getentropy(3) — Linux manual page

getentropy(3)            Library Functions Manual           getentropy(3)

NAME         top

       getentropy - fill a buffer with random bytes

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <unistd.h>

       int getentropy(size_t length;
                      void buffer[length], size_t length);

   Feature Test Macro Requirements for glibc (see
   feature_test_macros(7)):

       getentropy():
           _DEFAULT_SOURCE

DESCRIPTION         top

       The getentropy() function writes length bytes of high-quality
       random data to the buffer starting at the location pointed to by
       buffer.  The maximum permitted value for the length argument is
       256.

       A successful call to getentropy() always provides the requested
       number of bytes of entropy.

RETURN VALUE         top

       On success, this function returns zero.  On error, -1 is returned,
       and errno is set to indicate the error.

ERRORS         top

       EFAULT Part or all of the buffer specified by buffer and length is
              not in valid addressable memory.

       EIO    length is greater than 256.

       EIO    An unspecified error occurred while trying to overwrite
              buffer with random data.

       ENOSYS This kernel version does not implement the getrandom(2)
              system call required to implement this function.

STANDARDS         top

       None.

HISTORY         top

       glibc 2.25.  OpenBSD.

NOTES         top

       The getentropy() function is implemented using getrandom(2).

       Whereas the glibc wrapper makes getrandom(2) a cancelation point,
       getentropy() is not a cancelation point.

       getentropy() is also declared in <sys/random.h>.  (No feature test
       macro need be defined to obtain the declaration from that header
       file.)

       A call to getentropy() may block if the system has just booted and
       the kernel has not yet collected enough randomness to initialize
       the entropy pool.  In this case, getentropy() will keep blocking
       even if a signal is handled, and will return only once the entropy
       pool has been initialized.

SEE ALSO         top

       getrandom(2), urandom(4), random(7)

COLOPHON         top

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Linux man-pages 6.15            2025-06-28                  getentropy(3)

Pages that refer to this page: getrandom(2),  random(7)

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