main.c

#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main(void) {
    const char *src = "config";
    const char *dst = "config.lock";
    const char *src_content =
        "[remote \"origin\"]\n"
        "\turl = https://example.invalid/repo.git\n";

    int fd;
    ssize_t n;
    struct stat st;
    void *addr;

    /* Create a small source file like .git/config */
    fd = open(src, O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd < 0) {
        perror("open src for write");
        return 1;
    }

    n = write(fd, src_content, strlen(src_content));
    if (n < 0 || (size_t)n != strlen(src_content)) {
        perror("write src");
        close(fd);
        return 1;
    }

    if (close(fd) != 0) {
        perror("close src after create");
        return 1;
    }

    /* Open lock file in a Git-like way */
    int lockfd = open(dst, O_RDWR | O_CREAT | O_EXCL | O_CLOEXEC, 0666);
    if (lockfd < 0) {
        perror("open lock");
        return 1;
    }

    /* Redundant after O_CLOEXEC, but matches the Git trace pattern */
    if (fcntl(lockfd, F_SETFD, FD_CLOEXEC) < 0) {
        perror("fcntl(F_SETFD)");
        close(lockfd);
        return 1;
    }

    /* Open source file read-only and mmap it */
    int srcfd = open(src, O_RDONLY);
    if (srcfd < 0) {
        perror("open src for read");
        close(lockfd);
        return 1;
    }

    if (fstat(srcfd, &st) != 0) {
        perror("fstat");
        close(srcfd);
        close(lockfd);
        return 1;
    }

    if (st.st_size == 0) {
        fprintf(stderr, "source file is empty\n");
        close(srcfd);
        close(lockfd);
        return 1;
    }

    addr = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, srcfd, 0);
    if (addr == MAP_FAILED) {
        perror("mmap");
        close(srcfd);
        close(lockfd);
        return 1;
    }

    printf("mmap returned %p\n", addr);

    /* Close the source fd after mmap: mapping remains valid */
    if (close(srcfd) != 0) {
        perror("close srcfd");
        munmap(addr, st.st_size);
        close(lockfd);
        return 1;
    }

    if (chmod(dst, 0644) != 0) {
        perror("chmod lock");
        munmap(addr, st.st_size);
        close(lockfd);
        return 1;
    }

    n = write(lockfd, "[core]\n", 7);
    if (n < 0) {
        perror("write prefix 1");
        munmap(addr, st.st_size);
        close(lockfd);
        return 1;
    }
    printf("wrote prefix 1: %zd bytes\n", n);

    n = write(lockfd, "\tbare = false\n", 14);
    if (n < 0) {
        perror("write prefix 2");
        munmap(addr, st.st_size);
        close(lockfd);
        return 1;
    }
    printf("wrote prefix 2: %zd bytes\n", n);

    /* Write directly from the mmap'd address */
    n = write(lockfd, addr, st.st_size);
    if (n < 0) {
        perror("write from mmap");
        munmap(addr, st.st_size);
        close(lockfd);
        return 1;
    }
    printf("wrote mapped content: %zd bytes\n", n);

    if (close(lockfd) != 0) {
        perror("close lockfd");
        munmap(addr, st.st_size);
        return 1;
    }

    if (munmap(addr, st.st_size) != 0) {
        perror("munmap");
        return 1;
    }

    /* Show result */
    fd = open(dst, O_RDONLY);
    if (fd < 0) {
        perror("open dst for readback");
        return 1;
    }

    printf("--- %s contents ---\n", dst);
    char buf[4096];
    while ((n = read(fd, buf, sizeof(buf))) > 0) {
        ssize_t off = 0;
        while (off < n) {
            ssize_t wn = write(STDOUT_FILENO, buf + off, (size_t)(n - off));
            if (wn < 0) {
                perror("write stdout");
                close(fd);
                return 1;
            }
            off += wn;
        }
    }
    if (n < 0) {
        perror("read dst");
        close(fd);
        return 1;
    }

    if (close(fd) != 0) {
        perror("close dst readback");
        return 1;
    }

    return 0;
}

strace.txt

rook-ceph-demo-nfs-auxtel-jhoblitt-test1:/data/auxtel/gittest$ strace ./a.out 
execve("./a.out", ["./a.out"], 0x7ffc1e9c2a50 /* 16 vars */) = 0
arch_prctl(ARCH_SET_FS, 0x7f54b8fc47b8) = 0
set_tid_address(0x7f54b8fc48f0)         = 563
brk(NULL)                               = 0xbc6000
brk(0xbc7000)                           = 0xbc7000
readlink("/proc/self/exe", "/data/auxtel/gittest/a.out", 4096) = 26
execve("/lib/ld-musl-x86_64.so.1", ["ld-linux-x86-64.so.2", "--argv0", "./a.out", "--preload", "/lib/libgcompat.so.0 ", "--", "/data/auxtel/gittest/a.out"], 0x7ffe3e83dc78 /* 16 vars */) = 0
arch_prctl(ARCH_SET_FS, 0x7fd1b4ddfb28) = 0
set_tid_address(0x7fd1b4ddff90)         = 563
open("/data/auxtel/gittest/a.out", O_RDONLY|O_LARGEFILE) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\2\0>\0\1\0\0\0@\4@\0\0\0\0\0"..., 960) = 960
mmap(0x400000, 16384, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) = 0x400000
mmap(0x401000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED, 3, 0x1000) = 0x401000
mmap(0x402000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x1000) = 0x402000
close(3)                                = 0
brk(NULL)                               = 0x555556877000
brk(0x555556879000)                     = 0x555556879000
mmap(0x555556877000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x555556877000
open("/lib/libgcompat.so.0", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
fstat(3, {st_mode=S_IFREG|0755, st_size=67184, ...}) = 0
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
mmap(NULL, 73728, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd1b4d28000
mmap(0x7fd1b4d2f000, 20480, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0x7000) = 0x7fd1b4d2f000
mmap(0x7fd1b4d34000, 12288, PROT_READ, MAP_PRIVATE|MAP_FIXED, 3, 0xc000) = 0x7fd1b4d34000
mmap(0x7fd1b4d37000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0xf000) = 0x7fd1b4d37000
mmap(0x7fd1b4d39000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7fd1b4d39000
close(3)                                = 0
open("/etc/ld-musl-x86_64.path", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/lib/libucontext.so.1", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/usr/local/lib/libucontext.so.1", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/usr/lib/libucontext.so.1", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
fstat(3, {st_mode=S_IFREG|0755, st_size=13936, ...}) = 0
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
mmap(NULL, 20480, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd1b4d23000
mmap(0x7fd1b4d24000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0x1000) = 0x7fd1b4d24000
mmap(0x7fd1b4d25000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = 0x7fd1b4d25000
mmap(0x7fd1b4d26000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = 0x7fd1b4d26000
close(3)                                = 0
open("/lib/libobstack.so.1", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/usr/local/lib/libobstack.so.1", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/usr/lib/libobstack.so.1", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
fstat(3, {st_mode=S_IFREG|0755, st_size=13944, ...}) = 0
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\0\0\0\0\0\0\0"..., 960) = 960
mmap(NULL, 20480, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fd1b4d1e000
mmap(0x7fd1b4d1f000, 4096, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0x1000) = 0x7fd1b4d1f000
mmap(0x7fd1b4d20000, 4096, PROT_READ, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = 0x7fd1b4d20000
mmap(0x7fd1b4d21000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED, 3, 0x2000) = 0x7fd1b4d21000
close(3)                                = 0
mprotect(0x7fd1b4d37000, 4096, PROT_READ) = 0
mprotect(0x7fd1b4ddc000, 4096, PROT_READ) = 0
mprotect(0x7fd1b4d26000, 4096, PROT_READ) = 0
mprotect(0x7fd1b4d21000, 4096, PROT_READ) = 0
mprotect(0x402000, 4096, PROT_READ)     = 0
open("config", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0644) = 3
write(3, "[remote \"origin\"]\n\turl = https:/"..., 58) = 58
close(3)                                = 0
open("config.lock", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE|O_CLOEXEC, 0666) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
open("config", O_RDONLY|O_LARGEFILE)    = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=58, ...}) = 0
mmap(NULL, 58, PROT_READ, MAP_PRIVATE, 4, 0) = 0x7fd1b4d1d000
ioctl(1, TIOCGWINSZ, {ws_row=48, ws_col=190, ws_xpixel=0, ws_ypixel=0}) = 0
writev(1, [{iov_base="mmap returned 0x7fd1b4d1d000", iov_len=28}, {iov_base="\n", iov_len=1}], 2mmap returned 0x7fd1b4d1d000
) = 29
close(4)                                = 0
chmod("config.lock", 0644)              = 0
write(3, "[core]\n", 7)                 = 7
writev(1, [{iov_base="wrote prefix 1: 7", iov_len=17}, {iov_base=" bytes\n", iov_len=7}], 2wrote prefix 1: 7 bytes
) = 24
write(3, "\tbare = false\n", 14)        = 14
writev(1, [{iov_base="wrote prefix 2: 14", iov_len=18}, {iov_base=" bytes\n", iov_len=7}], 2wrote prefix 2: 14 bytes
) = 25
write(3, "[remote \"origin\"]\n\turl = https:/"..., 58) = 58
writev(1, [{iov_base="wrote mapped content: 58", iov_len=24}, {iov_base=" bytes\n", iov_len=7}], 2wrote mapped content: 58 bytes
) = 31
close(3)                                = 0
munmap(0x7fd1b4d1d000, 58)              = 0
open("config.lock", O_RDONLY|O_LARGEFILE) = 3
writev(1, [{iov_base="--- config.lock", iov_len=15}, {iov_base=" contents ---\n", iov_len=14}], 2--- config.lock contents ---
) = 29
read(3, "[core]\n\tbare = false\n[remote \"or"..., 4096) = 79
write(1, "[core]\n\tbare = false\n[remote \"or"..., 79[core]
	bare = false
[remote "origin"]
	url = https://example.invalid/repo.git
) = 79
read(3, "", 4096)                       = 0
close(3)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++