Implementation status: to be implemented


#include <grp.h>

struct group *getgrgid(gid_t gid);

int getgrgid_r(gid_t gid, struct group *grp, char *buffer, size_t bufsize, struct group **result);


The getgrgid() and getgrgid_r() functions return the group database entry for the group ID.


gid - the group ID,
grp - a pointer to the group structure,
buffer - a pointer to the memory provided,
bufsize - a size of the provided memory,
result - the updated group structure.

The getgrgid() function searches the group database for an entry with a matching gid. The getgrgid() function is not thread-safe.

To check for error situations you must set errno to 0 before calling getgrgid(). If getgrgid() returns a null pointer and errno is set to non-zero, an error occurred.

The getgrgid_r() function updates the group structure pointed to by grp and stores a pointer to that structure at the location pointed to by result. The structure contains an entry from the group database with a matching gid. Storage referenced by the group structure is allocated from the memory provided with the buffer parameter, which is bufsize bytes in size. A call to sysconf(_SC_GETGR_R_SIZE_MAX) returns either -1 without changing errno or an initial value suggested for the size of this buffer. A null pointer is returned at the location pointed to by result on error or if the requested entry is not found.

The getgrgid_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_GETGR_R_SIZE_MAX.

Return value

Upon successful completion, getgrgid() returns a pointer to a struct group with the structure defined in <grp.h> with a matching entry if one is found. The getgrgid() function returns a null pointer if either the requested entry was not found, or an error occurred. If the requested entry was not found, errno is not changed. On error, errno is set to indicate the error.

The application does 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 getgrent(), getgrgid(), or getgrnam(). The returned pointer, and pointers within the structure, might also be invalidated if the calling thread is terminated.

If successful, the getgrgid_r() function returns zero; otherwise, an error number is returned to indicate the error.


[EIO] An I/O error has occurred.
[EINTR] A signal was caught during getgrgid().
[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 getgrgid_r() function fails if:

[ERANGE] Insufficient storage was supplied via buffer and bufsize to contain the data to be referenced by the resulting group structure.

Implementation tasks

  • Implement the getgrgid() function.
  • Implement the getgrgid_r() function.