GETPWUID(3P)

GETPWUID(3P) — hey bestie, let's go find a user 💖

okay so real talk: this page lives in the POSIX Programmer's Manual, and it's all about getpwuid and getpwuid_r — the two functions that go digging in the user database for an entry that matches a uid. 🕵️♀️✨

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🌸 PROLOG 🌸

So this manual page is part of the POSIX Programmer's Manual, which means… bestie, listen. The Linux implementation of this interface may differ (please, please go consult the corresponding Linux manual page for the actual Linux behavior, iykyk), or — plot twist 😭 — the interface might not even be implemented on Linux at all. Vibe check your platform, okay?

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👑 NAME 👑

getpwuid, getpwuid_r — search the user database for a user ID 💅

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✨ SYNOPSIS ✨

#include <pwd.h>

struct passwd *getpwuid(uid_t uid);
int getpwuid_r(uid_t uid, struct passwd *pwd, char *buffer,
    size_t bufsize, struct passwd **result);

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🦋 DESCRIPTION 🦋

So the getpwuid() function shall search the user database for an entry with a matching uid. That's the whole mission. Go find the one whose uid matches. 🎯

Heads up bestie: the getpwuid() function need not be thread-safe. Not me being dramatic, that's literally the spec. 😭

If your app wants to actually check for error situations (and you do, bestie, you do), set errno to 0 before calling getpwuid(). Then: if getpwuid() returns a null pointer and errno is set to non-zero, an error occurred. That's the tell. 👀

Now the _r version, which is the well-behaved sibling:

The getpwuid_r() function shall update the passwd structure pointed to by pwd and store a pointer to that structure at the location pointed to by result. That structure shall contain an entry from the user database with a matching uid. The storage referenced by the structure is allocated from the memory provided with the buffer parameter, which is bufsize bytes in size. A call to sysconf(_SC_GETPW_R_SIZE_MAX) returns either -1 without changing errno, or an initial value suggested for the size of this buffer. And a null pointer shall be returned at the location pointed to by result on error or if the requested entry is not found. Periodt. 💅

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💖 RETURN VALUE 💖

The getpwuid() function shall return a pointer to a struct passwd — with the structure as defined in <pwd.h> — with a matching entry if found. 🎀 A null pointer shall be returned if the requested entry is not found, or if an error occurs.

Careful though, these two are not the same thing:

• If the requested entry was not found, errno shall not be changed. ✨
• On error, errno shall be set to indicate the error. 🚨

Okay now the big one, bestie, read this twice: the application shall not modify the structure to which the return value points, nor any storage areas pointed to by pointers within the structure. The returned pointer, and pointers within the structure, might be invalidated — or the structure or the storage areas might be overwritten — by a subsequent call to getpwent(), getpwnam(), or getpwuid(). 😱 And the returned pointer, and pointers within the structure, might also be invalidated if the calling thread is terminated. So like… copy what you need, don't hoard the pointer. fr fr.

If successful, the getpwuid_r() function shall return zero; otherwise, an error number shall be returned to indicate the error. Zero = slay, non-zero = error number. Simple. 🌈

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🔥 ERRORS 🔥

These functions may fail if:

• EIO — An I/O error has occurred. 💀
• EINTR — A signal was caught during getpwuid().
• EMFILE — All file descriptors available to the process are currently open.
• ENFILE — The maximum allowable number of files is currently open in the system.

The getpwuid_r() function may fail if:

• ERANGE — Insufficient storage was supplied via buffer and bufsize to contain the data to be referenced by the resulting passwd structure. (Basically: your buffer was too small, bestie. Grow it. 🌱)

The following sections are informative.

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🎀 EXAMPLES 🎀

Quick note before we dive in: sysconf(_SC_GETPW_R_SIZE_MAX) may return -1 if there is no hard limit on the size of the buffer needed to store all the groups returned. This example shows how an application can allocate a buffer of sufficient size to work with getpwuid_r(). 🫶

long int initlen = sysconf(_SC_GETPW_R_SIZE_MAX);
size_t len;
if (initlen == -1)
    /* Default initial length. */
    len = 1024;
else
    len = (size_t) initlen;
struct passwd result;
struct passwd *resultp;
char *buffer = malloc(len);
if (buffer == NULL)
    ...handle error...
int e;
while ((e = getpwuid_r(42, &result, buffer, len, &resultp)) == ERANGE)
    {
    size_t newlen = 2 * len;
    if (newlen < len)
        ...handle error...
    len = newlen;
    char *newbuffer = realloc(buffer, len);
    if (newbuffer == NULL)
        ...handle error...
    buffer = newbuffer;
    }
if (e != 0)
    ...handle error...
free (buffer);

⭐ Getting an Entry for the Root User ⭐

The following example gets the user database entry for the user with user ID 0 (root). Iconic. 👑

#include <sys/types.h>
#include <pwd.h>
...
uid_t id = 0;
struct passwd *pwd;

pwd = getpwuid(id);

🌸 Finding the Name for the Effective User ID 🌸

The following example defines pws as a pointer to a structure of type passwd, which is used to store the structure pointer returned by the call to the getpwuid() function. The geteuid() function shall return the effective user ID of the calling process; this is used as the search criteria for the getpwuid() function. The call to getpwuid() shall return a pointer to the structure containing that user ID value.

#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>
...
struct passwd *pws;
pws = getpwuid(geteuid());

🦋 Finding an Entry in the User Database 🦋

The following example uses getpwuid() to search the user database for a user ID that was previously stored in a stat structure, then prints out the user name if it is found. If the user is not found, the program prints the numeric value of the user ID for the entry. No cap, it just prints the number instead. 😌

#include <sys/types.h>
#include <pwd.h>
#include <stdio.h>
...
struct stat statbuf;
struct passwd *pwd;
...
if ((pwd = getpwuid(statbuf.st_uid)) != NULL)
    printf(" %-8.8s", pwd->pw_name);
else
    printf(" %-8d", statbuf.st_uid);

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💜 APPLICATION USAGE 💜

Three names associated with the current process can be determined, and honestly the way that they're all slightly different is so important:

• getpwuid(geteuid()) returns the name associated with the effective user ID of the process.
• getlogin() returns the name associated with the current login activity.
• getpwuid(getuid()) returns the name associated with the real user ID of the process.

The getpwuid_r() function is thread-safe and returns values in a user-supplied buffer instead of possibly using a static data area that may be overwritten by each call. That's the upgrade, bestie. 🧚

Portable applications should take into account that it is usual for an implementation to return -1 from sysconf(), indicating that there is no maximum for _SC_GETPW_R_SIZE_MAX. So don't panic when you see that -1, it's just vibes. ✨

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🌈 RATIONALE 🌈

None.

(Yeah. Nothing. It's giving minimal. 💀)

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🎀 FUTURE DIRECTIONS 🎀

None.

(Also nothing. The future is wide open, bestie. 🦋)

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🌟 SEE ALSO 🌟

• getpwnam()
• geteuid()
• getuid()
• getlogin()
• sysconf()

The Base Definitions volume of POSIX.1-2017, <pwd.h>, <sys_types.h>

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📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page, bestie!! 🫶 brought to you by IEEE/The Open Group, 2017 — ok bye, go getpwuid() responsibly, mwah 💋✨