confstr(3)

💖 confstr(3) — get configuration dependent string variables 💖

okay bestie, welcome to the confstr page from the Library Functions Manual (yes that's the section name, we're being ✨official✨ today). this one's a libc baddie and she's here to hand you the config strings your system is hiding. let's get into it 🫶

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🌸 NAME 🌸

confstr — get configuration dependent string variables

📚 LIBRARY 📚

Standard C library (libc, -lc)

✨ SYNOPSIS ✨

#include <unistd.h>

size_t confstr(size_t size;
               int name, char buf[size], size_t size);

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

• confstr() — _POSIX_C_SOURCE >= 2 || _XOPEN_SOURCE

yeah bestie there's a feature test macro situation, you knew there would be. love that for glibc.

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💅 DESCRIPTION 💅

confstr() gets the value of configuration-dependent string variables. it's literally just that. ask it about a system variable, it tells you the string.

The name argument is the system variable to be queried. The following variables are supported:

• _CS_GNU_LIBC_VERSION (GNU C library only; since glibc 2.3.2) — a string which identifies the GNU C library version on this system (e.g., "glibc 2.3.4"). she's telling you her own name and version, iconic behavior 💁‍♀️
• _CS_GNU_LIBPTHREAD_VERSION (GNU C library only; since glibc 2.3.2) — a string which identifies the POSIX implementation supplied by this C library (e.g., "NPTL 2.3.4" or "linuxthreads-0.10").
• _CS_PATH — a value for the PATH variable which indicates where all the POSIX.2 standard utilities can be found. the classic, the moment, the one everyone actually wants.

okay now the plot twist that everyone forgets 🫠

If buf is not NULL and size is not zero, confstr() copies the value of the string to buf truncated to size - 1 bytes if necessary, with a null byte ('\0') as terminator. This can be detected by comparing the return value of confstr() against size.

If size is zero and buf is NULL, confstr() just returns the value as defined below.

note the and energy up there, bestie. both conditions. the logic is picky and we respect her for it. 🙈

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🔥 RETURN VALUE 🔥

If name is a valid configuration variable, confstr() returns the number of bytes (including the terminating null byte) that would be required to hold the entire value of that variable. This value may be greater than size, which means that the value in buf is truncated.

If name is a valid configuration variable, but that variable does not have a value, then confstr() returns 0.

If name does not correspond to a valid configuration variable, confstr() returns 0, and errno is set to EINVAL.

so like... three different situations and two of them return 0?? 💀 check errno bestie, that's the vibe check right there.

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💀 ERRORS 💀

• EINVAL — the value of name is invalid.

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👑 ATTRIBUTES 👑

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

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

MT-Safe?? multithreaded-safe, no lock drama, she's main character energy in every thread. periodt. 🌈

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⭐ STANDARDS ⭐

POSIX.1-2008.

🎀 HISTORY 🎀

POSIX.1-2001.

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🦋 EXAMPLES 🦋

The following code fragment determines the path where to find the POSIX.2 system utilities:

char *pathbuf;
size_t n;

n = confstr(_CS_PATH, NULL, (size_t) 0);
pathbuf = malloc(n);
if (pathbuf == NULL)
    abort();
confstr(_CS_PATH, pathbuf, n);

see what she did there?? first call with NULL and (size_t) 0 just to ask how much room she needs, then malloc(n), then the real call. two-pass girlie. so smart. so clean. we're obsessed. 🍓

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🌈 SEE ALSO 🌈

getconf(1), sh(1), exec(3), fpathconf(3), pathconf(3), sysconf(3), system(3)

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and that's the whole page bestie, you ate it 💖 this was Linux man-pages 6.20, dated 2026-02-08, so if you're reading this in the far future... hi, hope your glibc is thriving. confstr(3) forever. mwah 💋✨🎀