A linux C API function that repositions the offset of the open file associated with the file descriptor to the argument offset according to the directive.
Questions tagged [lseek]
81 questions
2
votes
3 answers
SEEK_HOLE and SEEK_DATA not working in Ubuntu 12.04.2 LTS
When compiling I get the error:
cc holetest.c -o holetest
holetest.c: In function ‘test_seek’:
holetest.c:48:19: error: ‘SEEK_HOLE’ undeclared (first use in this function)
holetest.c:48:19: note: each undeclared identifier is reported only…
fadedbee
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1
vote
1 answer
lseek SEEK_DATA appears to be not working
Running on Tumbleweed with kernel 6.2.12-1-default
Using the _GNU_SOURCE define should allow the use of SEEK_DATA as the whence value for lseek.
According to the man page this should work for a range of file systems. I have tested this code on btrfs…
Rashid
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- 2
1
vote
0 answers
C POSIX function lseek returns -1 but errno is not set
I have encountered a situation where after calling lseek, I would get -1 as a return value but errno would remain unset.
From what I understand by reading the documentation for lseek, -1 return value would indicate an error.
Upon successful…
Long Claw
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1
vote
1 answer
Unexpected behaviour of lseek
I am using lseek just to find the size of a file and lseek returns less bytes than the file's actual size. I think the code is right and I can't explain why that happens.
When i ran the program for the first time it worked well. Then I added some…
Grigorios Garoufalis
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- 6
1
vote
1 answer
C - Wrong read after lseek with define expression
I have a binary file from which I need to read timestamps. In particular, I'm trying to read the last record. Every record is 24 bytes long, the timestamp is in the first 8 bytes. Here's the code I wrote:
#define IDX_RS_LEN (unsigned…
bui3
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- 11
1
vote
1 answer
C lseek misbehaving
Im trying to write a program that emulates the tail behavior in C. When searching the file for '/n' chars, im getting a strange behavior from lseek. Here is the relevant piece of code:
FILE *myfile = fopen(argv[i], "r");
if (myfile == NULL)…
Rob
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1
vote
0 answers
What's linux kernel equivalent of lseek?
I'm writing a linux device driver with linux kernel modules, and I want handle situation when user writes after size of the file. I saw there is vfs_lseek(), but it takes an fd, insted I have only struct* file pointer.
I could use an equivalent of…
Claudio Santoro
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1
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1 answer
Is there a way to find the position of a special character '\n' in a file in C?
I want to find the position of a '\n' in a file and print out the remaining characters after \n
I've tried to use lseek to find the number of times '\n' occurred but I cannot seem to find the position at which the special character '\n' occurs…
alfie
- 157
- 10
1
vote
0 answers
difference between reading the /proc/stat files via syscall fucntion read () or via glibc function fread()
Program reading /proc/stat files via read(like cat) or via fread (like nl) return different values. read() /proc/stat return right result, while fread() return wrong result.
I had write simple C programs that just read or fread /proc/stat. read()…
shaun
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- 5
1
vote
1 answer
if parent and child processes append to same file, do lseek() and write() need to be atomic?
Problem Statement
One process opens a file for appending (assume there is no O_APPEND, appending here means first lseek() to the file end then write()), and then forks a child. The two dependent processes append to the file simultaneously. Assume…
David Chen
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1 answer
Why lseek with offset `-2` and not `-1` to read in reverse?
Why is it -2 and not -1? This is a part of a code that has to write a string from a file to another in reverse. Can you help me understand why it is -2 and not -1?
while ( n >= 0) {
read(fdin, &c, 1);
write(fdout, &c, 1);
…
user10568310
- 29
- 6
1
vote
2 answers
Alternative to lseek to tail a file (Posix)
I had to implement a version of tail (posix system call). I did it using lseek and pread. (I reach the end of my file then I search for the right offset position, and then read from this position with pread, and writes to stdout till the end of the…
oajdwd
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- 2
1
vote
2 answers
lseek() Trying to use with a byte file but pointer is of FILE type
This is for a project for everyone's awareness. It's my first project in C and have a question regarding lseek() and moving the file pointer.
Right now I'm able to read the bitmap and DIB header of a bitmap file. I need to now traverse the…
Pwrcdr87
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1
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1 answer
Why do we need lseek when pread accepts offset value?
I am having trouble to understand why lseek function is useful.
Assuming I got a parameter like this given to me from the user:
off_t offset = 10;
And I wanted to read from the offset 100 bytes.
I can use pread like this:
void * buf =…
Eyzuky
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1
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1 answer
lseek not working with append mode of file
int main(int argc, char *argv[])
{
int i, j, count;
int f1;
char buf[16];
f1 = open(argv[1], O_RDWR | O_APPEND);
if(f1 == -1)
perror("open failed");
if(lseek(f1, 0, SEEK_SET) == -1)
printf("lseek failed\n");
while(count = read(0, buf,…
sasha00
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