linux以二进制读数据,linux读写二进制大文件-优快云博客

这篇博客详细介绍了在C语言中实现的一系列安全的文件读写、预读写以及可能涉及的 splice 函数操作。这些函数在遇到错误如 EINTR 时能进行重试，确保数据的完整性和可靠性。同时，还提供了一个用于检查和更新文件内容的实用函数 `safe_write_file`，该函数在确保文件内容正确后进行原子替换。

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#include #include "common/safe_io.h"

#include "include/compat.h"

ssize_t safe_read(int fd, void *buf, size_t count)

{

size_t cnt = 0;

while (cnt < count) {

ssize_t r = read(fd, buf, count - cnt);

if (r <= 0) {

if (r == 0) {

// EOF

return cnt;

}

if (errno == EINTR)

continue;

return -errno;

}

cnt += r;

buf = (char *)buf + r;

}

return cnt;

}

ssize_t safe_read_exact(int fd, void *buf, size_t count)

{

ssize_t ret = safe_read(fd, buf, count);

if (ret < 0)

return ret;

if ((size_t)ret != count)

return -EDOM;

return 0;

}

ssize_t safe_write(int fd, const void *buf, size_t count)

{

while (count > 0) {

ssize_t r = write(fd, buf, count);

if (r < 0) {

if (errno == EINTR)

continue;

return -errno;

}

count -= r;

buf = (char *)buf + r;

}

return 0;

}

ssize_t safe_pread(int fd, void *buf, size_t count, off_t offset)

{

size_t cnt = 0;

char *b = (char*)buf;

while (cnt < count) {

ssize_t r = pread(fd, b + cnt, count - cnt, offset + cnt);

if (r <= 0) {

if (r == 0) {

// EOF

return cnt;

}

if (errno == EINTR)

continue;

return -errno;

}

cnt += r;

}

return cnt;

}

ssize_t safe_pread_exact(int fd, void *buf, size_t count, off_t offset)

{

ssize_t ret = safe_pread(fd, buf, count, offset);

if (ret < 0)

return ret;

if ((size_t)ret != count)

return -EDOM;

return 0;

}

ssize_t safe_pwrite(int fd, const void *buf, size_t count, off_t offset)

{

while (count > 0) {

ssize_t r = pwrite(fd, buf, count, offset);

if (r < 0) {

if (errno == EINTR)

continue;

return -errno;

}

count -= r;

buf = (char *)buf + r;

offset += r;

}

return 0;

}

#ifdef CEPH_HAVE_SPLICE

ssize_t safe_splice(int fd_in, loff_t *off_in, int fd_out, loff_t *off_out,

size_t len, unsigned int flags)

{

size_t cnt = 0;

while (cnt < len) {

ssize_t r = splice(fd_in, off_in, fd_out, off_out, len - cnt, flags);

if (r <= 0) {

if (r == 0) {

// EOF

return cnt;

}

if (errno == EINTR)

continue;

return -errno;

}

cnt += r;

}

return cnt;

}

ssize_t safe_splice_exact(int fd_in, loff_t *off_in, int fd_out,

loff_t *off_out, size_t len, unsigned int flags)

{

ssize_t ret = safe_splice(fd_in, off_in, fd_out, off_out, len, flags);

if (ret < 0)

return ret;

if ((size_t)ret != len)

return -EDOM;

return 0;

}

#endif

int safe_write_file(const char *base, const char *file,

const char *val, size_t vallen)

{

int ret;

char fn[PATH_MAX];

char tmp[PATH_MAX];

int fd;

// does the file already have correct content?

char oldval[80];

ret = safe_read_file(base, file, oldval, sizeof(oldval));

if (ret == (int)vallen && memcmp(oldval, val, vallen) == 0)

return 0; // yes.

snprintf(fn, sizeof(fn), "%s/%s", base, file);

snprintf(tmp, sizeof(tmp), "%s/%s.tmp", base, file);

fd = open(tmp, O_WRONLY|O_CREAT|O_TRUNC, 0644);

if (fd < 0) {

ret = errno;

return -ret;

}

ret = safe_write(fd, val, vallen);

if (ret) {

VOID_TEMP_FAILURE_RETRY(close(fd));

return ret;

}

ret = fsync(fd);

if (ret < 0) ret = -errno;

VOID_TEMP_FAILURE_RETRY(close(fd));

if (ret < 0) {

unlink(tmp);

return ret;

}

ret = rename(tmp, fn);

if (ret < 0) {

ret = -errno;

unlink(tmp);

return ret;

}

fd = open(base, O_RDONLY);

if (fd < 0) {

ret = -errno;

return ret;

}

ret = fsync(fd);

if (ret < 0) ret = -errno;

VOID_TEMP_FAILURE_RETRY(close(fd));

return ret;

}

int safe_read_file(const char *base, const char *file,

char *val, size_t vallen)

{

char fn[PATH_MAX];

int fd, len;

snprintf(fn, sizeof(fn), "%s/%s", base, file);

fd = open(fn, O_RDONLY);

if (fd < 0) {

return -errno;

}

len = safe_read(fd, val, vallen);

if (len < 0) {

VOID_TEMP_FAILURE_RETRY(close(fd));

return len;

}

// close sometimes returns errors, but only after write()

VOID_TEMP_FAILURE_RETRY(close(fd));

return len;

}