matherr(3)

💅 matherr(3) — SVID math library exception handling 💅

<span style="color:pink">okay bestie, buckle up</span> — this is a Library Functions Manual page, section 3, and it's about matherr, the old-school SVID way of catching math exceptions. spoiler: it's a fossil. But you need to know it if you're porting ancient code, so let's go 🦋

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🌸 NAME 🌸

matherr — SVID math library exception handling

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📚 LIBRARY 📚

Math library (libm, -lm) 💖

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✨ SYNOPSIS ✨

#include <math.h>

[[deprecated]] int matherr(struct exception *exc);

[[deprecated]] extern _LIB_VERSION_TYPE _LIB_VERSION;

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🎀 DESCRIPTION 🎀

Note: the mechanism described in this page is no longer supported by glibc. Before glibc 2.27, it had been marked as obsolete. Since glibc 2.27, the mechanism has been removed altogether. New applications should use the techniques described in math_error(7) and fenv(3). This page documents the matherr() mechanism as an aid for maintaining and porting older applications.

🚨 bestie, this is literally a museum exhibit. It's here to help you keep old code alive, not to build anything new on top of. Do not start a fresh project with this, I am begging 🙈

The System V Interface Definition (SVID) specifies that various math functions should invoke a function called matherr() if a math exception is detected. This function is called before the math function returns; after matherr() returns, the system then returns to the math function, which in turn returns to the caller. so it's like, the math function stops mid-step, whispers to your function, and then finishes. iconic behavior tbh 💫

To employ matherr(), the programmer must define the _SVID_SOURCE feature test macro (before including any header files), and assign the value _SVID_ to the external variable _LIB_VERSION.

The system provides a default version of matherr(). This version does nothing, and returns zero (see below for the significance of this). The default matherr() can be overridden by a programmer-defined version, which will be invoked when an exception occurs. The function is invoked with one argument, a pointer to an exception structure, defined as follows:

struct exception {
    int    type;      /* Exception type */
    char  *name;      /* Name of function causing exception */
    double arg1;      /* 1st argument to function */
    double arg2;      /* 2nd argument to function */
    double retval;    /* Function return value */
}

The type field has one of the following values:

• DOMAIN — a domain error occurred (the function argument was outside the range for which the function is defined). The return value depends on the function; errno is set to EDOM.
• SING — a pole error occurred (the function result is an infinity). The return value in most cases is HUGE (the largest single precision floating-point number), appropriately signed. In most cases, errno is set to EDOM.
• OVERFLOW — an overflow occurred. In most cases, the value HUGE is returned, and errno is set to ERANGE.
• UNDERFLOW — an underflow occurred. 0.0 is returned, and errno is set to ERANGE.
• TLOSS — total loss of significance. 0.0 is returned, and errno is set to ERANGE.
• PLOSS — partial loss of significance. This value is unused on glibc (and many other systems).

The arg1 and arg2 fields are the arguments supplied to the function (arg2 is undefined for functions that take only one argument). single-argument girlies, don't go looking at arg2, it's not for you 💅

The retval field specifies the return value that the math function will return to its caller. The programmer-defined matherr() can modify this field to change the return value of the math function. yesss, you get to rewrite the ending of the math function's story. main character energy.

If the matherr() function returns zero, then the system sets errno as described above, and may print an error message on standard error (see below).

If the matherr() function returns a nonzero value, then the system does not set errno, and doesn't print an error message. ✨ full silence, full control ✨

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🧮 Math functions that employ matherr() 🧮

The table below lists the functions and circumstances in which matherr() is called. The "Type" column indicates the value assigned to exc->type when calling matherr(). The "Result" column is the default return value assigned to exc->retval.

The "Msg?" and "errno" columns describe the default behavior if matherr() returns zero. If the "Msg?" column contains "y", then the system prints an error message on standard error.

The table uses the following notations and abbreviations:

• x — first argument to function
• y — second argument to function
• fin — finite value for argument
• neg — negative value for argument
• int — integral value for argument
• o/f — result overflowed
• u/f — result underflowed
• |x| — absolute value of x
• X_TLOSS — is a constant defined in <math.h>

here comes the big one, bestie. every single row, no skips 💖

Function             Type        Result         Msg?   errno
acos(|x|>1)          DOMAIN      HUGE            y     EDOM
asin(|x|>1)          DOMAIN      HUGE            y     EDOM
atan2(0,0)           DOMAIN      HUGE            y     EDOM
acosh(x<1)           DOMAIN      NAN             y     EDOM
atanh(|x|>1)         DOMAIN      NAN             y     EDOM
atanh(|x|==1)        SING        (x>0.0)?        y     EDOM
                                 HUGE_VAL :
                                 -HUGE_VAL
cosh(fin) o/f        OVERFLOW    HUGE            n     ERANGE
sinh(fin) o/f        OVERFLOW    (x>0.0) ?       n     ERANGE
                                 HUGE : -HUGE
sqrt(x<0)            DOMAIN      0.0             y     EDOM
hypot(fin,fin) o/f   OVERFLOW    HUGE            n     ERANGE
exp(fin) o/f         OVERFLOW    HUGE            n     ERANGE
exp(fin) u/f         UNDERFLOW   0.0             n     ERANGE
exp2(fin) o/f        OVERFLOW    HUGE            n     ERANGE
exp2(fin) u/f        UNDERFLOW   0.0             n     ERANGE
exp10(fin) o/f       OVERFLOW    HUGE            n     ERANGE
exp10(fin) u/f       UNDERFLOW   0.0             n     ERANGE
j0(|x|>X_TLOSS)      TLOSS       0.0             y     ERANGE
j1(|x|>X_TLOSS)      TLOSS       0.0             y     ERANGE
jn(|x|>X_TLOSS)      TLOSS       0.0             y     ERANGE
y0(x>X_TLOSS)        TLOSS       0.0             y     ERANGE
y1(x>X_TLOSS)        TLOSS       0.0             y     ERANGE
yn(x>X_TLOSS)        TLOSS       0.0             y     ERANGE
y0(0)                DOMAIN      -HUGE           y     EDOM
y0(x<0)              DOMAIN      -HUGE           y     EDOM
y1(0)                DOMAIN      -HUGE           y     EDOM
y1(x<0)              DOMAIN      -HUGE           y     EDOM
yn(n,0)              DOMAIN      -HUGE           y     EDOM
yn(x<0)              DOMAIN      -HUGE           y     EDOM
lgamma(fin) o/f      OVERFLOW    HUGE            n     ERANGE
lgamma(-int) or      SING        HUGE            y     EDOM
  lgamma(0)
tgamma(fin) o/f      OVERFLOW    HUGE_VAL        n     ERANGE
tgamma(-int)         SING        NAN             y     EDOM
tgamma(0)            SING        copysign(       y     ERANGE
                                 HUGE_VAL,x)
log(0)               SING        -HUGE           y     EDOM
log(x<0)             DOMAIN      -HUGE           y     EDOM
log2(0)              SING        -HUGE           n     EDOM
log2(x<0)            DOMAIN      -HUGE           n     EDOM
log10(0)             SING        -HUGE           y     EDOM
log10(x<0)           DOMAIN      -HUGE           y     EDOM
pow(0.0,0.0)         DOMAIN      0.0             y     EDOM
pow(x,y) o/f         OVERFLOW    HUGE            n     ERANGE
pow(x,y) u/f         UNDERFLOW   0.0             n     ERANGE
pow(NaN,0.0)         DOMAIN      x               n     EDOM
0**neg               DOMAIN      0.0             y     EDOM
neg**non-int         DOMAIN      0.0             y     EDOM
scalb() o/f          OVERFLOW    (x>0.0) ?       n     ERANGE
                                 HUGE_VAL :
                                 -HUGE_VAL
scalb() u/f          UNDERFLOW   copysign(       n     ERANGE
                                   0.0,x)
fmod(x,0)            DOMAIN      x               y     EDOM
remainder(x,0)       DOMAIN      NAN             y     EDOM

look at all those EDOMs and ERANGEs. the Msg? column is basically a yes/no on whether the system is gonna yell at you in stderr 😭

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🌈 ATTRIBUTES 🌈

For an explanation of the terms used in this section, see attributes(7).

• matherr() — Thread safety: MT-Safe 🫶

that's the whole vibe check on this one, bestie — MT-Safe, so it plays nice with threads. 💜

💖 EXAMPLES 💖

okay bestie, here's the whole tea ☕ — this lil example program shows you what matherr() is doing when you call log(3). It takes up to three command-line arguments, and yes we're counting them out:

• the first argument is the floating-point number you're feeding to log(3)
• if you pass the optional second argument, then _LIB_VERSION gets set to _SVID_ so that matherr() actually gets called, and that integer you typed gets used as the return value from matherr()
• if you toss in the optional third command-line argument too, that one specifies an alternative return value that matherr() should assign as the return value of the math function ✨

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Here's a run where log(3) gets an argument of 0.0 — and no matherr(), no drama:

$ ./a.out 0.0
errno: Numerical result out of range
x=-inf

Next up: matherr() is called, and it returns 0:

$ ./a.out 0.0 0
matherr SING exception in log() function
        args:   0.000000, 0.000000
        retval: -340282346638528859811704183484516925440.000000
log: SING error
errno: Numerical argument out of domain
x=-340282346638528859811704183484516925440.000000

That "log: SING error" message? Yeah, that was the C library printing it, not the program. 🫶

And now matherr() is called again, but this time it returns a nonzero value:

$ ./a.out 0.0 1
matherr SING exception in log() function
        args:   0.000000, 0.000000
        retval: -340282346638528859811704183484516925440.000000
x=-340282346638528859811704183484516925440.000000

In this case the C library did not print a message, and errno was not set. iconic silence honestly 🤫

Last one, bestie — matherr() gets called, changes the return value of the math function, and returns a nonzero value:

$ ./a.out 0.0 1 12345.0
matherr SING exception in log() function
        args:   0.000000, 0.000000
        retval: -340282346638528859811704183484516925440.000000
x=12345.000000

🎀 Program source 🎀

here's the whole thing, character for character, do not touch a single one of them 💅

#define _SVID_SOURCE
#include <errno.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>

static int matherr_ret = 0;     /* Value that matherr()
                                   should return */
static int change_retval = 0;   /* Should matherr() change
                                   function's return value? */
static double new_retval;       /* New function return value */

int
matherr(struct exception *exc)
{
    fprintf(stderr, "matherr %s exception in %s() function\n",
            (exc->type == DOMAIN) ?    "DOMAIN" :
            (exc->type == OVERFLOW) ?  "OVERFLOW" :
            (exc->type == UNDERFLOW) ? "UNDERFLOW" :
            (exc->type == SING) ?      "SING" :
            (exc->type == TLOSS) ?     "TLOSS" :
            (exc->type == PLOSS) ?     "PLOSS" : "???",
            exc->name);
    fprintf(stderr, "        args:   %f, %f\n",
            exc->arg1, exc->arg2);
    fprintf(stderr, "        retval: %f\n", exc->retval);

    if (change_retval)
        exc->retval = new_retval;

    return matherr_ret;
}

int
main(int argc, char *argv[])
{
    double x;

    if (argc < 2) {
        fprintf(stderr, "Usage: %s <argval>"
                " [<matherr-ret> [<new-func-retval>]]\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    if (argc > 2) {
        _LIB_VERSION = _SVID_;
        matherr_ret = atoi(argv[2]);
    }

    if (argc > 3) {
        change_retval = 1;
        new_retval = atof(argv[3]);
    }

    x = log(atof(argv[1]));
    if (errno != 0)
        perror("errno");

    printf("x=%f\n", x);
    exit(EXIT_SUCCESS);
}

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👀 SEE ALSO 👀

• fenv(3)
• math_error(7)
• standards(7)

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and that's the whole page, bestie!! 🦋 you ate that. this one's from Linux man-pages 6.20, dated 2025-05-17 — go forth and log() responsibly 💜✨