strcat / strncat (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 <string.h>

char *strcat(char *restrict dest, const char *restrict src);
char *strncat(char *restrict dest, const char *restrict src, size_t n);

No feature-test gates: both are ISO C (/usr/include/string.h lines 148-153).

Semantics

strcat appends src to dest, overwriting dest’s terminating null and adding a new one; the strings may not overlap and dest must have room for the result (strcat(3)). Both return dest. strncat uses at most n bytes from src (which then need not be null-terminated) and always appends one terminating null, so it writes n + 1 bytes when src has n or more bytes; dest must be at least strlen(dest) + n + 1 bytes (strcat(3); manual Truncating Strings node). A compliant sketch from the manual is memcpy(to + len, from, strnlen(from, size)); to[len + strnlen(from, size)] = '\0';.

Gotchas and portability

• dest size arithmetic is the classic overflow source: strcat(3) states outright that if dest is not large enough “buffer overruns are a favorite avenue for attacking secure programs”.
• strncat is the safer of the pair but is not a bounded strcat with the same argument order of risk: its n bounds the source, not the destination, and it still assumes dest already holds a valid string.
• Quadratic cost: both functions re-scan dest from its start to find the terminator, so repeated concatenation into one buffer is O(n^2); strcat(3) ships a benchmark program demonstrating this, and the manual’s Truncating Strings node lists “significant performance issues” for both. For many appends, track the end pointer yourself or use stpcpy.
• Overlap is undefined (manual, Truncating Strings).
• BSD strlcat: added to glibc in 2.38 (glibc NEWS under Version 2.38); declared on this host under __USE_MISC in string.h. The cached strcat(3) page (2021) predates that and still says it is absent.
• musl 1.2.5 has both plus strlcat (musl src/string/strcat.c, src/string/strlcat.c).

References

• glibc manual: string.texi, nodes Concatenating Strings, Truncating Strings
• man3: strcat(3) (covers strncat via .so alias strncat.3)
• header: /usr/include/string.h
• symbols: nm -D --with-symbol-versions libc.so.6