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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ATTRIBUTES |VERSIONS |STANDARDS |HISTORY |EXAMPLES |SEE ALSO |COLOPHON | |
rand(3) Library Functions Manualrand(3)rand, rand_r, srand - pseudo-random number generator
Standard C library (libc,-lc)
#include <stdlib.h>int rand(void);void srand(unsigned intseed);[[deprecated]] int rand_r(unsigned int *seedp); Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):rand_r(): Since glibc 2.24: _POSIX_C_SOURCE >= 199506L glibc 2.23 and earlier _POSIX_C_SOURCE
Therand() function returns a pseudo-random integer in the range 0 toRAND_MAXinclusive (i.e., the mathematical range [0,RAND_MAX]). Thesrand() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned byrand(). These sequences are repeatable by callingsrand() with the same seed value. If no seed value is provided, therand() function is automatically seeded with a value of 1. The functionrand() is not reentrant, since it uses hidden state that is modified on each call. This might just be the seed value to be used by the next call, or it might be something more elaborate. In order to get reproducible behavior in a threaded application, this state must be made explicit; this can be done using the reentrant functionrand_r(). Likerand(),rand_r() returns a pseudo-random integer in the range [0,RAND_MAX]. Theseedp argument is a pointer to anunsigned int that is used to store state between calls. Ifrand_r() is called with the same initial value for the integer pointed to byseedp, and that value is not modified between calls, then the same pseudo-random sequence will result. The value pointed to by theseedp argument ofrand_r() provides only a very small amount of state, so this function will be a weak pseudo-random generator. Trydrand48_r(3) instead.
Therand() andrand_r() functions return a value between 0 andRAND_MAX(inclusive). Thesrand() function returns no value.
For an explanation of the terms used in this section, seeattributes(7). ┌──────────────────────────────────────┬───────────────┬─────────┐ │Interface│Attribute│Value│ ├──────────────────────────────────────┼───────────────┼─────────┤ │rand(),rand_r(),srand() │ Thread safety │ MT-Safe │ └──────────────────────────────────────┴───────────────┴─────────┘
The versions ofrand() andsrand() in the Linux C Library use the same random number generator asrandom(3) andsrandom(3), so the lower-order bits should be as random as the higher-order bits. However, on olderrand() implementations, and on current implementations on different systems, the lower-order bits are much less random than the higher-order bits. Do not use this function in applications intended to be portable when good randomness is needed. (Userandom(3) instead.)
rand()srand() C11, POSIX.1-2008.rand_r() POSIX.1-2008.
rand()srand() SVr4, 4.3BSD, C89, POSIX.1-2001.rand_r() POSIX.1-2001. Obsolete in POSIX.1-2008.
POSIX.1-2001 gives the following example of an implementation ofrand() andsrand(), possibly useful when one needs the same sequence on two different machines. static unsigned long next = 1; /* RAND_MAX assumed to be 32767 */ int myrand(void) { next = next * 1103515245 + 12345; return((unsigned) (next/65536) % 32768); } void mysrand(unsigned int seed) { next = seed; } The following program can be used to display the pseudo-random sequence produced byrand() when given a particular seed. When the seed is-1, the program uses a random seed. #include <stdio.h> #include <stdlib.h> int main(int argc, char *argv[]) { int r; unsigned int seed, nloops; if (argc != 3) { fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]); exit(EXIT_FAILURE); } seed = atoi(argv[1]); nloops = atoi(argv[2]); if (seed == -1) { seed = arc4random(); printf("seed: %u\n", seed); } srand(seed); for (unsigned int j = 0; j < nloops; j++) { r = rand(); printf("%d\n", r); } exit(EXIT_SUCCESS); }drand48(3),random(3)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17rand(3)Pages that refer to this page:mcookie(1), arc4random(3), drand48(3), drand48_r(3), random(3), random_r(3)
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