Simple create - lseek - read program in C
0
I'm trying to create a simple program which creates a file, writes on it, then moves back the pointer and finally reads it.
#include<stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
int main(){
int fd = creat("/home/alum/Class/ej",S_IRWXU);
if(fd==-1){
printf("Error %dn",errno);
return 1;
}
ssize_t size = write(fd, "Halo",4);
if(size==-1){
printf("Error %dn",errno);
return 1;
}
char string[50];
lseek(fd,0,SEEK_SET);
while((size = read(fd, string, 49)) >0){
printf("Read[%d]: %sn",size,string);
}
printf("Size: %dn",size);
if(size==-1){
printf("Error %dn",errno);
return 1;
}
int c=close(fd);
if(c==-1){
printf("Error %dn",errno);
return 1;
}
return 0;
}
My problem is that "lseek" seems not to be working. I always get size "-1" when I try to read, so I assume I'm not going back to the beginning of the file... Any suggestions?
I've seen some questions related to lseek but I haven't found a solution to my problem.
EDIT:
I changed
int fd = creat("/home/alum/Class/ej",S_IRWXU);
to
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDONLY|O_WRONLY|O_TRUNC);
and also tried
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDONLY|O_WRONLY|O_TRUNC,0700);
As suggested but I keep getting the same error.
c file
asked Nov 27 '18 at 9:07
DgrmDgrm
589
|
show 1 more comment
0
I'm trying to create a simple program which creates a file, writes on it, then moves back the pointer and finally reads it.
#include<stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
int main(){
int fd = creat("/home/alum/Class/ej",S_IRWXU);
if(fd==-1){
printf("Error %dn",errno);
return 1;
}
ssize_t size = write(fd, "Halo",4);
if(size==-1){
printf("Error %dn",errno);
return 1;
}
char string[50];
lseek(fd,0,SEEK_SET);
while((size = read(fd, string, 49)) >0){
printf("Read[%d]: %sn",size,string);
}
printf("Size: %dn",size);
if(size==-1){
printf("Error %dn",errno);
return 1;
}
int c=close(fd);
if(c==-1){
printf("Error %dn",errno);
return 1;
}
return 0;
}
My problem is that "lseek" seems not to be working. I always get size "-1" when I try to read, so I assume I'm not going back to the beginning of the file... Any suggestions?
I've seen some questions related to lseek but I haven't found a solution to my problem.
EDIT:
I changed
int fd = creat("/home/alum/Class/ej",S_IRWXU);
to
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDONLY|O_WRONLY|O_TRUNC);
and also tried
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDONLY|O_WRONLY|O_TRUNC,0700);
As suggested but I keep getting the same error.
c file
asked Nov 27 '18 at 9:07
DgrmDgrm
589
5
According to the manpage, "creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC", so it seems you can't read from a file opened withcreat(flagO_WRONLY)
– Karsten Koop
Nov 27 '18 at 9:12
I tried using open and I get the same error (changes in the post)
– Dgrm
Nov 27 '18 at 9:21
Fixed it using O_RDWR, Thanks!
– Dgrm
Nov 27 '18 at 9:27
1
useperrorinstead ofprintf. It will print a human-readable error message.
– Antti Haapala
Nov 27 '18 at 10:11
1
Note:stringis not a string whenprintf("Read[%d]: %sn",size,string)is called asstringlacks a certain null character. (UB)
– chux
Nov 27 '18 at 11:08
|
show 1 more comment
0
0
0
I'm trying to create a simple program which creates a file, writes on it, then moves back the pointer and finally reads it.
#include<stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
int main(){
int fd = creat("/home/alum/Class/ej",S_IRWXU);
if(fd==-1){
printf("Error %dn",errno);
return 1;
}
ssize_t size = write(fd, "Halo",4);
if(size==-1){
printf("Error %dn",errno);
return 1;
}
char string[50];
lseek(fd,0,SEEK_SET);
while((size = read(fd, string, 49)) >0){
printf("Read[%d]: %sn",size,string);
}
printf("Size: %dn",size);
if(size==-1){
printf("Error %dn",errno);
return 1;
}
int c=close(fd);
if(c==-1){
printf("Error %dn",errno);
return 1;
}
return 0;
}
My problem is that "lseek" seems not to be working. I always get size "-1" when I try to read, so I assume I'm not going back to the beginning of the file... Any suggestions?
I've seen some questions related to lseek but I haven't found a solution to my problem.
EDIT:
I changed
int fd = creat("/home/alum/Class/ej",S_IRWXU);
to
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDONLY|O_WRONLY|O_TRUNC);
and also tried
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDONLY|O_WRONLY|O_TRUNC,0700);
As suggested but I keep getting the same error.
c file
asked Nov 27 '18 at 9:07
DgrmDgrm
589
I'm trying to create a simple program which creates a file, writes on it, then moves back the pointer and finally reads it.
#include<stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
int main(){
int fd = creat("/home/alum/Class/ej",S_IRWXU);
if(fd==-1){
printf("Error %dn",errno);
return 1;
}
ssize_t size = write(fd, "Halo",4);
if(size==-1){
printf("Error %dn",errno);
return 1;
}
char string[50];
lseek(fd,0,SEEK_SET);
while((size = read(fd, string, 49)) >0){
printf("Read[%d]: %sn",size,string);
}
printf("Size: %dn",size);
if(size==-1){
printf("Error %dn",errno);
return 1;
}
int c=close(fd);
if(c==-1){
printf("Error %dn",errno);
return 1;
}
return 0;
}
My problem is that "lseek" seems not to be working. I always get size "-1" when I try to read, so I assume I'm not going back to the beginning of the file... Any suggestions?
I've seen some questions related to lseek but I haven't found a solution to my problem.
EDIT:
I changed
int fd = creat("/home/alum/Class/ej",S_IRWXU);
to
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDONLY|O_WRONLY|O_TRUNC);
and also tried
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDONLY|O_WRONLY|O_TRUNC,0700);
As suggested but I keep getting the same error.
c file
c file
asked Nov 27 '18 at 9:07
DgrmDgrm
589
asked Nov 27 '18 at 9:07
DgrmDgrm
589
edited Nov 27 '18 at 9:20
Dgrm
asked Nov 27 '18 at 9:07
DgrmDgrm
589
asked Nov 27 '18 at 9:07
DgrmDgrm
589
asked Nov 27 '18 at 9:07
DgrmDgrm
589
589
5
According to the manpage, "creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC", so it seems you can't read from a file opened withcreat(flagO_WRONLY)
– Karsten Koop
Nov 27 '18 at 9:12
I tried using open and I get the same error (changes in the post)
– Dgrm
Nov 27 '18 at 9:21
Fixed it using O_RDWR, Thanks!
– Dgrm
Nov 27 '18 at 9:27
1
useperrorinstead ofprintf. It will print a human-readable error message.
– Antti Haapala
Nov 27 '18 at 10:11
1
Note:stringis not a string whenprintf("Read[%d]: %sn",size,string)is called asstringlacks a certain null character. (UB)
– chux
Nov 27 '18 at 11:08
|
show 1 more comment
5
According to the manpage, "creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC", so it seems you can't read from a file opened withcreat(flagO_WRONLY)
– Karsten Koop
Nov 27 '18 at 9:12
I tried using open and I get the same error (changes in the post)
– Dgrm
Nov 27 '18 at 9:21
Fixed it using O_RDWR, Thanks!
– Dgrm
Nov 27 '18 at 9:27
1
useperrorinstead ofprintf. It will print a human-readable error message.
– Antti Haapala
Nov 27 '18 at 10:11
1
Note:stringis not a string whenprintf("Read[%d]: %sn",size,string)is called asstringlacks a certain null character. (UB)
– chux
Nov 27 '18 at 11:08
5
5
According to the manpage, "creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC", so it seems you can't read from a file opened with
creat (flag O_WRONLY)– Karsten Koop
Nov 27 '18 at 9:12
According to the manpage, "creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC", so it seems you can't read from a file opened with
creat (flag O_WRONLY)– Karsten Koop
Nov 27 '18 at 9:12
I tried using open and I get the same error (changes in the post)
– Dgrm
Nov 27 '18 at 9:21
I tried using open and I get the same error (changes in the post)
– Dgrm
Nov 27 '18 at 9:21
Fixed it using O_RDWR, Thanks!
– Dgrm
Nov 27 '18 at 9:27
Fixed it using O_RDWR, Thanks!
– Dgrm
Nov 27 '18 at 9:27
1
1
use
perror instead of printf. It will print a human-readable error message.– Antti Haapala
Nov 27 '18 at 10:11
use
perror instead of printf. It will print a human-readable error message.– Antti Haapala
Nov 27 '18 at 10:11
1
1
Note:
string is not a string when printf("Read[%d]: %sn",size,string) is called as string lacks a certain null character. (UB)– chux
Nov 27 '18 at 11:08
Note:
string is not a string when printf("Read[%d]: %sn",size,string) is called as string lacks a certain null character. (UB)– chux
Nov 27 '18 at 11:08
|
show 1 more comment
1 Answer
1
active
oldest
votes
1
I had to use
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDWR|O_TRUNC,0700);
to create the file since 'creat' does not work for reading (manpage) and 'open' didn't work as O_RDWR for me if I used O_RDONLY|O_WRONLY.
answered Nov 27 '18 at 9:29
DgrmDgrm
589
3
I used O_RDONLY|O_WRONLY. And it is completely wrong to do that: "Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag: O_EXEC ... O_RDONLY ... O_RDWR ... O_SEARCH ... O_WRONLY"
– Andrew Henle
Nov 27 '18 at 10:24
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
1
I had to use
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDWR|O_TRUNC,0700);
to create the file since 'creat' does not work for reading (manpage) and 'open' didn't work as O_RDWR for me if I used O_RDONLY|O_WRONLY.
answered Nov 27 '18 at 9:29
DgrmDgrm
589
3
I used O_RDONLY|O_WRONLY. And it is completely wrong to do that: "Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag: O_EXEC ... O_RDONLY ... O_RDWR ... O_SEARCH ... O_WRONLY"
– Andrew Henle
Nov 27 '18 at 10:24
add a comment |
1
I had to use
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDWR|O_TRUNC,0700);
to create the file since 'creat' does not work for reading (manpage) and 'open' didn't work as O_RDWR for me if I used O_RDONLY|O_WRONLY.
answered Nov 27 '18 at 9:29
DgrmDgrm
589
3
I used O_RDONLY|O_WRONLY. And it is completely wrong to do that: "Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag: O_EXEC ... O_RDONLY ... O_RDWR ... O_SEARCH ... O_WRONLY"
– Andrew Henle
Nov 27 '18 at 10:24
add a comment |
1
1
1
I had to use
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDWR|O_TRUNC,0700);
to create the file since 'creat' does not work for reading (manpage) and 'open' didn't work as O_RDWR for me if I used O_RDONLY|O_WRONLY.
answered Nov 27 '18 at 9:29
DgrmDgrm
589
I had to use
int fd = open("/home/alum/Class/ej",O_CREAT|O_RDWR|O_TRUNC,0700);
to create the file since 'creat' does not work for reading (manpage) and 'open' didn't work as O_RDWR for me if I used O_RDONLY|O_WRONLY.
answered Nov 27 '18 at 9:29
DgrmDgrm
589
answered Nov 27 '18 at 9:29
DgrmDgrm
589
answered Nov 27 '18 at 9:29
DgrmDgrm
589
answered Nov 27 '18 at 9:29
DgrmDgrm
589
589
3
I used O_RDONLY|O_WRONLY. And it is completely wrong to do that: "Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag: O_EXEC ... O_RDONLY ... O_RDWR ... O_SEARCH ... O_WRONLY"
– Andrew Henle
Nov 27 '18 at 10:24
add a comment |
3
I used O_RDONLY|O_WRONLY. And it is completely wrong to do that: "Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag: O_EXEC ... O_RDONLY ... O_RDWR ... O_SEARCH ... O_WRONLY"
– Andrew Henle
Nov 27 '18 at 10:24
3
3
I used O_RDONLY|O_WRONLY. And it is completely wrong to do that: "Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag: O_EXEC ... O_RDONLY ... O_RDWR ... O_SEARCH ... O_WRONLY"
– Andrew Henle
Nov 27 '18 at 10:24
I used O_RDONLY|O_WRONLY. And it is completely wrong to do that: "Applications shall specify exactly one of the first five values (file access modes) below in the value of oflag: O_EXEC ... O_RDONLY ... O_RDWR ... O_SEARCH ... O_WRONLY"
– Andrew Henle
Nov 27 '18 at 10:24
add a comment |
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5
According to the manpage, "creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC", so it seems you can't read from a file opened with
creat(flagO_WRONLY)– Karsten Koop
Nov 27 '18 at 9:12
I tried using open and I get the same error (changes in the post)
– Dgrm
Nov 27 '18 at 9:21
Fixed it using O_RDWR, Thanks!
– Dgrm
Nov 27 '18 at 9:27
1
use
perrorinstead ofprintf. It will print a human-readable error message.– Antti Haapala
Nov 27 '18 at 10:11
1
Note:
stringis not a string whenprintf("Read[%d]: %sn",size,string)is called asstringlacks a certain null character. (UB)– chux
Nov 27 '18 at 11:08