Effect of filling memory allocated by malloc()

Although I was able to correct my program and make it run properly, the reason why it wasn't working left me really curious. I made a string with malloc and initialized it...then I did several strcat with it...and then I declares a file pointer...after that and if my string had more than approx. 26 chars the rest would be garbage...but if i declared the pointer previously to the string malloc it worked great. I just can't understand why, here it is some of the code if anyone thinks it's better to see more pls say so: code: char* holder=(char*)malloc(sizeof(char)*100); for(i=0;i<100;i++) *(holder+i)='\0'; strcat(holder,"set xtics ("); //will ignore until the last n lines for(i=0;i<26-n;i++) readline(sfd,line,29); //will manage the last lines char n_column[2]; char freq[3]={0}; for(i;i<26;i++) { readline(sfd,line,29); sscanf(line+4,"%s",freq); write(out,freq,strlen(freq)); write(out,"\n",1); strcat(holder,"'"); sscanf(line,"%s",temp); strcat(holder,temp); strcat(holder,"'"); sprintf(n_column,"%d",counter); strcat(holder," "); strcat(holder,n_column); //for the las one which won't have the , if(i==25) strcat(holder,")"); else strcat(holder,", "); counter++; } //sending to gnuplot using pipe printf("Before: %s\n",holder); FILE *pipe = popen("gnuplot -persist","w"); //why can't it be here!!!! printf("After: %s\n",holder); output: Before: set xtics ('a' 0, 'b' 1, 'c' 2, 'd' 3, 'e' 4, 'f' 5, 'g' 6, 'j' 7, 'k' 8, 'l' 9, 'm' 10, 'n' 11, 'o' 12, 'p' 13, 'q' 14, 'r' 15, 'u' 16, 'v' 17, 'w' 18, 'x' 19, 'y' 20, 'z' 21, 'h' 22, 'i' 23, 't' 24, 's' 25) After: set xtics ('a' 0, 'b' 1, 'c' 2, 'd' 3, 'e' 4, 'f' 5, 'g' 6, 'j' 7, 'k' 8, 'l' 9, 'm' 10, 'n' 11, 'o'� But if I change to: FILE *pipe = popen("gnuplot -persist","w"); char* holder=(char*)malloc(sizeof(char)*100); for(i=0;i<100;i++) *(holder+i)='\0'; Output is fine. So why declaring the file pointer made such a difference? Or is something besides that? Many thanks for you patience.
Why are you using C? You seem like you're doing a lot of harm using it while not understanding memory management. Try using a language like Python or Java where you can't screw up such simple things like building strings out of smaller strings :)

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