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explain_pread(3) FreeBSD Library Functions Manual explain_pread(3)

explain_pread - explain pread(2) errors

#include <libexplain/pread.h>

const char *explain_pread(int fildes, void *data, size_t data_size, off_t offset);
const char *explain_errno_pread(int errnum, int fildes, void *data, size_t data_size, off_t offset);
void explain_message_pread(char *message, int message_size, int fildes, void *data, size_t data_size, off_t offset);
void explain_message_errno_pread(char *message, int message_size, int errnum, int fildes, void *data, size_t data_size, off_t offset);

These functions may be used to obtain explanations for errors returned by the pread(2) system call.

const char *explain_pread(int fildes, void *data, size_t data_size, off_t offset);

The explain_pread function is used to obtain an explanation of an error returned by the pread(2) 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.

fildes
The original fildes, exactly as passed to the pread(2) system call.
data
The original data, exactly as passed to the pread(2) system call.
data_size
The original data_size, exactly as passed to the pread(2) system call.
offset
The original offset, exactly as passed to the pread(2) 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:


ssize_t result = pread(fildes, data, data_size, offset);
if (result < 0) 
{

fprintf(stderr, "%s\n", explain_pread(fildes, data, data_size, offset)); 
    exit(EXIT_FAILURE);
}

The above code example is available pre‐packaged as the explain_pread_or_die(3) function.

const char *explain_errno_pread(int errnum, int fildes, void *data, size_t data_size, off_t offset);

The explain_errno_pread function is used to obtain an explanation of an error returned by the pread(2) 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.
fildes
The original fildes, exactly as passed to the pread(2) system call.
data
The original data, exactly as passed to the pread(2) system call.
data_size
The original data_size, exactly as passed to the pread(2) system call.
offset
The original offset, exactly as passed to the pread(2) 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:


ssize_t result = pread(fildes, data, data_size, offset);
if (result < 0) 
{
    int err = errno;

fprintf(stderr, "%s\n", explain_errno_pread(err, fildes, data, data_size, offset)); 
    exit(EXIT_FAILURE);
}

The above code example is available pre‐packaged as the explain_pread_or_die(3) function.

void explain_message_pread(char *message, int message_size, int fildes, void *data, size_t data_size, off_t offset);

The explain_message_pread function is used to obtain an explanation of an error returned by the pread(2) 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.
fildes
The original fildes, exactly as passed to the pread(2) system call.
data
The original data, exactly as passed to the pread(2) system call.
data_size
The original data_size, exactly as passed to the pread(2) system call.
offset
The original offset, exactly as passed to the pread(2) system call.

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


ssize_t result = pread(fildes, data, data_size, offset);
if (result < 0) 
{
    char message[3000];
explain_message_pread(message, sizeof(message), fildes, data, data_size, offset); 



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

The above code example is available pre‐packaged as the explain_pread_or_die(3) function.

void explain_message_errno_pread(char *message, int message_size, int errnum, int fildes, void *data, size_t data_size, off_t offset);

The explain_message_errno_pread function is used to obtain an explanation of an error returned by the pread(2) 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.
fildes
The original fildes, exactly as passed to the pread(2) system call.
data
The original data, exactly as passed to the pread(2) system call.
data_size
The original data_size, exactly as passed to the pread(2) system call.
offset
The original offset, exactly as passed to the pread(2) system call.

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


ssize_t result = pread(fildes, data, data_size, offset);
if (result < 0) 
{
    int err = errno;
    char message[3000];
explain_message_errno_pread(message, sizeof(message), err, fildes, data, data_size, offset); 



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

The above code example is available pre‐packaged as the explain_pread_or_die(3) function.

pread(2)
read from or write to a file descriptor at a given offset
explain_pread_or_die(3)
read from or write to a file descriptor at a given offset and report errors

libexplain version 1.3
Copyright (C) 2009 Peter Miller
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