What is the purpose of this function in assembly?
I am reading an assembly code, and I do not have the source code available. There is one function called next_text in the assembly that I am struggling to understand. Can anyone help me on this?
next_text()
push %rbp
push %rbx
sub $0x108,%rsp
mov %rdi,%rbp
mov %esi,%ebx
jmp <next_text+45>
16 mov %rbp,%rdx
mov $0x100,%esi
mov %rsp,%rdi
callq <fgets@plt>
test %rax,%rax
jne <next_text+42>
callq <func>
42 sub $0x1,%ebx
45 test %ebx,%ebx
jg <next_text+16>
add $0x108,%rsp
pop %rbx
pop %rbp
retq
Here %ebx starts with certain large integer like 16000. My understanding of what this function is doing is it takes an argument which is the file stream. It then reads the file line by line. The maximum length of the line is 100 hex which is 256 characters. It then subtracts ebx by 1, and as long as the file is long enough, keeps on reading the file for 16000 lines, and at the end when ebx is 0, it exits. Is my understanding correct? Also, is the command sub $0x108,%rsp particularly important here, or is it just setting up the stack memory?
c assembly x86-64
edited Nov 23 at 2:50
melpomene
58.4k54489
asked Nov 23 at 2:46
dipankar
214
add a comment |
I am reading an assembly code, and I do not have the source code available. There is one function called next_text in the assembly that I am struggling to understand. Can anyone help me on this?
next_text()
push %rbp
push %rbx
sub $0x108,%rsp
mov %rdi,%rbp
mov %esi,%ebx
jmp <next_text+45>
16 mov %rbp,%rdx
mov $0x100,%esi
mov %rsp,%rdi
callq <fgets@plt>
test %rax,%rax
jne <next_text+42>
callq <func>
42 sub $0x1,%ebx
45 test %ebx,%ebx
jg <next_text+16>
add $0x108,%rsp
pop %rbx
pop %rbp
retq
Here %ebx starts with certain large integer like 16000. My understanding of what this function is doing is it takes an argument which is the file stream. It then reads the file line by line. The maximum length of the line is 100 hex which is 256 characters. It then subtracts ebx by 1, and as long as the file is long enough, keeps on reading the file for 16000 lines, and at the end when ebx is 0, it exits. Is my understanding correct? Also, is the command sub $0x108,%rsp particularly important here, or is it just setting up the stack memory?
c assembly x86-64
edited Nov 23 at 2:50
melpomene
58.4k54489
asked Nov 23 at 2:46
dipankar
214
add a comment |
I am reading an assembly code, and I do not have the source code available. There is one function called next_text in the assembly that I am struggling to understand. Can anyone help me on this?
next_text()
push %rbp
push %rbx
sub $0x108,%rsp
mov %rdi,%rbp
mov %esi,%ebx
jmp <next_text+45>
16 mov %rbp,%rdx
mov $0x100,%esi
mov %rsp,%rdi
callq <fgets@plt>
test %rax,%rax
jne <next_text+42>
callq <func>
42 sub $0x1,%ebx
45 test %ebx,%ebx
jg <next_text+16>
add $0x108,%rsp
pop %rbx
pop %rbp
retq
Here %ebx starts with certain large integer like 16000. My understanding of what this function is doing is it takes an argument which is the file stream. It then reads the file line by line. The maximum length of the line is 100 hex which is 256 characters. It then subtracts ebx by 1, and as long as the file is long enough, keeps on reading the file for 16000 lines, and at the end when ebx is 0, it exits. Is my understanding correct? Also, is the command sub $0x108,%rsp particularly important here, or is it just setting up the stack memory?
c assembly x86-64
edited Nov 23 at 2:50
melpomene
58.4k54489
asked Nov 23 at 2:46
dipankar
214
I am reading an assembly code, and I do not have the source code available. There is one function called next_text in the assembly that I am struggling to understand. Can anyone help me on this?
next_text()
push %rbp
push %rbx
sub $0x108,%rsp
mov %rdi,%rbp
mov %esi,%ebx
jmp <next_text+45>
16 mov %rbp,%rdx
mov $0x100,%esi
mov %rsp,%rdi
callq <fgets@plt>
test %rax,%rax
jne <next_text+42>
callq <func>
42 sub $0x1,%ebx
45 test %ebx,%ebx
jg <next_text+16>
add $0x108,%rsp
pop %rbx
pop %rbp
retq
Here %ebx starts with certain large integer like 16000. My understanding of what this function is doing is it takes an argument which is the file stream. It then reads the file line by line. The maximum length of the line is 100 hex which is 256 characters. It then subtracts ebx by 1, and as long as the file is long enough, keeps on reading the file for 16000 lines, and at the end when ebx is 0, it exits. Is my understanding correct? Also, is the command sub $0x108,%rsp particularly important here, or is it just setting up the stack memory?
c assembly x86-64
c assembly x86-64
edited Nov 23 at 2:50
melpomene
58.4k54489
asked Nov 23 at 2:46
dipankar
214
edited Nov 23 at 2:50
melpomene
58.4k54489
asked Nov 23 at 2:46
dipankar
214
edited Nov 23 at 2:50
melpomene
58.4k54489
edited Nov 23 at 2:50
melpomene
58.4k54489
edited Nov 23 at 2:50
melpomene
58.4k54489
58.4k54489
asked Nov 23 at 2:46
dipankar
214
asked Nov 23 at 2:46
dipankar
214
asked Nov 23 at 2:46
dipankar
214
214
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
Looks to me like it is this:
void next_text(FILE *f, count) {
char buf[256];
while (count-- > 0) {
if (fgets(buf, sizeof buf, f)) {
func();
}
}
}
answered Nov 23 at 3:03
mevets
2,030618
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Looks to me like it is this:
void next_text(FILE *f, count) {
char buf[256];
while (count-- > 0) {
if (fgets(buf, sizeof buf, f)) {
func();
}
}
}
answered Nov 23 at 3:03
mevets
2,030618
add a comment |
Looks to me like it is this:
void next_text(FILE *f, count) {
char buf[256];
while (count-- > 0) {
if (fgets(buf, sizeof buf, f)) {
func();
}
}
}
answered Nov 23 at 3:03
mevets
2,030618
add a comment |
Looks to me like it is this:
void next_text(FILE *f, count) {
char buf[256];
while (count-- > 0) {
if (fgets(buf, sizeof buf, f)) {
func();
}
}
}
answered Nov 23 at 3:03
mevets
2,030618
Looks to me like it is this:
void next_text(FILE *f, count) {
char buf[256];
while (count-- > 0) {
if (fgets(buf, sizeof buf, f)) {
func();
}
}
}
answered Nov 23 at 3:03
mevets
2,030618
answered Nov 23 at 3:03
mevets
2,030618
answered Nov 23 at 3:03
mevets
2,030618
answered Nov 23 at 3:03
mevets
2,030618
2,030618
add a comment |
add a comment |
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