===== Why Alpha Has libc.so.6.1 =====

Every other currently supported Linux architecture that uses glibc 2.x has ''libc.so.6''. Alpha has ''libc.so.6.1'', and ''libm.so.6.1'' with it. The dynamic linker, ''ld-linux.so.2'', was not renamed.

==== Consequences for portable code ====

Code that names the C library by its file name breaks on Alpha. Typical cases are a ''dlopen("libc.so.6", ...)'' call, a language runtime or foreign function interface that loads ''libc.so.6'' by name, and a test suite or build script that looks for ''libc.so.6'' to find the C library or decide that it is glibc. All of them fail on Alpha, where no file of that name exists. [(>[[https://github.com/martinpitt/umockdev/commit/a52fb88c855708508a16f1d1c03fcdd5101866f3|umockdev commit a52fb88]], Michael Cree, 6 Jul 2026)] The same applies to ''libm.so.6''.

Portable code should not hard-code the name. In C, glibc's ''<gnu/lib-names.h>'' defines ''LIBC_SO'' and ''LIBM_SO'' as the correct names for the target, ''"libc.so.6.1"'' and ''"libm.so.6.1"'' on Alpha. Alternatively, ''dlopen(NULL, ...)'' returns a handle for the main program, through which the C library's symbols can be found without naming it. Python's ''ctypes.util.find_library("c")'' also returns the right name.

Debian names its packages after the soname, so on Alpha the C library is ''libc6.1'' and its development files ''libc6.1-dev'', not ''libc6'' and ''libc6-dev''. [(>[[https://packages.debian.org/search?keywords=libc6.1|libc6.1]], packages.debian.org)] Build dependencies and packaging scripts that name ''libc6-dev'' directly fail on Alpha.

==== Why ====

The reason is an ABI break in January 1997, in glibc 2.0.1. ''dev_t'' became 64 bits wide and glibc's ''sigset_t'' changed from an array of ''unsigned int'' to an array of ''unsigned long'' (1024 bits in either case). [(>[[https://github.com/alphalinux/mailing-list-archives/blob/main/axp-list-mbox/1997-January.mbox#L26971|"Re: glibc upgrade advice?"]], Richard Henderson, axp-list, 23 Jan 1997)] [(>[[https://github.com/alphalinux/mailing-list-archives/blob/main/axp-list-mbox/1997-May.mbox#L5740|"Re: Glibc 2.0.3 and LibX11"]], Richard Henderson, axp-list, 5 May 1997)] Alpha binaries had already been built against ''libc.so.6'', and to keep them from loading the incompatible library, Alpha's soname was changed to 6.1. [(>[[https://sourceware.org/git/?p=glibc.git;a=commit;h=0c5ecdc449ce581b56090758d5bb5673763ea909|"update from main arhive 970119"]], glibc, 20 Jan 1997)] [(>[[https://sourceware.org/git/?p=glibc.git;a=commit;h=831372e7c1|"update from main archive 970127"]], glibc, 28 Jan 1997)] Binaries built against one library do not work with the other. [(>[[https://github.com/alphalinux/mailing-list-archives/blob/main/axp-list-mbox/1997-January.mbox#L26756|"Re: glibc upgrade advice?"]], Philip Blundell, axp-list, 23 Jan 1997)]

Nothing in current Alpha glibc requires the ''.1''. It is a remnant of the 1997 ABI change, kept because changing it back would break every existing Alpha binary.

==== IA-64 ====

IA-64 also used ''libc.so.6.1'' and ''libm.so.6.1'', for undocumented reasons. It entered glibc at ''GLIBC_2.2'' already using 6.1, so it never shipped a ''libc.so.6'' and had no break to signal. IA-64 has since been removed from glibc, leaving Alpha as the only architecture using 6.1.

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