using sysfs show() and store() functions
I have 2 questions regarding using these functions. I don't completely understand the documentation written here:
sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the
method. Sysfs will call the method exactly once for each read or
write. This forces the following behavior on the method
implementations:
On read(2), the show() method should fill the entire buffer.
Recall that an attribute should only be exporting one value, or an
array of similar values, so this shouldn't be that expensive.
This allows userspace to do partial reads and forward seeks
arbitrarily over the entire file at will. If userspace seeks back to
zero or does a pread(2) with an offset of '0' the show() method will
be called again, rearmed, to fill the buffer.
On write(2), sysfs expects the entire buffer to be passed during the
first write. Sysfs then passes the entire buffer to the store() method.
A terminating null is added after the data on stores. This makes
functions like sysfs_streq() safe to use.
When writing sysfs files, userspace processes should first read the
entire file, modify the values it wishes to change, then write the
entire buffer back.
First, when I read/write to the sysfs attribute file with read/write, am I guaranteed that the buffer I read to / the buffer in the store function will have all of the bytes I wanted to read in that function, and not call it in several chunks?
Also, how is the null character added? That is, suppose I wrote n bytes, will the amount of written bytes be n in the function parameter, and the null char will be placed at n+1?
thanks
linux linux-kernel linux-device-driver sysfs
asked Nov 21 '18 at 15:35
Eloo
387
add a comment |
I have 2 questions regarding using these functions. I don't completely understand the documentation written here:
sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the
method. Sysfs will call the method exactly once for each read or
write. This forces the following behavior on the method
implementations:
On read(2), the show() method should fill the entire buffer.
Recall that an attribute should only be exporting one value, or an
array of similar values, so this shouldn't be that expensive.
This allows userspace to do partial reads and forward seeks
arbitrarily over the entire file at will. If userspace seeks back to
zero or does a pread(2) with an offset of '0' the show() method will
be called again, rearmed, to fill the buffer.
On write(2), sysfs expects the entire buffer to be passed during the
first write. Sysfs then passes the entire buffer to the store() method.
A terminating null is added after the data on stores. This makes
functions like sysfs_streq() safe to use.
When writing sysfs files, userspace processes should first read the
entire file, modify the values it wishes to change, then write the
entire buffer back.
First, when I read/write to the sysfs attribute file with read/write, am I guaranteed that the buffer I read to / the buffer in the store function will have all of the bytes I wanted to read in that function, and not call it in several chunks?
Also, how is the null character added? That is, suppose I wrote n bytes, will the amount of written bytes be n in the function parameter, and the null char will be placed at n+1?
thanks
linux linux-kernel linux-device-driver sysfs
asked Nov 21 '18 at 15:35
Eloo
387
add a comment |
I have 2 questions regarding using these functions. I don't completely understand the documentation written here:
sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the
method. Sysfs will call the method exactly once for each read or
write. This forces the following behavior on the method
implementations:
On read(2), the show() method should fill the entire buffer.
Recall that an attribute should only be exporting one value, or an
array of similar values, so this shouldn't be that expensive.
This allows userspace to do partial reads and forward seeks
arbitrarily over the entire file at will. If userspace seeks back to
zero or does a pread(2) with an offset of '0' the show() method will
be called again, rearmed, to fill the buffer.
On write(2), sysfs expects the entire buffer to be passed during the
first write. Sysfs then passes the entire buffer to the store() method.
A terminating null is added after the data on stores. This makes
functions like sysfs_streq() safe to use.
When writing sysfs files, userspace processes should first read the
entire file, modify the values it wishes to change, then write the
entire buffer back.
First, when I read/write to the sysfs attribute file with read/write, am I guaranteed that the buffer I read to / the buffer in the store function will have all of the bytes I wanted to read in that function, and not call it in several chunks?
Also, how is the null character added? That is, suppose I wrote n bytes, will the amount of written bytes be n in the function parameter, and the null char will be placed at n+1?
thanks
linux linux-kernel linux-device-driver sysfs
asked Nov 21 '18 at 15:35
Eloo
387
I have 2 questions regarding using these functions. I don't completely understand the documentation written here:
sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the
method. Sysfs will call the method exactly once for each read or
write. This forces the following behavior on the method
implementations:
On read(2), the show() method should fill the entire buffer.
Recall that an attribute should only be exporting one value, or an
array of similar values, so this shouldn't be that expensive.
This allows userspace to do partial reads and forward seeks
arbitrarily over the entire file at will. If userspace seeks back to
zero or does a pread(2) with an offset of '0' the show() method will
be called again, rearmed, to fill the buffer.
On write(2), sysfs expects the entire buffer to be passed during the
first write. Sysfs then passes the entire buffer to the store() method.
A terminating null is added after the data on stores. This makes
functions like sysfs_streq() safe to use.
When writing sysfs files, userspace processes should first read the
entire file, modify the values it wishes to change, then write the
entire buffer back.
First, when I read/write to the sysfs attribute file with read/write, am I guaranteed that the buffer I read to / the buffer in the store function will have all of the bytes I wanted to read in that function, and not call it in several chunks?
Also, how is the null character added? That is, suppose I wrote n bytes, will the amount of written bytes be n in the function parameter, and the null char will be placed at n+1?
thanks
linux linux-kernel linux-device-driver sysfs
linux linux-kernel linux-device-driver sysfs
asked Nov 21 '18 at 15:35
Eloo
387
asked Nov 21 '18 at 15:35
Eloo
387
asked Nov 21 '18 at 15:35
Eloo
387
asked Nov 21 '18 at 15:35
Eloo
387
asked Nov 21 '18 at 15:35
Eloo
387
387
add a comment |
add a comment |
1 Answer
1
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The answer to first questions yes - partial writes are not supported and the buffer is always filled in one call of show method.
The answer to 2nd question is also yes. See implementation kernfs_fop_write() which is used by sysfs - it will allocate up to PAGE_SIZE+1 bytes so that there is enough space to fit .
answered Nov 22 '18 at 22:24
Krzysztof Adamski
1,663611
Actually PAGE_SIZE or 2*PAGE_SIZE (so null terminator in that case takes full page).
– 0andriy
Dec 1 '18 at 9:49
add a comment |
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1 Answer
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active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
The answer to first questions yes - partial writes are not supported and the buffer is always filled in one call of show method.
The answer to 2nd question is also yes. See implementation kernfs_fop_write() which is used by sysfs - it will allocate up to PAGE_SIZE+1 bytes so that there is enough space to fit .
answered Nov 22 '18 at 22:24
Krzysztof Adamski
1,663611
Actually PAGE_SIZE or 2*PAGE_SIZE (so null terminator in that case takes full page).
– 0andriy
Dec 1 '18 at 9:49
add a comment |
The answer to first questions yes - partial writes are not supported and the buffer is always filled in one call of show method.
The answer to 2nd question is also yes. See implementation kernfs_fop_write() which is used by sysfs - it will allocate up to PAGE_SIZE+1 bytes so that there is enough space to fit .
answered Nov 22 '18 at 22:24
Krzysztof Adamski
1,663611
Actually PAGE_SIZE or 2*PAGE_SIZE (so null terminator in that case takes full page).
– 0andriy
Dec 1 '18 at 9:49
add a comment |
The answer to first questions yes - partial writes are not supported and the buffer is always filled in one call of show method.
The answer to 2nd question is also yes. See implementation kernfs_fop_write() which is used by sysfs - it will allocate up to PAGE_SIZE+1 bytes so that there is enough space to fit .
answered Nov 22 '18 at 22:24
Krzysztof Adamski
1,663611
The answer to first questions yes - partial writes are not supported and the buffer is always filled in one call of show method.
The answer to 2nd question is also yes. See implementation kernfs_fop_write() which is used by sysfs - it will allocate up to PAGE_SIZE+1 bytes so that there is enough space to fit .
answered Nov 22 '18 at 22:24
Krzysztof Adamski
1,663611
edited Nov 24 '18 at 6:53
answered Nov 22 '18 at 22:24
Krzysztof Adamski
1,663611
answered Nov 22 '18 at 22:24
Krzysztof Adamski
1,663611
answered Nov 22 '18 at 22:24
Krzysztof Adamski
1,663611
1,663611
Actually PAGE_SIZE or 2*PAGE_SIZE (so null terminator in that case takes full page).
– 0andriy
Dec 1 '18 at 9:49
add a comment |
Actually PAGE_SIZE or 2*PAGE_SIZE (so null terminator in that case takes full page).
– 0andriy
Dec 1 '18 at 9:49
Actually PAGE_SIZE or 2*PAGE_SIZE (so null terminator in that case takes full page).
– 0andriy
Dec 1 '18 at 9:49
Actually PAGE_SIZE or 2*PAGE_SIZE (so null terminator in that case takes full page).
– 0andriy
Dec 1 '18 at 9:49
add a comment |
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