The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group


getpwuid, getpwuid_r - search user database for a user ID


#include <pwd.h>

struct passwd *getpwuid(uid_t
int getpwuid_r(uid_t
uid, struct passwd *pwd, char *buffer,
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. 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.


These functions may fail if:

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

The getpwuid_r() function may fail if:

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;
    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);
    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.






getpwnam, geteuid, getuid, getlogin, sysconf

XBD <pwd.h>, <sys/types.h>


First released in Issue 1. Derived from System V Release 2.0.

Issue 5

Normative text previously in the APPLICATION USAGE section is moved to the RETURN VALUE section.

The getpwuid_r() function is included for alignment with the POSIX Threads Extension.

A note indicating that the getpwuid() function need not be reentrant is added to the DESCRIPTION.

Issue 6

The getpwuid_r() function is marked as part of the Thread-Safe Functions option.

The Open Group Corrigendum U028/3 is applied, correcting text in the DESCRIPTION describing matching the uid.

In the SYNOPSIS, the optional include of the <sys/types.h> header is removed.

In the DESCRIPTION, the note about reentrancy is expanded to cover thread-safety.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The APPLICATION USAGE section is updated to include a note on the thread-safe function and its avoidance of possibly using a static data area.

IEEE PASC Interpretation 1003.1 #116 is applied, changing the description of the size of the buffer from bufsize characters to bytes.

Issue 7

Austin Group Interpretation 1003.1-2001 #156 is applied.

SD5-XBD-ERN-4 is applied, changing the definition of the [EMFILE] error.

SD5-XSH-ERN-166 is applied.

The getpwuid_r() function is moved from the Thread-Safe Functions option to the Base.

A minor addition is made to the EXAMPLES section, reminding the application developer to free memory allocated as if by malloc().

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0256 [75] is applied.

End of informative text.


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