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POC - Inaccurate read() position obtained with lseek()
 

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  1. POC - Inaccurate read() position obtained with lseek()
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    <p>Creating a file, moving to location 128 and writing data but, while reading i need to read it from offset 0 rather than 128 though i am writing in 128. Can someone please point out where i am going wrong.</p> <p>After writing, printing the hex of the file. the data should be printed in dump 1 (location 128) as I am writing to that page. But it is displayed in dump 0 (location 0).</p> <p>Hexdump of the file from outside shows the data to be the location that i have written to (128).</p> <p>Is it something to do with modes or file permissions? I am working on linux os.</p> <pre><code> void readyCache() { fd = open("database.dat",O_RDWR|O_TRUNC|O_CREAT , S_IRUSR|S_IWUSR); } char* getPage(long pageNumber, int fd) { long offset; char* buffer = (char *)malloc(pageSize); offset = (pageNumber)*pageSize; lseek(fd, offset+pageSize, SEEK_SET); lseek(fd, offset, SEEK_SET); read(fd, buffer, pageSize); return buffer; } void setPage(long pageNumber,char* pageData, int fd) { long offset; offset = (pageNumber)*pageSize; lseek(fd, offset, SEEK_SET); write(fd, pageData, pageSize); } void hexdump(int fileDescriptor1, long pageNumber) { cout &lt;&lt; endl; unsigned char readChar; int iterator = 0, j = 0; char * tempBuffer = (char *)malloc(pageSize); tempBuffer = getPage(pageNumber, fileDescriptor1); for(int i=0;i&lt;pageSize;i++) { readChar = tempBuffer[i]; iterator++; j++; printf("%02x ", readChar);//%02x if (iterator == 16) { iterator = 0; cout&lt;&lt;endl; } } } int main() { readyCache(); char * tempBuffer = getPage(1, fd); int a = 1000; memcpy(tempBuffer, &amp;a, sizeof(int)); setPage(1,tempBuffer, fd); cout&lt;&lt;"\nDump 0\n"; hexdump(fd, 0); cout&lt;&lt;"\nDump 1\n"; hexdump(fd, 1); close(fd); return 0; } </code></pre>
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    1. COcut your example code down just little, tl;dr - also, don't cast the result of `malloc()`, it's bad form (and masks errors)
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    2. CO1) Eliminate the double-spacing. 2) Why do you do lseek twice in a row?
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    3. COhttp://www.gnu.org/software/indent/
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      1. USDave Jarvis
 

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