How to add data to txt file in C?

Question

in a C program I want to append data to a text file. Used the fopen function like this:

FILE* fileLog;
char logFile_name[] = "C:\\pg\\log.txt";
fileLog = fopen(logFile_name, "r+");
int j = 0;
while (j < 4)
{
    fprintf(fileLog, "%u,%s", GetLastError(), "1_aba_1\n");
    j++;
}

GetLastError sometimes returns (ok), but the file is overwritten and not added.

Used the fopen function like this:

FILE* fileLog;
char logFile_name[] = "C:\\pg\\log.txt";
fileLog = fopen(logFile_name, "a+");
std::cout << GetLastError() << " LOG \n";

int j = 0;
while (j < 3)
{
    fprintf(fileLog, "%u,%s", GetLastError(), "56_aba_4\n");
    j++;
}

Data is added but GetLastErrorgives an error 183. Programm continues work correctly in both cases, but i use this code in postgre extension, and it crashes and lose connection server for unknown reasons. How to correctly add data to a file without error?

Answer 1

from Microsoft System Error Codes:

ERROR_ALREADY_EXISTS 183 (0xB7). Cannot create a file when that file already exists

I think that this is just a reminder that the file is already there.

Answer 2

If you want your code to work as a PostgreSQL extension you should try to use the routines already available in PostgreSQL code that you can find in postgres/src/include/storage/fd.h:

/*
 * calls:
 *
 *  File {Close, Read, Write, Size, Sync}
 *  {Path Name Open, Allocate, Free} File
 *
 * These are NOT JUST RENAMINGS OF THE UNIX ROUTINES.
 * Use them for all file activity...
 *
 *  File fd;
 *  fd = PathNameOpenFile("foo", O_RDONLY);
 *
 *  AllocateFile();
 *  FreeFile();
 *
 * Use AllocateFile, not fopen, if you need a stdio file (FILE*); then
 * use FreeFile, not fclose, to close it.  AVOID using stdio for files
 * that you intend to hold open for any length of time, since there is
 * no way for them to share kernel file descriptors with other files.
 *
 * Likewise, use AllocateDir/FreeDir, not opendir/closedir, to allocate
 * open directories (DIR*), and OpenTransientFile/CloseTransientFile for an
 * unbuffered file descriptor.
 *
 * If you really can't use any of the above, at least call AcquireExternalFD
 * or ReserveExternalFD to report any file descriptors that are held for any
 * length of time.  Failure to do so risks unnecessary EMFILE errors.
 */

This code is available on Linux and Windows.

You can find examples in pg_stat_statements extension source code: postgres/contrib/pg_stat_statements/pg_stat_statements.c