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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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GETPWUID(3P) POSIX Programmer's Manual GETPWUID(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
getpwuid, getpwuid_r — search user database for a user ID
#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);
The getpwuid() function shall search the user database for an
entry with a matching uid.
The getpwuid() function need not be thread-safe.
Applications wishing to check for error situations should set
errno to 0 before calling getpwuid(). If getpwuid() returns a
null pointer and errno is set to non-zero, an error occurred.
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. The structure shall contain an
entry from the user database with a matching uid. 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. A null pointer shall be returned at the location pointed
to by result on error or if the requested entry is not found.
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 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.
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(). The returned pointer, and pointers
within the structure, might also be invalidated if the calling
thread is terminated.
If successful, the getpwuid_r() function shall return zero;
otherwise, an error number shall be returned to indicate the
error.
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.
The following sections are informative.
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 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).
#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.
#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);
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.
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.
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.
None.
None.
getpwnam(3p), geteuid(3p), getuid(3p), getlogin(3p), sysconf(3p)
The Base Definitions volume of POSIX.1‐2017, pwd.h(0p),
sys_types.h(0p)
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 .
IEEE/The Open Group 2017 GETPWUID(3P)
Pages that refer to this page: pwd.h(0p), find(1p), endpwent(3p), getlogin(3p), getpwnam(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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