explain_fseeko(3)

💖 explain_fseeko — bestie's guide to fseeko(3) errors 💖

okay so this is a page from the Library Functions Manual, and it's literally all about explain_fseeko — the glow-up your error messages deserve. it lives in libexplain, and it's here to tell you why your fseeko(3) call said no. 🫶

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✨ SYNOPSIS ✨

#include <libexplain/fseeko.h>
const char *explain_fseeko(FILE *fp, off_t offset, int whence);
const char *explain_errno_fseeko(int errnum, FILE *fp, off_t offset, int
whence);
void explain_message_fseeko(char *message, int message_size, FILE *fp,
off_t offset, int whence);
void explain_message_errno_fseeko(char *message, int message_size, int
errnum, FILE *fp, off_t offset, int whence);

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🌸 DESCRIPTION 🌸

so basically these functions are your translators. they take the errors that fseeko(3) throws back at you and turn them into actual explanations. no more squinting at a cryptic number wondering what you did wrong. 💅

🎀 explain_fseeko

const char *explain_fseeko(FILE *fp, off_t offset, int whence);

explain_fseeko gives you the tea on an error that came back from the fseeko(3) system call. the least the message will have in it is the value of strerror(errno) — but ngl, it usually does way better than that and tells you the actual underlying cause in more detail. it's giving detective energy. 🔍

the errno global variable is what gets used to grab the error value that needs decoding.

• fp — the original fp, exactly as passed to the fseeko(3) system call.
• offset — the original offset, exactly as passed to the fseeko(3) system call.
• whence — the original whence, exactly as passed to the fseeko(3) system call.

Returns:

the message explaining the error. bestie, listen — this message buffer is shared by all libexplain functions that don't supply a buffer in their argument list. it will be overwritten by the next call to any libexplain function that shares this buffer, including other threads. so don't get attached. 😭

Note: this function is not thread safe, because it shares a return buffer across all threads, and many other functions in this library do too.

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

if (fseeko(fp, offset, whence) < 0)
{
    fprintf(stderr, "%s\n", explain_fseeko(fp, offset, whence));
    exit(EXIT_FAILURE);
}

the above code example is available pre-packaged as the explain_fseeko_or_die(3) function. ✨

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🦋 explain_errno_fseeko

const char *explain_errno_fseeko(int errnum, FILE *fp, off_t offset, int
whence);

explain_errno_fseeko also hands you an explanation of an error returned by the fseeko(3) system call. the least the message will contain is the value of strerror(errno), but usually it does much better, and points at 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. (so like, grab errno immediately, no side quests. 🏃♀️)
• fp — the original fp, exactly as passed to the fseeko(3) system call.
• offset — the original offset, exactly as passed to the fseeko(3) system call.
• whence — the original whence, exactly as passed to the fseeko(3) 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 (fseeko(fp, offset, whence) < 0)
{
    int err = errno;
    fprintf(stderr, "%s\n", explain_errno_fseeko(err, fp, offset,
    whence));
    exit(EXIT_FAILURE);
}

the above code example is available pre-packaged as the explain_fseeko_or_die(3) function. 💖

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🌈 explain_message_fseeko

void explain_message_fseeko(char *message, int message_size, FILE *fp,
off_t offset, int whence);

explain_message_fseeko gets you an explanation of an error returned by the fseeko(3) 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. iconic behaviour, honestly.

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. (look at her go 👑)
• 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 fseeko(3) system call.
• offset — the original offset, exactly as passed to the fseeko(3) system call.
• whence — the original whence, exactly as passed to the fseeko(3) system call.

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

if (fseeko(fp, offset, whence) < 0)
{
    char message[3000];
    explain_message_fseeko(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_fseeko_or_die(3) function. 🎀

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🧚 explain_message_errno_fseeko

void explain_message_errno_fseeko(char *message, int message_size, int
errnum, FILE *fp, off_t offset, int whence);

explain_message_errno_fseeko obtains an explanation of an error returned by the fseeko(3) 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. she's thorough, she's helpful, she ate. 🔥

• 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 fseeko(3) system call.
• offset — the original offset, exactly as passed to the fseeko(3) system call.
• whence — the original whence, exactly as passed to the fseeko(3) system call.

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

if (fseeko(fp, offset, whence) < 0)
{
    int err = errno;
    char message[3000];
    explain_message_errno_fseeko(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_fseeko_or_die(3) function. ✨

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👀 SEE ALSO 👀

• fseeko(3) — seek to or report file position
• explain_fseeko_or_die(3) — seek to or report file position and report errors

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💜 COPYRIGHT 💜

this whole page is libexplain version 1.4, and the copyright belongs to Copyright (C) 2013 Peter Miller.

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okay bestie that's the whole page, you're officially fluent in explain_fseeko now. go forth and have the most descriptive error messages of your life. no cap. 💅🌸🦋 slay 🫶