explain_fprintf(3)

💖 explain_fprintf(3) — bestie edition 💖

okay so listen. this is a page from the Library Functions Manual, and it's all about explain_fprintf(3) — the lil glow-up that turns your fprintf(3) errors into actual, readable explanations instead of cryptic nonsense. let's get into it 🫶

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✨ NAME ✨

explain_fprintf — explain fprintf(3) errors 💅

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🌸 SYNOPSIS 🌸

#include <libexplain/fprintf.h>
const char *explain_fprintf(FILE *fp, const char *format, ...);
const char *explain_errno_fprintf(int errnum, FILE *fp, const char
*format, ...);
void explain_message_fprintf(char *message, int message_size, FILE *fp,
const char *format, ....);
void explain_message_errno_fprintf(char *message, int message_size, int
errnum, FILE *fp, const char *format, ...);

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🦋 DESCRIPTION 🦋

so these functions?? they're your besties when fprintf(3) does the system-call thing and comes back with an error. instead of just leaving you hanging, they hand you a whole explanation. iconic behaviour, honestly. ✨

🌈 explain_fprintf

const char *explain_fprintf(FILE *fp, const char *format, ...);

explain_fprintf is the one you call to get an explanation of an error that fprintf(3) threw back at you. the least you're gonna get in that message is the value of strerror(errno) — but usually it does way better than that and tells you the actual underlying cause in more detail. slay. 💅

the errno global variable is what it reads to get the error value that needs decoding.

• fp — the original fp, exactly as you passed it to the fprintf(3) system call. no edits, no remixes.
• format — the original format, exactly as you passed it to the fprintf(3) system call.

Returns:

the message explaining the error. heads up though — this message buffer is shared by every libexplain function that doesn't supply its own buffer in its argument list. so the very next call to any libexplain function that shares this buffer will overwrite it, other threads included. 😭

Note: this function is not thread safe, because it shares a return buffer across all threads, and so do many other functions in this library. no cap.

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

errno = EINVAL;
int result = fprintf(fp, format, ...);
if (result < 0)
{
    fprintf(stderr, "%s\n", explain_fprintf(fp, format, ...));
    exit(EXIT_FAILURE);
}

the above code example is available pre-packaged as the explain_fprintf_or_die(3) function. 🎀

💜 explain_errno_fprintf

const char *explain_errno_fprintf(int errnum, FILE *fp, const char
*format, ...);

explain_errno_fprintf is the one you call to get an explanation of an error returned by the fprintf(3) system call. again, 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. queen behaviour 👑

• errnum — the error value to be decoded, usually grabbed from the errno global variable right before this function gets called. this is necessary if you need to run any code between the system call you're explaining and this function, because many libc functions will alter the value of errno. so catch it early, bestie.
• fp — the original fp, exactly as passed to the fprintf(3) system call.
• format — the original format, exactly as passed to the fprintf(3) system call.

Returns:

the message explaining the error. same tea as before: this message buffer is shared by all libexplain functions that don't supply a buffer in their argument list, and it gets overwritten by the next call to any libexplain function that shares this buffer — other threads included. 🙈

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:

errno = EINVAL;
int result = fprintf(fp, format, ...);
if (result < 0)
{
    int err = errno;
    fprintf(stderr, "%s\n", explain_errno_fprintf(err, fp,
    format, ...));
    exit(EXIT_FAILURE);
}

the above code example is available pre-packaged as the explain_fprintf_or_die(3) function. 🌟

🎀 explain_message_fprintf

void explain_message_fprintf(char *message, int message_size, FILE *fp,
const char *format, ...);

explain_message_fprintf gets you an explanation of an error returned by the fprintf(3) system call. the least the message will contain is the value of strerror(errno), but usually it goes much harder and indicates the underlying cause in more detail. 🔥

the errno global variable is used to obtain the error value that gets decoded.

• message — the location where the returned message gets stored. if you supply a suitable message return buffer, this function is thread safe. ✨
• message_size — the size in bytes of the location where the returned message gets stored.
• fp — the original fp, exactly as passed to the fprintf(3) system call.
• format — the original format, exactly as passed to the fprintf(3) system call.

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

errno = EINVAL;
int result = fprintf(fp, format, ...);
if (result < 0)
{
    char message[3000];
    explain_message_fprintf(message, sizeof(message), fp, format,
    ...);
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}

the above code example is available pre-packaged as the explain_fprintf_or_die(3) function. 🍓

🧚 explain_message_errno_fprintf

void explain_message_errno_fprintf(char *message, int message_size, int
errnum, FILE *fp, const char *format, ...);

explain_message_errno_fprintf is used to obtain an explanation of an error returned by the fprintf(3) system call. 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. periodt. 💅

• 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 fprintf(3) system call.
• format — the original format, exactly as passed to the fprintf(3) system call.

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

errno = EINVAL;
int result = fprintf(fp, format, ...);
if (result < 0)
{
    int err = errno;
    char message[3000];
    explain_message_errno_fprintf(message, sizeof(message), err,
    fp, format, ...);
    fprintf(stderr, "%s\n", message);
    exit(EXIT_FAILURE);
}

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

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👑 SEE ALSO 👑

• fprintf(3) — formatted output conversion
• explain_fprintf_or_die(3) — formatted output conversion and report errors

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💀 COPYRIGHT 💀

libexplain version 1.4 Copyright (C) 2010 Peter Miller

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and that's the whole page bestie, you ate that 🫶 stay hydrated, check your errno, and remember: it's giving thread unsafe unless you brought your own buffer. mwah 💋

libexplain version 1.4 🌸 Copyright (C) 2010 Peter Miller — signing off, explain_fprintf(3) style, iykyk ✨