preadv_pwritev_test.c

// preadv_pwritev_test.c
// run like below
// strace -ff -vv -o trace.log -e trace=pread64,preadv,pwrite64,pwritev ./a.out
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/uio.h>

int main(void) {
    const char *fname = "testfile.bin";
    int fd = open(fname, O_CREAT | O_RDWR, 0644);
    if (fd < 0) {
        perror("open");
        return 1;
    }

    // Write some data at offset 0
    const char *msg = "hello world\n";
    ssize_t w = pwrite(fd, msg, strlen(msg), 0);
    if (w < 0) { perror("pwrite"); return 1; }

    // Read it back with pread64 at offset 0
    char buf[64];
    ssize_t r = pread(fd, buf, sizeof(buf)-1, 0);
    if (r < 0) { perror("pread"); return 1; }
    buf[r] = '\0';
    printf("pread got: %s", buf);

    // Now do a vectored write at offset 100
    struct iovec iovw[2];
    iovw[0].iov_base = "abc";
    iovw[0].iov_len  = 3;
    iovw[1].iov_base = "XYZ\n";
    iovw[1].iov_len  = 4;
    ssize_t wv = pwritev(fd, iovw, 2, 100);
    if (wv < 0) { perror("pwritev"); return 1; }

    // And a vectored read at offset 100
    char buf1[8], buf2[8];
    struct iovec iovr[2];
    iovr[0].iov_base = buf1;
    iovr[0].iov_len  = sizeof(buf1);
    iovr[1].iov_base = buf2;
    iovr[1].iov_len  = sizeof(buf2);
    ssize_t rv = preadv(fd, iovr, 2, 100);
    if (rv < 0) { perror("preadv"); return 1; }
    printf("preadv got: %.*s%.*s", (int)iovr[0].iov_len, buf1,
                                   (int)iovr[1].iov_len, buf2);

    close(fd);
    return 0;
}