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File locking for read and write

zheavensky (5)
Hi all,

I have two threads, one reads a file and one writes to the same file.
I want to lock the file while writing so read is blocked until the file is unlocked.

I tried using flockfile amd funlockfile without success.

Can someone please show the correct way to achieve this.


thread 1:
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FILE * F;
string outFile = "myfile";
string out2File = "string to be stored";

F = fopen(outFile.c_str(), "a");

flockfile(F);
fputs(out2File.c_str(), F);
funlockfile(F);

fclose(FileStream);


thread 2:

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FILE * F2;
unsigned long length;
char * FileContent;
size_t result;
string retContent = "";

string inFile = "myfile";

F2 = fopen(inFile.c_str(), "r");
fseek (F2 , 0 , SEEK_END);
length = ftell (F2);
rewind (F2);

FileContent = (char*) malloc (sizeof(char)*length);
result = fread (FileContent, 1, length, F2);
//.
//.
free (FileContent);
fclose(FileStream);
Disch (13742)
In my experience, fopen fails if the file has been opened with write permissions (presumably for this very reason). Although in append mode it might be different (I never use append).

Anyway -- "blocking" and "mutually exclusive access to a file" sounds an awful lot like you want a Mutex. Make a mutex and lock it during critical sections (file accesses). Mutexes are fundamental for multithreading and any threading lib should have them.
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