftell(fp);`\
 `if (result < 0)`

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

`fprintf(stderr, "%s\n", explain_errno_ftell(err, fp));`

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

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

### explain\_message\_ftell

void explain\_message\_ftell(char \*message, int message\_size, FILE \*fp);

The **explain\_message\_ftell** function is used to obtain an explanation of an error returned by the [*ftell*(3)](/ftell/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 [*ftell*(3)](/ftell/3.md) system call.

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

`long result = ftell(fp);`\
 `if (result < 0)`

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

`explain_message_ftell(message, sizeof(message), fp);`

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

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

### explain\_message\_errno\_ftell

void explain\_message\_errno\_ftell(char \*message, int message\_size, int errnum, FILE \*fp);

The **explain\_message\_errno\_ftell** function is used to obtain an explanation of an error returned by the [*ftell*(3)](/ftell/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 [*ftell*(3)](/ftell/3.md) system call.

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

`long result = ftell(fp);`\
 `if (result < 0)`

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

`explain_message_errno_ftell(message, sizeof(message), err, fp);`

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

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

## SEE ALSO

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

  reposition a stream

- [*explain\_ftell\_or\_die*(3)](/explain_ftell_or_die/3.md)

  reposition a stream and report errors

## COPYRIGHT

libexplain version 1.4\
 Copyright (C) 2010 Peter Miller