Improper reading of file in C - c

Question

I am doing a socket programming for hospital chanelling.
I am trying to read a text file like this
1 Kavi card 0 0 0
2 Anparasanesan gene 0 0 0
3 Thilak card 0 0 0
4 Akilanesan immu 0 0 0
5 Aravinthanesan derm 0 0 0
6 Akalya derm 0 0 0
7 Vishvapriya derm 0 0 0
8 Kavinga immu 0 0 0
9 Anjalie andr 0 0 0
10 Tom andr 0 0 0
but when i am reading that file it gives me the output as :
1 Kavi cardgenecardimmudermdermdermimmuandrandr
2 Anparasanesan genecardimmudermdermdermimmuandrandr
3 Thilak cardimmudermdermdermimmuandrandr
4 Akilanesan immudermdermdermimmuandrandr
5 Aravinthanesan dermdermdermimmuandrandr
6 Akalya dermdermimmuandrandr
7 Vishvapriya dermimmuandrandr
8 Kavinga immuandrandr
9 Anjalie andrandr
10 Tom andr
Here is my code segment
char line[MAXCHAR];
int x = 0;
while (fgets(line, sizeof(line), fp)){
sscanf(line,"%d\t%s\t%s\t%d\t%d\t%d",&dno,&dname,&dspl,&ti1,&ti2,&ti3);
id[x]=dno;
strncpy(name[x], dname, 50);
strncpy(spl[x], dspl, 4);
times[x][0]=ti1;
times[x][1]=ti2;
times[x][2]=ti3;
x++;
}
int z=0;
for(z=0;z<10;z++)
{
snprintf(line, sizeof(line),"%d\t%s\t%s\n",id[z],name[z],spl[z]);
n = strlen(line);
Writen(sockfd,line,n);
}

Answer 1

Let us look at one of the problems.
evil strncpy
Code is using strncpy with a magic number 4. This does not insure spl[x] is a string as the characters may lack a final null character.
strncpy(spl[x], dspl, 4); // Avoid code like this
Later code tries to print a string with "%s" and spl[z] and gets "cardgene..." rather than the expected "card". When spl[z] is not a string, the result is undefined behavior (UB) - anything may happen.
// Alternative: could limit output with
snprintf(line, sizeof(line),"%.*s\n",(int) (sizeof spl[z]), spl[z]);
How to fix?
Do not use sscanf(line,"%s",&dspl); as it lacks either a width limit, or it is not known that dspl is about the same size of line. I'd expect
char dspl[4+1];
sscanf(line,"%4s", dspl);
Better to insure the source string and destination array are sufficient than use strncpy() without tests.
char spl[X_N][4+1];
char dspl[sizeof spl[0]];
// strncpy(spl[x], dspl, 4);
strcpy(spl[x], dspl);
Others fixes include make certain the sscanf() completed as expected. A simple approach uses " %n" to record the scan offset, if it got that far and then look for extra garbage. Unnecessary "\t" removed.
// Insure dname is at least 50+1, dspl is 4+1 or as big as the line
char dname[sizeof line];
char dspl[sizeof line];
// sscanf(line,"%d\t%s\t%s\t%d\t%d\t%d",&dno,&dname,&dspl,&ti1,&ti2,&ti3);
int n = 0;
sscanf(line,"%d%50s%4s%d%d%d %n",&dno,dname,dspl,&ti1,&ti2,&ti3, &n);
if (n==0 || line[n]) {
puts("Bad Input"); // Handle bad input in some fashion
exit(RETURN_FAILURE);
}