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

# explain\_shmat(3)

explain shmat(2) errors

## NAME

explain\_shmat - explain [*shmat*(2)](/shmat/2.md) errors

## SYNOPSIS

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

const char \*explain\_shmat(int shmid, const void \*shmaddr, int shmflg);\
 const char \*explain\_errno\_shmat(int errnum, int shmid, const void \*shmaddr, int shmflg);\
 void explain\_message\_shmat(char \*message, int message\_size, int shmid, const void \*shmaddr, int shmflg);\
 void explain\_message\_errno\_shmat(char \*message, int message\_size, int errnum, int shmid, const void \*shmaddr, int shmflg);

## DESCRIPTION

These functions may be used to obtain explanations for errors returned by the [*shmat*(2)](/shmat/2.md) system call.

### explain\_shmat

const char \*explain\_shmat(int shmid, const void \*shmaddr, int shmflg);

The **explain\_shmat** function is used to obtain an explanation of an error returned by the [*shmat*(2)](/shmat/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 *errno* global variable will be used to obtain the error value to be decoded.

- *shmid*

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

- *shmaddr*

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

- *shmflg*

  The original shmflg, exactly as passed to the [*shmat*(2)](/shmat/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.

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

`void *result = shmat(shmid, shmaddr, shmflg);`\
 `if (!result)`

```
{
```

`fprintf(stderr, "%s\n", explain_shmat(shmid, shmaddr, shmflg));`

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

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

### explain\_errno\_shmat

const char \*explain\_errno\_shmat(int errnum, int shmid, const void \*shmaddr, int shmflg);

The **explain\_errno\_shmat** function is used to obtain an explanation of an error returned by the [*shmat*(2)](/shmat/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.

- *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*.

- *shmid*

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

- *shmaddr*

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

- *shmflg*

  The original shmflg, exactly as passed to the [*shmat*(2)](/shmat/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.

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

`void *result = shmat(shmid, shmaddr, shmflg);`\
 `if (!result)`

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

`fprintf(stderr, "%s\n", explain_errno_shmat(err, shmid, shmaddr, shmflg));`

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

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

### explain\_message\_shmat

void explain\_message\_shmat(char \*message, int message\_size, int shmid, const void \*shmaddr, int shmflg);

The **explain\_message\_shmat** function is used to obtain an explanation of an error returned by the [*shmat*(2)](/shmat/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 *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.

- *shmid*

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

- *shmaddr*

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

- *shmflg*

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

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

`void *result = shmat(shmid, shmaddr, shmflg);`\
 `if (!result)`

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

`explain_message_shmat(message, sizeof(message), shmid, shmaddr, shmflg);`

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

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

### explain\_message\_errno\_shmat

void explain\_message\_errno\_shmat(char \*message, int message\_size, int errnum, int shmid, const void \*shmaddr, int shmflg);

The **explain\_message\_errno\_shmat** function is used to obtain an explanation of an error returned by the [*shmat*(2)](/shmat/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.

- *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*.

- *shmid*

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

- *shmaddr*

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

- *shmflg*

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

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

`void *result = shmat(shmid, shmaddr, shmflg);`\
 `if (!result)`

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

`explain_message_errno_shmat(message, sizeof(message), err, shmid, shmaddr,` `shmflg);`

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

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

## SEE ALSO

- [*shmat*(2)](/shmat/2.md)

  shared memory attach

- [*explain\_shmat\_or\_die*(3)](/explain_shmat_or_die/3.md)

  shared memory attach and report errors

## COPYRIGHT

libexplain version 1.4\
 Copyright (C) 2011 Peter Miller