3. Library calls (functions within program libraries)
RAND
Section: Linux Programmer's Manual (3)Updated: 20170713
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NAME
rand, rand_r, srand  pseudorandom number generatorSYNOPSIS
#include <stdlib.h> int rand(void); int rand_r(unsigned int *seedp); void srand(unsigned int seed);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
rand_r():

Since glibc 2.24:
_POSIX_C_SOURCE >= 199506L
Glibc 2.23 and earlier
_POSIX_C_SOURCE
DESCRIPTION
The rand() function returns a pseudorandom integer in the range 0 to RAND_MAX inclusive (i.e., the mathematical range [0, RAND_MAX]).The srand() function sets its argument as the seed for a new sequence of pseudorandom integers to be returned by rand(). These sequences are repeatable by calling srand() with the same seed value.
If no seed value is provided, the rand() function is automatically seeded with a value of 1.
The function rand() 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 function rand_r().
Like rand(), rand_r() returns a pseudorandom integer in the range [0, RAND_MAX]. The seedp argument is a pointer to an unsigned int that is used to store state between calls. If rand_r() is called with the same initial value for the integer pointed to by seedp, and that value is not modified between calls, then the same pseudorandom sequence will result.
The value pointed to by the seedp argument of rand_r() provides only a very small amount of state, so this function will be a weak pseudorandom generator. Try drand48_r(3) instead.
RETURN VALUE
The rand() and rand_r() functions return a value between 0 and RAND_MAX (inclusive). The srand() function returns no value.ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).Interface  Attribute  Value 
rand(), rand_r(), srand()  Thread safety  MTSafe 
CONFORMING TO
The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99, POSIX.12001. The function rand_r() is from POSIX.12001. POSIX.12008 marks rand_r() as obsolete.NOTES
The versions of rand() and srand() in the Linux C Library use the same random number generator as random(3) and srandom(3), so the lowerorder bits should be as random as the higherorder bits. However, on older rand() implementations, and on current implementations on different systems, the lowerorder bits are much less random than the higherorder bits. Do not use this function in applications intended to be portable when good randomness is needed. (Use random(3) instead.)EXAMPLE
POSIX.12001 gives the following example of an implementation of rand() and srand(), 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 pseudorandom sequence produced by rand() when given a particular seed.
#include <stdlib.h> #include <stdio.h>
int
main(int argc, char *argv[])
{
int j, r, nloops;
unsigned int seed;
if (argc != 3) {
fprintf(stderr, "Usage: %s <seed> <nloops>\n", argv[0]);
exit(EXIT_FAILURE);
}
seed = atoi(argv[1]);
nloops = atoi(argv[2]);
srand(seed);
for (j = 0; j < nloops; j++) {
r = rand();
printf("%d\n", r);
}
SEE ALSO
drand48(3), random(3)COLOPHON
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