RAND(3P)

💖 RAND(3P) 💖

okay bestie, listen up — this is the POSIX Programmer's Manual page for rand, and we're gonna get you fluent in pseudo-randomness, no cap. ✨

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🌸 PROLOG 🌸

so this page is part of the POSIX Programmer's Manual, periodt. but heads up: the Linux implementation of this interface may differ (go consult the corresponding Linux manual page for details of Linux behavior, fr), or honestly the interface may not even be implemented on Linux at all. 💀

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👑 NAME 👑

rand, rand_r, srand — pseudo-random number generator. the trio. the moment. the icons. 🎀

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✨ SYNOPSIS ✨

#include <stdlib.h>

int rand(void);
int rand_r(unsigned *seed);
void srand(unsigned seed);

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🦋 DESCRIPTION 🦋

okay so for rand() and srand(): the functionality described on this reference page is aligned with the ISO C standard. any conflict between the requirements described here and the ISO C standard is unintentional — not me causing drama, it's just a disclaimer. this volume of POSIX.1‐2017 defers to the ISO C standard, and that's that. 💅

the rand() function shall compute a sequence of pseudo-random integers in the range [0,{RAND_MAX}] with a period of at least 2^32. that's a LOT of numbers bestie.

the rand() function need not be thread-safe. lowkey a red flag but go off. 🚩

the rand_r() function shall compute a sequence of pseudo-random integers in the range [0,{RAND_MAX}]. (the value of the {RAND_MAX} macro shall be at least 32767.)

if rand_r() is called with the same initial value for the object pointed to by seed and that object is not modified between successive returns and calls to rand_r(), the same sequence shall be generated. iconic reproducibility behavior. 🫶

the srand() function uses the argument as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand(). if srand() is then called with the same seed value, the sequence of pseudo-random numbers shall be repeated. and if rand() is called before any calls to srand() are made, the same sequence shall be generated as when srand() is first called with a seed value of 1. so like, 1 is the default bestie, remember that. ⭐

the implementation shall behave as if no function defined in this volume of POSIX.1‐2017 calls rand() or srand(). periodt. 💜

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💫 RETURN VALUE 💫

• the rand() function shall return the next pseudo-random number in the sequence. 🎲
• the rand_r() function shall return a pseudo-random integer.
• the srand() function shall not return a value. it gives nothing back and that's on purpose, we respect it. 🙈

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💀 ERRORS 💀

no errors are defined. literally none. flawless. ✨

the following sections are informative. 📖

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🍓 EXAMPLES 🍓

🌈 Generating a Pseudo-Random Number Sequence

the following example demonstrates how to generate a sequence of pseudo-random numbers. slay.

#include <stdio.h>
#include <stdlib.h>
...
    long count, i;
    char *keystr;
    int elementlen, len;
    char c;
...
/* Initial random number generator. */
    srand(1);

    /* Create keys using only lowercase characters */
    len = 0;
    for (i=0; i<count; i++) {
        while (len < elementlen) {
            c = (char) (rand() % 128);
            if (islower(c))
                keystr[len++] = c;
        }

        keystr[len] = '\0';
        printf("%s Element%0*ld\n", keystr, elementlen, i);
        len = 0;
    }

🧚 Generating the Same Sequence on Different Machines

the following code defines a pair of functions that could be incorporated into applications wishing to ensure that the same sequence of numbers is generated across different machines. cross-platform consistency? we love to see it. 💖

static unsigned long next = 1;
int myrand(void)  /* RAND_MAX assumed to be 32767. */
{
    next = next * 1103515245 + 12345;
    return((unsigned)(next/65536) % 32768);
}

void mysrand(unsigned seed)
{
    next = seed;
}

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🎀 APPLICATION USAGE 🎀

the drand48() and random() functions provide much more elaborate pseudo-random number generators. so if you want the fancy stuff, you know where to look. 👀

the limitations on the amount of state that can be carried between one function call and another mean the rand_r() function can never be implemented in a way which satisfies all of the requirements on a pseudo-random number generator. it's giving structurally limited, ngl. 😭

these functions should be avoided whenever non-trivial requirements (including safety) have to be fulfilled. okay that's a serious one bestie — if safety is on the line, walk away. 🚨

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🔥 RATIONALE 🔥

the ISO C standard rand() and srand() functions allow per-process pseudo-random streams shared by all threads. those two functions need not change, but there has to be mutual-exclusion that prevents interference between two threads concurrently accessing the random number generator. like, no cutting in line. 🚧

with regard to rand(), there are two different behaviors that may be wanted in a multi-threaded program:

1. a single per-process sequence of pseudo-random numbers that is shared by all threads that call rand()
2. a different sequence of pseudo-random numbers for each thread that calls rand()

this is provided by the modified thread-safe function based on whether the seed value is global to the entire process or local to each thread. main character energy per thread, if you will. 👑

this does not address the known deficiencies of the rand() function implementations, which have been approached by maintaining more state. in effect, this specifies new thread-safe forms of a deficient function. yeah... they said it, not me. 💀

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⭐ FUTURE DIRECTIONS ⭐

the rand_r() function may be removed in a future version. so... don't get too attached bestie. 🥲

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🌸 SEE ALSO 🌸

• drand48()
• initstate()

the Base Definitions volume of POSIX.1‐2017, <stdlib.h> — go read it, iykyk. 📚

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💜 COPYRIGHT 💜

portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. in the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. the original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. to report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page, bestie!! this was IEEE/The Open Group, 2017, page RAND(3P) — ty for coming to my ted talk, go seed responsibly 💅✨🎀