mktemp(3)

💖 mktemp(3) — the "make a unique temporary filename" page 💖

okay bestie, sit down, we need to talk. this one is a Library Functions Manual page for the function called mktemp, and it's lowkey a cautionary tale. no cap. 🫶

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✨ NAME ✨

mktemp — make a unique temporary filename

📚 LIBRARY 📚

Standard C library (libc, -lc) — she lives in libc, queen 💅

🌸 SYNOPSIS 🌸

#include <stdlib.h>

char *mktemp(char *template);

🧚 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

mktemp():

Since glibc 2.12:
    (_XOPEN_SOURCE >= 500) && ! (_POSIX_C_SOURCE >= 200112L)
        || /* glibc >= 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
Before glibc 2.12:
    _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500

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💀 DESCRIPTION 💀

Never use this function; see BUGS. literally. that's the whole vibe. 🙈

The mktemp() function generates a unique temporary filename from template. The last six characters of template must be XXXXXX and these are replaced with a string that makes the filename unique. And because it's gonna get modified, template must not be a string constant — declare it as a character array instead, bestie. do not pass it a literal, she will not survive it. 😭

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🔁 RETURN VALUE 🔁

The mktemp() function always returns template. always always. If a unique name was created, the last six bytes of template will have been modified in such a way that the resulting name is unique (i.e., does not exist already). If a unique name could not be created, template is made an empty string, and errno is set to indicate the error. so like… check errno, okay? 👑

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🚨 ERRORS 🚨

• EINVAL — The last six characters of template were not XXXXXX. that's it, that's the whole list. she's picky about her X's fr fr.

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🌈 ATTRIBUTES 🌈

For an explanation of the terms used in this section, see attributes(7).

The way that this table is giving "one row, no notes":

• Interface: mktemp()
• Attribute: Thread safety
• Value: MT-Safe

MT-Safe and iconic, periodt. ✨

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📜 STANDARDS 📜

None.

that's right. none. not a single one claims her. 💀

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🕰️ HISTORY 🕰️

4.3BSD, POSIX.1-2001. Removed in POSIX.1-2008. she got ✂️ removed, bestie. they said "we don't do that anymore."

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🛑 BUGS 🛑

Never use mktemp(). like, genuinely, for real for real. 🙅‍♀️

Some implementations follow 4.3BSD and replace XXXXXX by the current process ID and a single letter, so that at most 26 different names can be returned. twenty-six. that's it. that's the whole namespace. 😭

Since on the one hand the names are easy to guess, and on the other hand there is a race between testing whether the name exists and opening the file, every use of mktemp() is a security risk. a whole race condition. a TOCTOU situation. not the cute kind of race. 🏃‍♀️💨

The race is avoided by mkstemp(3) and mkdtemp(3). those are your girls. use those girls instead. 💖

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👀 SEE ALSO 👀

mktemp(1), mkdtemp(3), mkstemp(3), tempnam(3), tmpfile(3), tmpnam(3)

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and that's the page, bestie!! 🎀 huge shoutout to the Linux man-pages 6.20, dated 2026-02-08, for keeping this fossil documented so we all know exactly which function to never ever call. mwah. 💜✨