GETGRENT_R(3) - Linux man page online | Library functions

Get group file entry reentrantly.

GETGRENT_R(3) Linux Programmer's Manual GETGRENT_R(3)


getgrent_r, fgetgrent_r - get group file entry reentrantly


#include <grp.h> int getgrent_r(struct group *gbuf, char *buf, size_t buflen, struct group **gbufp); int fgetgrent_r(FILE *stream, struct group *gbuf, char *buf, size_t buflen, struct group **gbufp); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): getgrent_r(): _GNU_SOURCE fgetgrent_r(): Since glibc 2.19: _DEFAULT_SOURCE Glibc 2.19 and earlier: _SVID_SOURCE


The functions getgrent_r() and fgetgrent_r() are the reentrant versions of getgrent(3) and fgetgrent(3). The former reads the next group entry from the stream initialized by set‐ grent(3). The latter reads the next group entry from stream. The group structure is defined in <grp.h> as follows: struct group { char *gr_name; /* group name */ char *gr_passwd; /* group password */ gid_t gr_gid; /* group ID */ char **gr_mem; /* NULL-terminated array of pointers to names of group members */ }; For more information about the fields of this structure, see group(5). The nonreentrant functions return a pointer to static storage, where this static storage contains further pointers to group name, password and members. The reentrant functions described here return all of that in caller-provided buffers. First of all there is the buffer gbuf that can hold a struct group. And next the buffer buf of size buflen that can hold additional strings. The result of these functions, the struct group read from the stream, is stored in the provided buffer *gbuf, and a pointer to this struct group is returned in *gbufp.


On success, these functions return 0 and *gbufp is a pointer to the struct group. On error, these functions return an error value and *gbufp is NULL.


ENOENT No more entries. ERANGE Insufficient buffer space supplied. Try again with larger buffer.


For an explanation of the terms used in this section, see attributes(7). ┌──────────────┬───────────────┬─────────────────────────────┐ │Interface │ Attribute │ Value │ ├──────────────┼───────────────┼─────────────────────────────┤ │getgrent_r() │ Thread safety │ MT-Unsafe race:grent locale │ ├──────────────┼───────────────┼─────────────────────────────┤ │fgetgrent_r() │ Thread safety │ MT-Safe │ └──────────────┴───────────────┴─────────────────────────────┘ In the above table, grent in race:grent signifies that if any of the functions setgrent(), getgrent(), endgrent(), or getgrent_r() are used in parallel in different threads of a program, then data races could occur.


These functions are GNU extensions, done in a style resembling the POSIX version of func‐ tions like getpwnam_r(3). Other systems use the prototype struct group *getgrent_r(struct group *grp, char *buf, int buflen); or, better, int getgrent_r(struct group *grp, char *buf, int buflen, FILE **gr_fp);


The function getgrent_r() is not really reentrant since it shares the reading position in the stream with all other threads.


#define _GNU_SOURCE #include <grp.h> #include <stdio.h> #include <stdlib.h> #define BUFLEN 4096 int main(void) { struct group grp, *grpp; char buf[BUFLEN]; int i; setgrent(); while (1) { i = getgrent_r(&grp, buf, BUFLEN, &grpp); if (i) break; printf("%s (%d):", grpp->gr_name, grpp->gr_gid); for (i = 0; ; i++) { if (grpp->gr_mem[i] == NULL) break; printf(" %s", grpp->gr_mem[i]); } printf("\n"); } endgrent(); exit(EXIT_SUCCESS); }


fgetgrent(3), getgrent(3), getgrgid(3), getgrnam(3), putgrent(3), group(5)


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GNU 2017-09-15 GETGRENT_R(3)
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getgrent_r(3) referred by fgetgrent(3) | getgrent(3)
refer to attributes(7) | feature_test_macros(7) | fgetgrent(3) | getgrent(3) | getgrnam(3) | getpwnam(3) | group(5) | putgrent(3)
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