Changing file pointer position

Dear Friends

I need your kind help urgently.
I'm writing a piece of program under Ubuntu attempting to open an existing file to update (read/write).I need
to move file pointer to any position in the file based on some calculations and alter the content. But the problem is that when I open the file using (fopen and "a"-mode) it writes always at the end of file and I cannot move backward(fseek has no effect). If I open it in "w"-mode it would be truncated (actually destroyed). The action (forcing the file pointer to set at any location of file) is something I used to do under DOS perfectly. Could you please suggest any solution for the issue?

Have you considered using the "+r" mode?

Some documentation for the open modes of fopen.

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Originally Posted by jimblumberg

Have you considered using the "+r" mode?

Some documentation for the open modes of fopen.

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"r" combinations stand for "Read Only", don't they?

Quote:

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Originally Posted by Sanfter

"r" combinations stand for "Read Only", don't they?

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Please study the manpage for fopen() thoroughly, as jimblumberg gave you above! That will give you all the information you need!

Quote:

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Originally Posted by jimblumberg

Have you considered using the "+r" mode?

Some documentation for the open modes of fopen.

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Dear jimblumberg

Thank you so much for your advice. You solved my problem.