A larger buffer won't help. Setting the buffer size to zero won't help. The problem is that each vfprintf() call can generate multiple calls to write() - the calls that actually write the data to the file. Each individual write() call is guaranteed to be atomic, but a series of multiple write() calls from different processes and/or threads will wind up as you're seeing - all interleaved together.
You need to make certain each call to your writeLog() function results in one and only one underlying write() call.
write(). Not fwrite() or fprintf().
Code:
#define BUF_LEN 4096
void writeLog( const char *format, ... )
{
char buffer[ BUF_LEN ];
char *ptr;
int length;
va_list args;
va_start( args, format );
// expand the format and args into a single string
length = vsnprintf( buffer, BUF_LEN, format, args );
// if the buffer isn't big enough, allocate a temporary
// one on the stack (if strings can be really long and
// blow up the stack, use malloc() here - but then you
// have to add complexity to the code to make sure
// any malloc()'d buffer is freed.)
if ( length >= BUF_LEN )
{
// I don't recall offhand if the "+ 1" is needed, but
// it doesn't hurt to have it
ptr = ( char * ) alloca( length + 1 );
length = vsnprintf( ptr, length + 1, format, args );
}
else
{
ptr = buffer;
}
write( logFd, ptr, length );
va_end( args );
}
In theory, you could pass a zero length to the first call to vsnprintf() and get back the known length of the final string. In practice there are still vsnprintf() implementations that don't do that per current standards. (The old SUSv2 standard specified different return values.)
I need to create a shared library to access an in memory DB. The DB is not huge, but big enough to make it cumbersome to carry around in every single process using the shared library. Luckily, it is pretty static information, so I don't need to worry much about synchronizing the data between... (12 Replies)
Hi all,
Well I've had a bit more experience with Unix-like environments since my last post, now that I have started working on my website in earnest and am doing much of the file manipulation via the command line through SSH.
The thing is, I want to be able to log all console activity,... (4 Replies)
I am writing a shared library in Linux (but compatible with other UNIXes) and I want to allow multiple instances to share a piece of memory -- 1 byte is enough. What's the "best" way to do this? I want to optimize for speed and portability.
Obviously, I'll have to worry about mutual exclusion. (0 Replies)
I am trying to figure a way to have a log file and still keep the output in the terminal in a script.
The example below logs to a file nicely but i still want the output in the terminal as well
#!/bin/bash
#Create a log
exec >> /path/to/my/logfile
echo "hello world"
Any help would be... (3 Replies)
We are trying to install third party software on this unix server...
Here is the error message we are getting...
error while loading shared libraries: libodbc.so.1: cannot open shared object file: No such file or directory
It seems like odbc driver is not installed...
>rpm -q unixODBC... (1 Reply)
Hello.
I am new to this forum and I would like to ask for advice about low level POSIX programming.
I have to implement a POSIX compliant C shared library.
A file will have some variables and the shared library will have some functions which need those variables.
There is one special... (5 Replies)
Hi,
I've the following code to log the errors any after the command is executed.
# Ksh 88 Version
log_path=/home/etc/fls/fls_log.log
del_path=/home/etc/fls/to_day
rm $del_path/* >> $log_path 2>&1
But I even want to log if the rm command is success without any error along with... (1 Reply)
When unlocking a Linux server's console there's no event indicating successful logging
Is there a way I can fix this ?
I have the following in my rsyslog.conf
auth.info /var/log/secure
authpriv.info /var/log/secure (1 Reply)
Hi,
While running tcpdump command on my Fedora 16 machine I am get shared library issue.
# tcpdump
tcpdump: error while loading shared libraries: libcrypto.so.6: cannot open shared object file: No such file or directory
# which tcpdump
/usr/software/sbin/tcpdump
I have tried... (3 Replies)
Discussion started by: muzaffar.k
3 Replies
LEARN ABOUT DEBIAN
curl_mvaprintf
curl_printf(3) libcurl Manual curl_printf(3)NAME
curl_maprintf, curl_mfprintf, curl_mprintf, curl_msnprintf, curl_msprintf curl_mvaprintf, curl_mvfprintf, curl_mvprintf, curl_mvsnprintf,
curl_mvsprintf - formatted output conversion
SYNOPSIS
#include <curl/mprintf.h>
int curl_mprintf(const char *format, ...);
int curl_mfprintf(FILE *fd, const char *format, ...);
int curl_msprintf(char *buffer, const char *format, ...);
int curl_msnprintf(char *buffer, size_t maxlength, const char *format, ...);
int curl_mvprintf(const char *format, va_list args);
int curl_mvfprintf(FILE *fd, const char *format, va_list args);
int curl_mvsprintf(char *buffer, const char *format, va_list args);
int curl_mvsnprintf(char *buffer, size_t maxlength, const char *format, va_list args);
char *curl_maprintf(const char *format, ...);
char *curl_mvaprintf(const char *format, va_list args);
DESCRIPTION
These are all functions that produce output according to a format string and given arguments. These are mostly clones of the well-known C-
style functions and there will be no detailed explanation of all available formatting rules and usage here.
See this table for notable exceptions.
curl_mprintf()
Normal printf() clone.
curl_mfprintf()
Normal fprintf() clone.
curl_msprintf()
Normal sprintf() clone.
curl_msnprintf()
snprintf() clone. Many systems don't have this. It is just like sprintf but with an extra argument after the buffer that
specifies the length of the target buffer.
curl_mvprintf()
Normal vprintf() clone.
curl_mvfprintf()
Normal vfprintf() clone.
curl_mvsprintf()
Normal vsprintf() clone.
curl_mvsnprintf()
vsnprintf() clone. Many systems don't have this. It is just like vsprintf but with an extra argument after the buffer that
specifies the length of the target buffer.
curl_maprintf()
Like printf() but returns the output string as a malloc()ed string. The returned string must be free()ed by the receiver.
curl_mvaprintf()
Like curl_maprintf() but takes a va_list pointer argument instead of a variable amount of arguments.
To easily use all these cloned functions instead of the normal ones, #define _MPRINTF_REPLACE before you include the <curl/mprintf.h> file.
Then all the normal names like printf, fprintf, sprintf etc will use the curl-functions instead.
AVAILABILITY
These function will be removed from the public libcurl API in a near future. They will instead be made "available" by source code access
only, and then as curlx_-prefixed functions. See lib/README.curlx for further details.
RETURN VALUE
The curl_maprintf and curl_mvaprintf functions return a pointer to a newly allocated string, or NULL if it failed.
All other functions return the number of characters they actually outputted.
SEE ALSO printf(3), sprintf(3), fprintf(3), vprintf(3)libcurl 7.12 30 April 2004 curl_printf(3)