How do I wait on output from sysfs? - c

I tried the obvious (see below), but it does not catch new output to /var/log/syslog. I'm sure there is an answer somewhere already, but I have not been able to find it.
Here is my code (my best guess how to do this):
#include <stdio.h>
#include <stdlib.h> // provides fopen()
#include <unistd.h> // provides sleep()
int main() {
// *** This is failing to pick up new output on /var/log/syslog. I'm not sure
// how to do this properly.
// Open a read file handle on /sys/kernel/tracing/trace and wait for data to
// appear there. When it does, echo it to the screen. This is essentially an
// implementation of "tail -f /sys/kernel/tracing/trace".
//FILE *fp = fopen("/sys/kernel/tracing/trace", "r");
FILE *fp = fopen("/var/log/syslog", "r");
char c;
if (fp != NULL) {
printf("Opened the file successfully. Waiting...\n");
} else {
printf("Failed to open the file.\n");
exit(1);
}
// Check every second and output whatever is in the buffer.
while(1) {
c = fgetc(fp);
// We get back -1 when there is nothing to read.
if (c != -1) {
printf("%c", c);
} else {
printf("."); fflush(stdout);
sleep(1);
}
}
fclose(fp);
return 0;
}

You need to call clearerr(fp) to clear the feof indicator:
// Check every second and output whatever is in the buffer.
while(1) {
c = fgetc(fp);
// We get back -1 when there is nothing to read.
if (c != -1) {
printf("%c", c);
} else {
printf("."); fflush(stdout);
if(feof(fp)) clearerr(fp); // <-- clear the feof indicator
sleep(1);
}
}

Related

Why rewind() followed by fscanf() do not reflect changes made on disk?

I am repeatedly reading a previously-opened file using fscanf and rewind:
#include <stdio.h>
#include <errno.h>
#include <unistd.h>
void read_opened_file(FILE *file, int *value)
{
errno = 0;
fscanf(file, "%d", value);
if (errno != 0) perror("fscanf error");
errno = 0;
rewind(file);
if (errno != 0) perror("rewind error");
}
int main(int argc, char* argv[])
{
int value = 0;
FILE *file = fopen(argv[1], "r");
errno = 0;
if (setvbuf(file, NULL, _IONBF, 0) != 0) perror("setvbuf error");
for (int i = 0; i < 10; ++i)
{
read_opened_file(file, &value);
printf("value = %d\n", value); fflush(stdout);
sleep(1);
}
fclose(file);
return 0;
}
However, any changes made to the file on disk behind the scenes by gedit, echo >, cp are not reflected: the function keeps reporting the first (cached?) value read (and errno is not set).
When I setvbuf(file, NULL, _IONBUF, 0); as suggested here, changes made by cp and echo > are reflected, but changes made by gedit are still not reflected.
When I use fopen and fclose every loop iteration everything is as expected.
How do I fix the above code without fopen and fclose every time?
You can't. The gedit program does not change the file. That's simply not how it works. So you will not see any change to the file.
The changes gedit makes are done by replacing the old file with a new file. You must close the old file and open the new file to see the changes that gedit made.

How to change STDIN stream to binary

I'm making an upload form via a CGI interface. I'm writing it in C and don't want to use any outside libraries (ie. cgic).
I thought the program was complete, as the first test files uploaded correctly. But they were ASCII files. When I tested with a binary file (JPG). It seems that STDIN is trying to read the binary data as ASCII which creates a problem for characters like \0 which is present at the end of an ASCII file, but is a common character in binary files. The results of uploading a 1.9MB file end up with a 38kB file.
When searching how to change the STDIN stream to binary, I was referred to the command freopen and told to use NULL as the argument for the file. example 1
It says:
If filename is a null pointer, the freopen() function shall attempt to
change the mode of the stream to that specified by mode, as if the
name of the file currently associated with the stream had been used.
In this case, the file descriptor associated with the stream need not
be closed if the call to freopen() succeeds. It is
implementation-defined which changes of mode are permitted (if any),
and under what circumstances.
But when I check the man page on my system with man 3 freopen, it doesn't say any of
this at all. Furthermore, reading the man page, I find out the the
option for binary (adding 'b' to the mode) is no longer recognized and
only exists for archaic compliancy:
The mode string can also include
the letter 'b' either as a last character or as a character between
the characters in any of the two-character strings described above.
This is strictly for compatibility with C89 and has no effect; the 'b'
is ignored on all POSIX conforming systems, including Linux.
So right now I'm completely lost. How can I change the STDIN stream to read binary input?
Here is the code:
#include <stdio.h>
#include <stdlib.h>
#include <libgen.h>
#include <string.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
// Declare constants.
#define BUF_SIZE 4096
#define FILENAME_SIZE 500
#define MARKER_SIZE 100
#define RETURN_FAILURE 0
#define RETURN_SUCCESS 1
#define SEARCH_STRING_1 "filename=\""
#define SEARCH_STRING_2 "\r\n\r\n"
// Declare global variables.
char filename[FILENAME_SIZE + 1];
char *program_name;
// Declare function prototype.
void print_footer (void);
void print_header (void);
void process_input (char *data);
int main (int argc, char *argv[])
{
// Declare variables.
long long ret;
char buf[BUF_SIZE + 1];
// Get program name for error reporting.
program_name = basename(argv[0]);
// Prepare output for browser.
print_header();
// Protect variable against buffer overflow.
buf[BUF_SIZE] = '\0';
// Loop through all the file data.
while(1)
{
// Read in the next block of data.
if((ret = (long long) fread(buf, 1, BUF_SIZE, stdin)) != BUF_SIZE)
{
// Check for error.
if(ferror(stdin) != 0)
{
printf("%s: An error occurred while reading the input file.<br>\n", program_name);
process_input(NULL);
exit(EXIT_FAILURE);
}
// Check for EOF.
else if(feof(stdin) != 0)
break;
}
// Terminate and process uploaded data.
buf[ret] = '\0';
process_input(buf);
}
// Terminate and process uploaded data.
buf[ret] = '\0';
process_input(buf);
// Finish user output, close output file and exit.
print_footer();
process_input(NULL);
exit(EXIT_SUCCESS);
}
void process_input (char *data)
{
// Declare variables.
char *ptr1= NULL;
char *ptr2;
int x = 0;
static FILE *fp;
static int flag = 0;
static char marker[MARKER_SIZE + 1];
// If data is NULL, close output file.
if(data == NULL)
{
if(fclose(fp) == EOF)
{
printf("%s: process_input: close failed (%s)<br>\n", program_name, strerror(errno));
exit(EXIT_FAILURE);
}
return;
}
// Check if this is the first time through.
if(flag == 0)
{
// Get marker.
if((ptr1 = strchr(data, '\n')) == NULL)
{
printf("%s: process_input: strchr(1) failed (\n)<br>\n", program_name);
exit(EXIT_FAILURE);
}
ptr1[0] = '\0';
strcpy(marker, data);
ptr1[0] = '\n';
// Get filename.
if((ptr1 = strstr(data, SEARCH_STRING_1)) == NULL)
{
printf("%s: process_input: strstr(1) failed (%s)<br>\n", program_name, SEARCH_STRING_1);
exit(EXIT_FAILURE);
}
// Advance pointer to start of filename.
ptr1 += 10;
// Find end of filename.
if((ptr2 = strchr(ptr1, '"')) == NULL)
{
printf("%s: process_input: strchr(2) failed (\")<br>\n", program_name);
exit(EXIT_FAILURE);
}
// Terminate and store filename.
ptr2[0] = '\0';
strcpy(filename, ptr1);
ptr2[0] = '"';
// Remove spaces from filename.
while(filename[x] != '\0')
{
if(filename[x] == ' ')
filename[x] = '.';
x++;
}
// Open output file.
if((fp = fopen(filename, "wb")) == NULL)
{
printf("%s: process_input: fopen failed (%s) (%s)<br>\n", program_name, strerror(errno), filename);
exit(EXIT_FAILURE);
}
// Find start of file data.
if((ptr1 = strstr(data, SEARCH_STRING_2)) == NULL)
{
printf("%s: process_input: strstr(2) failed (%s)<br>\n", program_name, SEARCH_STRING_2);
fclose(fp);
exit(EXIT_FAILURE);
}
// Set flag.
flag++;
// Advance pointer to start of file data.
ptr1 += 4;
// Change STDIN stream to binary.
if(freopen(NULL, "rb", stdin) == NULL)
{
printf("%s: process_input: freopen failed (%s)<br>\n", program_name, strerror(errno));
fclose(fp);
exit(EXIT_FAILURE);
}
}
// Catch everything else.
else
{
ptr1 = data;
if((ptr2 = strstr(ptr1, marker)) != NULL)
ptr2[0 - 2] = '\0';
}
// Write file data.
if(fwrite(ptr1, 1, strlen(ptr1), fp) != strlen(ptr1))
{
printf("%s: process_input: write failed (%s)<br>\n", program_name, strerror(errno));
fclose(fp);
exit(EXIT_FAILURE);
}
}
void print_footer (void)
{
printf("\nMade it!\n");
}
void print_header (void)
{
printf("Content-type: text/plain\r\n\r\n");
}
Ok, it appears what #NominalAnimal said was correct. You can store binary data in a string, but the moment you use any function in the string.h library, it almost always changes what is stored in that string (if the data is binary).
The easy solution is to make a separate function that takes a pointer to the binary data and do your string searches in that function, returning what pertinent information is needed. That way, the original data is never changed.
'stdin' is a macro of STDIN_FILENO, which is egal to 0. See also 'unistd.h'.
You are not showing your code, but I think you stop when you encounter a '\0' or a non-ascii char, since you said you were using 'fread()'.
You have to stop when fread() function returns 0, which means it stopped to read : it encountered EOF.

I am trying to print a txt file and it doesn't work in C homework

I'm writing code that's supposed to verify that a .txt file is a certain format.
I wrote my code as I saw in a tutorial and in the website
and for some reason my program doesn't even print my file.
Can you tell me what I'm doing wrong?
The code will do something far more complex, but I'm still trying to work on my basics.
Here's my code so far:
int main(int argc, char *argv[]) {
/* argv[0] = name of my running file
* argv[1] = the first file that i receive
*/
define MAXBUFLEN 4096
char source[MAXBUFLEN + 1];
int badReturnValue = 1;
char *error = "Error! trying to open the file ";
if (argc != 2) {
printf("please supply a file \n");
return badReturnValue;
}
char *fileName = argv[1];
FILE *fp = fopen(argv[1], "r"); /* "r" = open for reading */
if (fp != NULL) {
size_t newLen = fread(&source, sizeof(char), MAXBUFLEN, fp);
if (ferror(fp) != 0) {
printf("%s %s", error, fileName);
return badReturnValue;
}
int symbol;
while ((symbol = getc(fp)) != EOF) {
putchar(symbol);
}
printf("finish");
fclose(fp);
}
else {
printf("%s %s", error, fileName);
return badReturnValue;
}
}
I think you need a bit more explanations:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
// there might be a macro BUFLEN defined in stdio
// which size is optimized for reading in chunks.
// Test if avaiable otherwise define it
#ifndef BUFLEN
# define BUFLEN 4096
#endif
int main(int argc, char *argv[])
{
char source[BUFLEN];
char *filename;
FILE *fp;
size_t fpread, written;
char c;
int ret_fclose;
if (argc != 2) {
fprintf(stderr, "Usage: %s filename\n", argv[0]);
exit(EXIT_FAILURE);
}
// reset errno, just in case
errno = 0;
// work on copy
filename = malloc(strlen(argv[1]) + 1);
if (filename == NULL) {
fprintf(stderr, "Allocating %zu bytes failed\n", strlen(argv[1]) + 1);
exit(EXIT_FAILURE);
}
filename = strcpy(filename, argv[1]);
// try to open the file at 'filename'
fp = fopen(filename, "r");
if (fp == NULL) {
fprintf(stderr, "Opening file \"%s\" filename failed\n", filename);
// errno might got set to something usable, check and print
if (errno != 0) {
fprintf(stderr, "Error: %s\n", strerror(errno));
}
exit(EXIT_FAILURE);
}
// You have two options here. One is to read in chunks of MAXBUFLEN
while ((fpread = fread(&source, 1, BUFLEN, fp)) > 0) {
// Do something with the stuff we read into "source"
// we do nothing with it here, we just write to stdout
written = fwrite(&source, 1, fpread, stdout);
// you can use 'written' for error check when writing to an actual file
// but it is unlikely (but not impossible!) with stdout
// test if we wrote what we read
if ((fpread - written) != 0) {
fprintf(stderr, "We did not write what we read. Diff: %d\n",
(int) (fpread - written));
}
}
// fread() does not distinguish between EOF and error, we have to check by hand
if (feof(fp)) {
// we have read all, exit
puts("\n\n\tfinish\n");
// No, wait, we want to do it again in a different way, so: no exit
// exit(EXIT_SUCCESS);
} else {
// some error may have occured, check
if (ferror(fp)) {
fprintf(stderr, "Something bad happend while reading \"%s\"\n", filename);
if (errno != 0) {
fprintf(stderr, "Error: %s\n", strerror(errno));
}
exit(EXIT_FAILURE);
}
}
// the other way is to read it byte by byte
// reset the filepointers/errors et al.
rewind(fp);
// rewind() should have reseted errno, but better be safe than sorry
errno = 0;
printf("\n\n\tread and print \"%s\" again\n\n\n\n", filename);
// read one byte and print it until end of file
while ((c = fgetc(fp)) != EOF) {
// just print. Gathering them into "source" is left as an exercise
fputc(c, stdout);
}
// clean up
errno = 0;
ret_fclose = fclose(fp);
// even fclose() might fail
if (ret_fclose == EOF) {
fprintf(stderr, "Something bad happend while closing \"%s\"\n", filename);
if (errno != 0) {
fprintf(stderr, "Error: %s\n", strerror(errno));
}
exit(EXIT_FAILURE);
}
// The macros EXIT_FAILURE and EXIT_SUCCESS are set to the correct values for
// the OS to tell it if we had an eror or not.
// Using exit() is noot necessary here but there exits teh function atexit()
// that runs a given function (e.g: clean up, safe content etc.) when called
exit(EXIT_SUCCESS);
}
You read from the file twice but only print once.
If the file is to small the first reading will read all of the contents, and the second reading will not produce anything so you don't print anything.
I believe you have to reset the pointer after using fread.
Try fseek(fp, SEEK_SET, 0) to reset the pointer to the beginning of the file. Then print the file.

My text for output file is not the same as input

#include <stdio.h>
#include <stdlib.h>
int main() {
int c;
FILE *poem = fopen("short.txt", "r");
FILE *html = fopen("index.html", "w");
if (poem == NULL){
perror("Error in opening file");
return(-1);
}
if (html == NULL){
perror("Error in opening file");
return(-1);
}
while((c = fgetc(poem)) != EOF) {
c = getc(poem);
fputc(c, html);
}
fclose (poem);
fclose (html);
return 0;
}
I've been searching and trying but I can't figure it out. My read file has less than a sentence of words, and then when it outputs it to index.html it's all jumbled up. I don't really understand know whats wrong with the code. Any help will be appreciated. Thanks!
You are doing 2 reads for each write
while((c = fgetc(poem)) != EOF) { // read
c = getc(poem); // read
fputc(c, html); // write
}

beginner in C under linux. write function doesn't work properly

I'm trying to write a C program, that make user able to write stuff in a file. My Problem is that after making and running the program the file stay empty ?? any idea how can I solve this.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
// the user should give a file to write the file
int main (int argc , char**argv)
{
int fd; // file descriptor
char ret; // the character
int offset;
if(argc != 2) {
printf("You have to give the name or the path of the file to work with \n");
printf("Exiting the program \n")
return -1;
}
fd = open (argv[1], O_WRONLY/*write*/|O_CREAT/*create if not found */, S_IRUSR|S_IWUSR/*user can read and write*/);
if (fd == -1) {
printf("can'T open the file ");
return -1;
}
printf("At wich position you want to start ");
scanf("%d",&offset);
lseek(fd,offset,SEEK_SET);
while(1) {
ret = getchar();
if(ret == '1') {
printf("closing the file");
close (fd);
return 1;
}
else
write (fd,red, sizeof(char));
}
return 0;
}
thanks in advance for you help.
I have made some changes,this should work:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
int main (int argc , char**argv)
{
int fd; // file descriptor
char ret; // the character
int offset;
if(argc != 2){
printf("You have to give the name or the path of the file to work with \n");
printf("Exiting the program \n"); **//There was ';' missing here**
return -1;
}
fd = open (argv[1], O_WRONLY|O_CREAT,S_IRUSR|S_IWUSR);
if (fd == -1) {
printf("can'T open the file ");
return -1;
}
printf("At wich position you want to start ");
scanf("%d",&offset);
lseek(fd,offset,SEEK_SET);
while(1){
ret = getchar();
if(ret == '1'){
printf("closing the file");
close (fd);
return 1;
}
else
write (fd,&ret, sizeof(char)); **//red has been changed to &ret**
}
return 0;
}
One error I can notice, the call of write function:
write (fd,red, sizeof(char));
should be:
write (fd, &red, sizeof(char));
You forgot & before red, write need address.
syntax of write: int write( int handle, void *buffer, int nbyte );
This will cause an undefined behavior in your code at run time
Edit: in write function you are using red that is not defined, I think it should be ret variable in your code. correct it as write (fd, &ret, sizeof(char));
second, you forgot ; after printf("Exiting the program \n") in if, but I also think its mistake while posting question as you says you are getting run time error.
side note: If you are using gcc compiler then you can use gcc -Wall -pedantic to generate warnings
It should be:
write (fd,&ret, sizeof(char));
write takes the pointer to the memory position, and since ret is a single char, you need to pass a pointer to it.

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