[PATCH v4 05/10] compat/posix: introduce writev(3p) wrapper
- From
Patrick Steinhardt <ps@pks.im>
- Date
- Mar 13, 2026, 06:45 UTC
- Message-ID
- <20260313-pks-upload-pack-write-contention-v4-5-7a9668061f7f@pks.im>
- In-Reply-To
- <20260313-pks-upload-pack-write-contention-v4-0-7a9668061f7f@pks.im>
In a subsequent commit we're going to add the first caller to writev(3p). Introduce a compatibility wrapper for this syscall that we can use on systems that don't have this syscall.
The syscall exists on modern Unixes like Linux and macOS, and seemingly even for NonStop according to [1]. It doesn't seem to exist on Windows though.
[1]: http://nonstoptools.com/manuals/OSS-SystemCalls.pdf [2]: https://www.gnu.org/software/gnulib/manual/html_node/writev.html
Signed-off-by: Patrick Steinhardt <ps@pks.im> --- Makefile | 4 ++++ compat/posix.h | 14 ++++++++++++++ compat/writev.c | 44 ++++++++++++++++++++++++++++++++++++++++++++ config.mak.uname | 2 ++ meson.build | 1 + 5 files changed, 65 insertions(+)
diff --git a/Makefile b/Makefile index f3264d0a37..493851162d 100644 --- a/Makefile +++ b/Makefile @@ -2021,6 +2021,10 @@ ifdef NO_PREAD COMPAT_CFLAGS += -DNO_PREAD COMPAT_OBJS += compat/pread.o endif +ifdef NO_WRITEV + COMPAT_CFLAGS += -DNO_WRITEV + COMPAT_OBJS += compat/writev.o +endif ifdef NO_FAST_WORKING_DIRECTORY BASIC_CFLAGS += -DNO_FAST_WORKING_DIRECTORY endif diff --git a/compat/posix.h b/compat/posix.h index 245386fa4a..3c611d2736 100644 --- a/compat/posix.h +++ b/compat/posix.h @@ -137,6 +137,9 @@ #include <sys/socket.h> #include <sys/ioctl.h> #include <sys/statvfs.h> +#ifndef NO_WRITEV +#include <sys/uio.h> +#endif #include <termios.h> #ifndef NO_SYS_SELECT_H #include <sys/select.h> @@ -323,6 +326,17 @@ int git_lstat(const char *, struct stat *); ssize_t git_pread(int fd, void *buf, size_t count, off_t offset); #endif +#ifdef NO_WRITEV +#define writev git_writev +#define iovec git_iovec +struct git_iovec { + void *iov_base; + size_t iov_len; +}; + +ssize_t git_writev(int fd, const struct iovec *iov, int iovcnt); +#endif + #ifdef NO_SETENV #define setenv gitsetenv int gitsetenv(const char *, const char *, int); diff --git a/compat/writev.c b/compat/writev.c new file mode 100644 index 0000000000..3a94870a2f --- /dev/null +++ b/compat/writev.c @@ -0,0 +1,44 @@ +#include "../git-compat-util.h" +#include "../wrapper.h" + +ssize_t git_writev(int fd, const struct iovec *iov, int iovcnt) +{ + size_t total_written = 0; + size_t sum = 0; + + /* + * According to writev(3p), the syscall shall error with EINVAL in case + * the sum of `iov_len` overflows `ssize_t`. + */ + for (int i = 0; i < iovcnt; i++) { + if (iov[i].iov_len > maximum_signed_value_of_type(ssize_t) || + iov[i].iov_len + sum > maximum_signed_value_of_type(ssize_t)) { + errno = EINVAL; + return -1; + } + + sum += iov[i].iov_len; + } + + for (int i = 0; i < iovcnt; i++) { + const char *bytes = iov[i].iov_base; + size_t iovec_written = 0; + + while (iovec_written < iov[i].iov_len) { + ssize_t bytes_written = xwrite(fd, bytes + iovec_written, + iov[i].iov_len - iovec_written); + if (bytes_written < 0) { + if (total_written) + goto out; + return bytes_written; + } + if (!bytes_written) + goto out; + iovec_written += bytes_written; + total_written += bytes_written; + } + } + +out: + return (ssize_t) total_written; +} diff --git a/config.mak.uname b/config.mak.uname index 5feb582558..ccb3f71881 100644 --- a/config.mak.uname +++ b/config.mak.uname @@ -459,6 +459,7 @@ ifeq ($(uname_S),Windows) SANE_TOOL_PATH ?= $(msvc_bin_dir_msys) HAVE_ALLOCA_H = YesPlease NO_PREAD = YesPlease + NO_WRITEV = YesPlease NEEDS_CRYPTO_WITH_SSL = YesPlease NO_LIBGEN_H = YesPlease NO_POLL = YesPlease @@ -674,6 +675,7 @@ ifeq ($(uname_S),MINGW) pathsep = ; HAVE_ALLOCA_H = YesPlease NO_PREAD = YesPlease + NO_WRITEV = YesPlease NEEDS_CRYPTO_WITH_SSL = YesPlease NO_LIBGEN_H = YesPlease NO_POLL = YesPlease diff --git a/meson.build b/meson.build index 4b536e0124..381974ab57 100644 --- a/meson.build +++ b/meson.build @@ -1414,6 +1414,7 @@ checkfuncs = { 'initgroups' : [], 'strtoumax' : ['strtoumax.c', 'strtoimax.c'], 'pread' : ['pread.c'], + 'writev' : ['writev.c'], } if host_machine.system() == 'windows'
-- 2.53.0.904.g2727be2e99.dirty