lib_mysqludf_fwrite.c

#ifdef STANDARD
#include <string.h>
#include <stdlib.h>
#include <time.h>
#else
#include <my_global.h>
#include <my_sys.h>
#endif
#include <mysql.h>
#include <m_ctype.h>
#include <m_string.h>
#include <stdlib.h>

#include <ctype.h>

#ifdef HAVE_DLOPEN
#define LIBVERSION "lib_mysqludf_fwrite version 0.1.0"
#define WRITE_ERROR "Error writing to file."

my_bool lib_mysqludf_fwrite_info_init(UDF_INIT *initid, UDF_ARGS *args, char *message){
    return 1;
}

char* lib_mysqludf_fwrite_info(UDF_INIT *initid, UDF_ARGS *args, char* result, unsigned long* length, char *is_null, char *error){
    strcpy(result,LIBVERSION);
    *length = strlen(LIBVERSION);
    return result;
}

my_bool udf_fwrite_init(UDF_INIT *initid, UDF_ARGS *args, char *message){
    unsigned int i = 0;
    if(args->arg_count >= 2){
        for (i = 0; i < args->arg_count; i++) {
            args->arg_type[i] = STRING_RESULT;
        }
        return 0;
    } else {
        strcpy(message,"Usage: fwrite(filename,string1,[string2,...,stringN]");
        return 1;
    }
}

char* udf_fwrite(UDF_INIT *initid, UDF_ARGS *args, char* result, unsigned long* length, char *is_null, char *error){
    FILE *file;
    unsigned int i = 0;
    *length = 0;

    result = malloc(1);

    if ((file = fopen(args->args[0],"w")) != NULL) {
        for (i = 1; i < args->arg_count; i++) {
            if (fwrite(args->args[i], 1, args->lengths[i], file) != args->lengths[i]) {
                *length = strlen(WRITE_ERROR);
                result = realloc(result,*length);
                strncpy(result,WRITE_ERROR,*length);
                fclose(file);
                return result;
            } else {
                result = realloc(result, *length + args->lengths[i] + 1);
                strncpy(result + *length,args->args[i],args->lengths[i]);
                *length += args->lengths[i] + 1;
            }
        }
        fclose(file);
    }

    if (!(*result) || result == NULL) {
        *is_null = 1;
    } else {
        result[*length] = 0x00;
    }

    return result;
}

#endif /* HAVE_DLOPEN */

for my own benefit:

$ gcc -fPIC -shared -I/usr/include/mysql/ -o lib_mysqludf_fwrite.so lib_mysqludf_fwrite.c

also:

$ sudo cp lib_mysqludf_fwrite.so /usr/lib/mysql/plugin/
$ sudo /etc/rc.d/mysqld restart
:: Stopping MySQL Server                                                                         [DONE] 
:: Starting MySQL Server                                                                         [DONE] 
$ mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
mysql> create function udf_fwrite returns string soname 'lib_mysqludf_fwrite.so';
Query OK, 0 rows affected (0.00 sec)
mysql> select udf_fwrite('/tmp/test','abc');
+-------------------------------+
| udf_fwrite('/tmp/test','abc') |
+-------------------------------+
| abc                           |
+-------------------------------+

LIBDIR=/usr/lib
INCLUDE=/usr/include
MYSQL=$(INCLUDE)/mysql
PLUGINDIR=/usr/lib/mysql/plugin
CC=/usr/bin/gcc

objects=$(patsubst %.c,%.so,$(wildcard *.c))

$(PLUGINDIR)/%.so: %.so
    install -d $(dir $@)
    install -t $(dir $@) $<

%.so: %.c
    $(CC) -Wall -I$(MYSQL) -I. -shared -fPIC $< -o $@

all: $(objects)


clean:
    rm $(PWD)/*.so || true

install: $(patsubst %,$(PLUGINDIR)/%,$(objects))
    mysql -u root < install.sql

DROP FUNCTION IF EXISTS lib_mysqludf_fwrite_info;
CREATE FUNCTION lib_mysqludf_fwrite_info RETURNS string SONAME 'lib_mysqludf_fwrite.so';

DROP FUNCTION IF EXISTS udf_fwrite;
CREATE FUNCTION udf_fwrite RETURNS string SONAME 'lib_mysqludf_fwrite.so';