PMEM_LOG_SET_FUNCTION(3)

💖 pmem_log_set_function(3) 💖

okay bestie, gather round, because we're talking about the PMDK Programmer's Manual and specifically about pmem_log_set_function() — the girl whose whole job is deciding who gets to hear the library's little gossip (aka its logging messages). 🎀

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✨ NAME ✨

pmem_log_set_function() — set the logging function 💅

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🌸 SYNOPSIS 🌸

#include <libpmem.h>

typedef void pmem_log_function(
    enum pmem_log_level level,
    const char *file_name,
    unsigned line_no,
    const char *function_name,
    const char *message);

int pmem_log_set_function(pmem_log_function *log_function);

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🦋 DESCRIPTION 🦋

so pmem_log_set_function() is literally the vibe check for your logs — it lets you choose the function that's gonna receive all the generated logging messages. no cap. 🫶

your log_function can be one of two things:

• PMEM_LOG_USE_DEFAULT_FUNCTION, which just uses the default logging function (it's built right into the library, it's giving pre-installed ✨), or
• a pointer to a user-defined function of your own making, because you're the main character here 👑

now, the parameters of a user-defined log function go like this:

• level — the log level of the message (peep pmem_log_set_threshold(3) for the available log levels 👀)
• file_name — name of the source file where the message is coming from. it could be set to NULL, and if that happens, neither line_no nor function_name are provided. so like, don't come for them, they're just not there 😭
• line_no — the source file line where the message is coming from
• function_name — the function name where the message is coming from
• message — the message string, the actual tea 🍓

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🌈 THE DEFAULT LOGGING FUNCTION 🌈

the library ships with a default logging function that writes messages to syslog(3) and to stderr(3). she's enabled during library initialization, and you can bring her back any time by passing PMEM_LOG_USE_DEFAULT_FUNCTION as the log_function argument. iconic behavior honestly.

here's the thing though — the default logging function sinks all the logging messages into syslog(3) unconditionally. every single one. no filter, no shame. 💀

additionally, it sinks a logging message into stderr(3) if:

• its logging level is not less severe than the value indicated by the PMEM_LOG_THRESHOLD_AUX threshold, and
• it is not of level PMEM_LOG_LEVEL_HARK.

also note that the PMEM_LOG_THRESHOLD threshold's value applies to the default logging function. please see the notes below, bestie, they matter 💜

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⭐ RETURN VALUE ⭐

pmem_log_set_function() returns 0 on success, or it returns a non-zero value and sets errno on failure. and on failure? the logging function remains unchanged. she stays exactly as she was, no drama. 🧚

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💀 ERRORS 💀

pmem_log_set_function() can set the following errno values on fail:

• EAGAIN — multiple threads attempted to change the logging function concurrently. a retry may fix the problem. this error is not expected when the function is called from just one thread at a time. so like, one at a time please, we're not doing chaos today 🙈

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👑 NOTE 👑

the logging messages of level less severe than indicated by the PMEM_LOG_THRESHOLD threshold's value won't make it to the logging function, no matter whether the logging function is the default one or a user-provided one. sorry babes, them's the rules. 🌸

the user-defined function must be thread-safe. periodt. 🔥

also, the library will call your user-defined function before returning from pmem_log_set_function(), just to deliver basic info about the library:

• “src version: 2.y.z”
• “compiled with support for shutdown state”
• “compiled with libndctl 63+”

so yeah, she introduces herself immediately. polite queen. 💅

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🎀 SEE ALSO 🎀

• pmem_log_get_threshold(3)
• pmem_log_set_threshold(3)

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and that's the whole page bestie!! this came to you straight from the PMDK crew, dated 2025-07-24 — love that for us, mwah 💖✨🦋