GETPWNAM(3P)

💖 GETPWNAM — the tea on finding users by name 💖

okay so bestie, welcome to the POSIX Programmer's Manual page for getpwnam, getpwnam_r — and before we go any further, real talk: this is the POSIX spec version. The Linux implementation of this interface may differ (so go check the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface might not even be implemented on Linux at all. 🙈

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✨ NAME ✨

getpwnam, getpwnam_r — search user database for a name 🔍

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🎀 SYNOPSIS 🎀

#include <pwd.h>

struct passwd *getpwnam(const char *name);
int getpwnam_r(const char *name, struct passwd *pwd, char *buffer,
    size_t bufsize, struct passwd **result);

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💅 DESCRIPTION 💅

So getpwnam() shall search the user database for an entry with a matching name. That's the whole vibe. 👑

ngl though, getpwnam() need not be thread-safe. It's giving "one person at a time, bestie." 😭

If you're an application that wants to actually check for error situations, you should set errno to 0 before calling getpwnam(). Then: if getpwnam() returns a null pointer and errno is non-zero, an error occurred. Periodt.

Now for the glow-up version: getpwnam_r() shall update the passwd structure pointed to by pwd and store a pointer to that structure at the location pointed to by result. The structure shall contain an entry from the user database with a matching name. 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. That's just how it is. 💀

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🌸 RETURN VALUE 🌸

The getpwnam() 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 an error occurs. If the requested entry was not found, errno shall not be changed. On error, errno shall be set to indicate the error.

🚨 BESTIE LISTEN. The application shall not modify the structure to which the return value points, nor any storage areas pointed to by pointers within the structure. Hands off, okay?? 🙅

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(). The returned pointer, and pointers within the structure, might also be invalidated if the calling thread is terminated. It's giving "this info is NOT permanent, screenshot it while you can." 📸

The getpwnam_r() function shall return zero on success or if the requested entry was not found and no error has occurred. If an error has occurred, an error number shall be returned to indicate the error.

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⚠️ ERRORS ⚠️

These functions may fail if:

• EIO — An I/O error has occurred. 💀
• EINTR — A signal was caught during getpwnam().
• 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 getpwnam_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. Not enough room, babe. 😭

The following sections are informative.

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🦋 EXAMPLES 🦋

Note that 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 getpwnam_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 = getpwnam_r("someuser", &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 Login Name 🌈

The following example uses the getlogin() function to return the name of the user who logged in; this information is passed to the getpwnam() function to get the user database entry for that user.

#include <sys/types.h>
#include <pwd.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
...
char *lgn;
struct passwd *pw;
...
if ((lgn = getlogin()) == NULL || (pw = getpwnam(lgn)) == NULL) {
    fprintf(stderr, "Get of user information failed.\n"); exit(1);
}
...

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👑 APPLICATION USAGE 👑

Three names associated with the current process can be determined: getpwuid(geteuid()) returns the name associated with the effective user ID of the process; getlogin() returns the name associated with the current login activity; and getpwuid(getuid()) returns the name associated with the real user ID of the process. Three names, three vibes, one process. 🫶

The getpwnam_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. She's safe, she's considerate, she's that girl. ✨

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.

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🍓 RATIONALE 🍓

None. (Not a single note. Silence. Iconic of her, honestly.) 🤐

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🌟 FUTURE DIRECTIONS 🌟

None.

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🔥 SEE ALSO 🔥

getpwuid(), 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, circa 2017, on the GETPWNAM(3P) tip. go forth and look up some users. mwah. 💖💅✨