abs / labs / llabs / imaxabs / div / ldiv / lldiv (3) - glibc

Split from the chapter glibc: strings, ctype and stdlib arithmetic into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <stdlib.h>

int       abs(int j);        long labs(long j);
long long llabs(long long j);
typedef struct { int quot, rem; } div_t;
typedef struct { long quot, rem; } ldiv_t;
typedef struct { long long quot, rem; } lldiv_t;
div_t  div(int numer, int denom);
ldiv_t ldiv(long numer, long denom);
lldiv_t lldiv(long long numer, long long denom);   /* C99 gate _ISOC99_SOURCE */

#include <inttypes.h>
intmax_t imaxabs(intmax_t j);
imaxdiv_t imaxdiv(intmax_t numer, intmax_t denom);

llabs/lldiv are gated __USE_ISOC99 in the installed header (/usr/include/stdlib.h lines 971-976, 985-990); the alias pairs labs == imaxabs and ldiv == imaxdiv are visible in nm as identical addresses with W marking on the intmax names.

Semantics

The absolute-value functions return the magnitude of j in the matching integer type (abs(3)); all are __attribute_const__ in the header. The div family computes numer / denom and numer % denom together and returns them in a *_t struct with members quot and rem (member order unspecified per div(3)); the quotient is rounded toward zero and the invariant quot * denom + rem == numer always holds (div(3), manual arith.texi Integer Division).

Gotchas and portability

• abs(INT_MIN) is undefined: in two’s complement the magnitude of the most negative value is not representable (abs(3) NOTES; manual Absolute Value states it plainly). Same trap for labs(LONG_MIN) and llabs(LLONG_MIN).
• Division by zero, and any unrepresentable division result, is undefined behavior for the div family (manual Integer Division: “as in a division by zero”).
• Why div exists: in ISO C the rounding of / for negative operands was implementation-defined; div pins the round-toward-zero semantics and the sign of the remainder follows the numerator. div(-5, 3) gives quot == -1, rem == -2 (div(3) EXAMPLES). In GNU C / and % already round toward zero, so the manual calls these functions redundant under GCC but useful for exactness and single-instruction codegen.
• GCC treats abs/labs/llabs/imaxabs as builtins and may fold them (abs(3) NOTES), so a breakpoint on abs is unreliable.
• The master manual documents uabs/ulabs/ullabs/umaxabs as new in ISO C2Y (arith.texi Absolute Value); none are exported by this host’s glibc 2.41 (nm), so treat them as unverified for this host.
• musl 1.2.5 keeps the same ISO semantics (trivial wrappers in musl src/stdlib/abs.c, div.c).

References

• glibc manual: arith.texi, nodes Integer Division, Absolute Value
• man3: abs(3) (covers labs, llabs, imaxabs), div(3) (covers ldiv, lldiv, imaxdiv)
• headers: /usr/include/stdlib.h, /usr/include/inttypes.h
• symbols: nm -D --with-symbol-versions libc.so.6