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PROLOG
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.
NAME
getpwuid, getpwuid_r — search user database for a user ID
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);
DESCRIPTION
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.
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 an error occurs. 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().
If successful, the getpwuid_r() function shall return zero; otherwise, an error number
shall be returned to indicate the error.
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.
The following sections are informative.
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 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);
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.
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.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
getpwnam(), geteuid(), getuid(), getlogin(), sysconf()
The Base Definitions volume of POSIX.1‐2008, <pwd.h>, <sys_types.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is
POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .