c - Overwrite a line in a file -

i'm trying overwrite line in file contains unsigned long numbers. contents of file this:

1 2 3 4 5 

i want replace specific number number 0. code wrote looks this:

file *f = fopen("timestamps", "r+");  unsigned long times = 0; int pos = 0; while(fscanf(f, "%lu\n", &times) != eof) {     if(times == 3)     {         fseek(f, pos, seek_set);         fprintf(f, "%lu\n", 0);     }     times = 0;     pos = ftell(f); } fclose(f);  f = fopen("timestamps", "r"); times = 0; while(fscanf(f, "%lu\n", &times) != eof) {     printf("%lu\n", times);     times = 0; } fclose(f); 

the output of program looks this:

1 2 10 5 

interestingly, if cat file, looks this:

1 2 10  5 

am making mistake in ftell? also, why didn't printf show missing line cat showed?

i reproduce , fix.

the present problem when open file in r+ must call fseek @ each time switch reading writing , writing reading.

here, correctly call fseek before writing 0, not after write , following read. file pointer not correctly positionned , undefined behaviour.

fix trivial, replace :

if(times == 3) {     fseek(f, pos, seek_set);     fprintf(f, "%lu\n", 0); } 

with

if(times == 3) {     fseek(f, pos, seek_set);     fprintf(f, "%lu\n", 0);     pos = ftell(f);     fseek(f, pos, seek_set); } 

but beware : works here because replace line line of same length. if tried replace line containing 1000 line containing 0 line containing 0 on windows system end of line \r\n , 00 on unix system end of line \n.

because here happen (windows case) :

before rewrite :

...  1  0  0  0 \r \n ... 

after :

...  0 \r \n  0 \r \n ... 

because sequential file ... sequential serie of byte !