strfry / memfrob (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

#define _GNU_SOURCE
#include <string.h>

char  *strfry(char *string);
void  *memfrob(void *s, size_t n);

Both live in the __USE_GNU block of /usr/include/string.h (gate line 517): strfry at lines 522-523, memfrob at lines 525-527. The man pages record the same _GNU_SOURCE requirement, and the manual’s @standards{GNU, string.h} agrees. There is no _l or bounded variant of either.

Semantics

strfry performs an in-place shuffle of string: each character is swapped to a random position within the substring starting at its own position - Fisher-Yates, per the manual, so the result is an unbiased anagram. It returns string itself and, notably, leaves every global random number generator undisturbed because it seeds from its own state (manual, Shuffling Bytes). memfrob reversibly obfuscates length bytes at mem by XORing each with the fixed byte 0x2A (42, binary 00101010); running it a second time restores the original. Both functions return their first argument (strfry(3), memfrob(3), manual Obfuscating Data).

Gotchas and portability

• Neither is a security primitive: the manual’s Obfuscating Data node says the memfrob result “still bears a clear relationship to the original” (Rot13-grade obfuscation, no key) and points real encryption at a crypto library; memfrob(3) says the same (“suitable only for hiding strings”).
• Distinguishing erase from obfuscate: the manual routes erasing to explicit_bzero and reversible obfuscation to memfrob (Shuffling Bytes intro) - strfry destroys data by permutation and is listed as a destroy-adjacent oddity, not a wipe.
• Both mutate in place and are GNU-only: musl 1.2.5 has neither (no strfry.c/memfrob.c under musl src/string/, no declarations in musl include/string.h).

References

• glibc manual: string.texi, nodes Shuffling Bytes, Obfuscating Data, Erasing Sensitive Data (cross-references)
• man3: strfry(3), memfrob(3)
• header: /usr/include/string.h
• symbols: nm -D --with-symbol-versions libc.so.6