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

# explain\_fread(3)

explain fread(3) errors

## NAME

explain\_fread - explain [fread(3)](/fread/3.md) errors

## SYNOPSIS

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

const char \*explain\_fread(void \*ptr, size\_t size, size\_t nmemb, FILE \*fp);\
 const char \*explain\_errno\_fread(int errnum, void \*ptr, size\_t size, size\_t nmemb, FILE \*fp);\
 void explain\_message\_fread(char \*message, int message\_size, void \*ptr, size\_t size, size\_t nmemb, FILE \*fp);\
 void explain\_message\_errno\_fread(char \*message, int message\_size, int errnum, void \*ptr, size\_t size, size\_t nmemb, FILE \*fp);

## DESCRIPTION

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

### explain\_fread

const char \*explain\_fread(void \*ptr, size\_t size, size\_t nmemb, FILE \*fp);

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

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

```
size_t how_many = fread(ptr, size, nmemb, fp);
if (how_many == 0 && ferror(fp))
{
    fprintf(stderr, "%s\n", explain_fread(ptr, size, nmemb, fp));
    exit(EXIT_FAILURE);
}
```

- *ptr*

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

- *size*

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

- *nmemb*

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

- *fp*

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

### explain\_errno\_fread

const char \*explain\_errno\_fread(int errnum, void \*ptr, size\_t size, size\_t nmemb, FILE \*fp);

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

```
size_t how_many = fread(ptr, size, nmemb, fp);
if (how_many == 0 && ferror(fp))
{
    int err = errno;
    fprintf(stderr, "%s\n", explain_errno_fread(err, ptr, size, nmemb, fp));
    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*.

- *ptr*

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

- *size*

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

- *nmemb*

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

- *fp*

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

### explain\_message\_fread

void explain\_message\_fread(char \*message, int message\_size, void \*ptr, size\_t size, size\_t nmemb, FILE \*fp);

The **explain\_message\_fread** function may be used to  obtain an explanation of an error returned by the [*fread*(3)](/fread/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.

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

```
size_t how_many = fread(ptr, size, nmemb, fp);
if (how_many == 0 && ferror(fp))
{
    char message[3000];
    explain_message_fread(message, sizeof(message), ptr, size, nmemb, fp);
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}
```

- *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.

- *ptr*

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

- *size*

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

- *nmemb*

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

- *fp*

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

### explain\_message\_errno\_fread

void explain\_message\_errno\_fread(char \*message, int message\_size, int errnum, void \*ptr, size\_t size, size\_t nmemb, FILE \*fp);

The **explain\_message\_errno\_fread** function may be used to obtain an explanation of an error returned by the [*fread*(3)](/fread/3.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:

```
size_t how_many = fread(ptr, size, nmemb, fp);
if (how_many == 0 && ferror(fp))
{
    int err = errno;
    char message[3000];
    explain_message_errno_fread(message, sizeof(message), err,
        ptr, size, nmemb, fp);
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}
```

- *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*.

- *ptr*

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

- *size*

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

- *nmemb*

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

- *fp*

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

## SEE ALSO

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

  binary stream input

- [*explain\_fread\_or\_die*(3)](/explain_fread_or_die/3.md)

  binary stream input and report errors

## COPYRIGHT

libexplain version 1.4\
 Copyright (C) 2008 Peter Miller