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| .TH GETENTROPY 3 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| getentropy \- fill a buffer with random bytes |
| .SH SYNOPSIS |
| .nf |
| .B #include <unistd.h> |
| .PP |
| .BI "int getentropy(void *" buffer ", size_t " length ); |
| .fi |
| .PP |
| .RS -4 |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .RE |
| .PP |
| .BR getentropy (): |
| .nf |
| _DEFAULT_SOURCE |
| .fi |
| .SH DESCRIPTION |
| The |
| .BR getentropy () |
| function writes |
| .I length |
| bytes of high-quality random data to the buffer starting |
| at the location pointed to by |
| .IR buffer . |
| The maximum permitted value for the |
| .I length |
| argument is 256. |
| .PP |
| A successful call to |
| .BR getentropy () |
| always provides the requested number of bytes of entropy. |
| .SH RETURN VALUE |
| On success, this function returns zero. |
| On error, \-1 is returned, and |
| .I errno |
| is set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EFAULT |
| Part or all of the buffer specified by |
| .I buffer |
| and |
| .I length |
| is not in valid addressable memory. |
| .TP |
| .B EIO |
| .I length |
| is greater than 256. |
| .TP |
| .B EIO |
| An unspecified error occurred while trying to overwrite |
| .I buffer |
| with random data. |
| .TP |
| .B ENOSYS |
| This kernel version does not implement the |
| .BR getrandom (2) |
| system call required to implement this function. |
| .SH VERSIONS |
| The |
| .BR getentropy () |
| function first appeared in glibc 2.25. |
| .SH CONFORMING TO |
| This function is nonstandard. |
| It is also present on OpenBSD. |
| .SH NOTES |
| The |
| .BR getentropy () |
| function is implemented using |
| .BR getrandom (2). |
| .PP |
| Whereas the glibc wrapper makes |
| .BR getrandom (2) |
| a cancellation point, |
| .BR getentropy () |
| is not a cancellation point. |
| .PP |
| .BR getentropy () |
| is also declared in |
| .BR <sys/random.h> . |
| (No feature test macro need be defined to obtain the declaration from |
| that header file.) |
| .PP |
| A call to |
| .BR 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, |
| .BR getentropy () |
| will keep blocking even if a signal is handled, |
| and will return only once the entropy pool has been initialized. |
| .SH SEE ALSO |
| .BR getrandom (2), |
| .BR urandom (4), |
| .BR random (7) |