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| .TH GETGROUPLIST 3 2021-03-22 "GNU" "Linux Programmer's Manual" |
| .SH NAME |
| getgrouplist \- get list of groups to which a user belongs |
| .SH SYNOPSIS |
| .nf |
| .B #include <grp.h> |
| .PP |
| .BI "int getgrouplist(const char *" user ", gid_t " group , |
| .BI " gid_t *" groups ", int *" ngroups ); |
| .fi |
| .PP |
| .RS -4 |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .RE |
| .PP |
| .BR getgrouplist (): |
| .nf |
| Since glibc 2.19: |
| _DEFAULT_SOURCE |
| Glibc 2.19 and earlier: |
| _BSD_SOURCE |
| .fi |
| .SH DESCRIPTION |
| The |
| .BR getgrouplist () |
| function scans the group database (see |
| .BR group (5)) |
| to obtain the list of groups that |
| .I user |
| belongs to. |
| Up to |
| .I *ngroups |
| of these groups are returned in the array |
| .IR groups . |
| .PP |
| If it was not among the groups defined for |
| .I user |
| in the group database, then |
| .I group |
| is included in the list of groups returned by |
| .BR getgrouplist (); |
| typically this argument is specified as the group ID from |
| the password record for |
| .IR user . |
| .PP |
| The |
| .I ngroups |
| argument is a value-result argument: |
| on return it always contains the number of groups found for |
| .IR user , |
| including |
| .IR group ; |
| this value may be greater than the number of groups stored in |
| .IR groups . |
| .SH RETURN VALUE |
| If the number of groups of which |
| .I user |
| is a member is less than or equal to |
| .IR *ngroups , |
| then the value |
| .I *ngroups |
| is returned. |
| .PP |
| If the user is a member of more than |
| .I *ngroups |
| groups, then |
| .BR getgrouplist () |
| returns \-1. |
| In this case, the value returned in |
| .IR *ngroups |
| can be used to resize the buffer passed to a further call |
| .BR getgrouplist (). |
| .SH VERSIONS |
| This function is present since glibc 2.2.4. |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .ad l |
| .nh |
| .TS |
| allbox; |
| lbx lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR getgrouplist () |
| T} Thread safety MT-Safe locale |
| .TE |
| .hy |
| .ad |
| .sp 1 |
| .SH CONFORMING TO |
| This function is nonstandard; it appears on most BSDs. |
| .SH BUGS |
| In glibc versions before 2.3.3, |
| the implementation of this function contains a buffer-overrun bug: |
| it returns the complete list of groups for |
| .IR user |
| in the array |
| .IR groups , |
| even when the number of groups exceeds |
| .IR *ngroups . |
| .SH EXAMPLES |
| The program below displays the group list for the user named in its |
| first command-line argument. |
| The second command-line argument specifies the |
| .I ngroups |
| value to be supplied to |
| .BR getgrouplist (). |
| The following shell session shows examples of the use of this program: |
| .PP |
| .in +4n |
| .EX |
| .RB "$" " ./a.out cecilia 0" |
| getgrouplist() returned \-1; ngroups = 3 |
| .RB "$" " ./a.out cecilia 3" |
| ngroups = 3 |
| 16 (dialout) |
| 33 (video) |
| 100 (users) |
| .EE |
| .in |
| .SS Program source |
| \& |
| .EX |
| #include <stdio.h> |
| #include <stdlib.h> |
| #include <grp.h> |
| #include <pwd.h> |
| |
| int |
| main(int argc, char *argv[]) |
| { |
| int ngroups; |
| struct passwd *pw; |
| struct group *gr; |
| |
| if (argc != 3) { |
| fprintf(stderr, "Usage: %s <user> <ngroups>\en", argv[0]); |
| exit(EXIT_FAILURE); |
| } |
| |
| ngroups = atoi(argv[2]); |
| |
| gid_t *groups = malloc(sizeof(*groups) * ngroups); |
| if (groups == NULL) { |
| perror("malloc"); |
| exit(EXIT_FAILURE); |
| } |
| |
| /* Fetch passwd structure (contains first group ID for user). */ |
| |
| pw = getpwnam(argv[1]); |
| if (pw == NULL) { |
| perror("getpwnam"); |
| exit(EXIT_SUCCESS); |
| } |
| |
| /* Retrieve group list. */ |
| |
| if (getgrouplist(argv[1], pw\->pw_gid, groups, &ngroups) == \-1) { |
| fprintf(stderr, "getgrouplist() returned \-1; ngroups = %d\en", |
| ngroups); |
| exit(EXIT_FAILURE); |
| } |
| |
| /* Display list of retrieved groups, along with group names. */ |
| |
| fprintf(stderr, "ngroups = %d\en", ngroups); |
| for (int j = 0; j < ngroups; j++) { |
| printf("%d", groups[j]); |
| gr = getgrgid(groups[j]); |
| if (gr != NULL) |
| printf(" (%s)", gr\->gr_name); |
| printf("\en"); |
| } |
| |
| exit(EXIT_SUCCESS); |
| } |
| .EE |
| .SH SEE ALSO |
| .BR getgroups (2), |
| .BR setgroups (2), |
| .BR getgrent (3), |
| .BR group_member (3), |
| .BR group (5), |
| .BR passwd (5) |